**Application of Clay Mineral-Iridium(III) Complexes Hybrid Langmuir-Blodgett Films for Photosensing**

Hisako Sato, Kenji Tamura and Akihiko Yamagishi

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/47793

## **1. Introduction**

258 Clay Minerals in Nature – Their Characterization, Modification and Application

challenge, Appl. Cat. A 211:1-17.

[44] Schuchardt U., Cardoso D., Sercheli R., Pereira R., da Cruz R. S., Guerreiro M.C., Mandelli D., Spinacé E. V., Pires E. L. (2000) Cyclohexane oxidation continues to be a

[45] Ebitani K., Ide M., Mitsudome T., Mizugaki T., Kaneda K. (2002) Creation of a chain-like cationic iron species in montmorillonite as a highly active heterogeneous catalyst for

alkane oxygenations using hydrogen peroxide, Chem. Comm.: 690-691.

There has been an extensive interest in developing photo-responsive devices based on luminescent transition metal complexes (Sato & Yamagishi, 2007). As a promising applicant for emitting composites, cyclometalated iridium(III) complexes are attracting a wide attention due to their highly emitting properties in a visible region (Lo et al., 2011, Ulbricht et al., 2009). The lifetime of the excited triplet states is very long (ca. 1 s) and the quantum yield attains a value as high as 10 ~ 100 %. These iridium(III) complexes are used for photoresponsive molecular devices such as photo-diodes and oxygen sensors (Lowry & Benhard, 2006, Sajoto et al., 2009 ). The attempts are based on the fact that energy transfer takes place efficiently from the triplet excited state of an iridium(III) complex to semiconductors or an oxygen molecule in the triplet ground state.

Clay is an environmentally-friendly ubiquitous material. They are characterized by layered structures with cation-exchange properties (Ogawa & Kuroda, 1995). Cationic molecules are intercalated in the narrow galleries between aluminosilicate layers. The materials are used as a host for various types of photochemical reactions. We recently studied the interactions of cationic iridium(III) complexes with a colloidally dispersed clay (Sato et al., 2009, 2011a). The adsorption of iridium(III) complexes by a clay was found to result in the drastic enhancement of emission intensity in an aqueous solution. The attempts demonstrate that emission behavior often provide a key to monitoring the delicate change of adsorption structures.

Recently the application of clay minerals for photochemical reactions was further extended to thin-film systems. For such purposes, luminescent Langmuir-Blodgett (LB) films were prepared by depositing the monolayers of amphiphilic iridium(III) complexes onto a glass substrate (Sato et al., 2010). The emission properties of a single layered film were studied under vacuum or

© 2012 Sato et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Sato et licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

under the atmosphere of various gases. As far as our literature survey is concerned, it was the first report on the Langmuir-Blodgett films consisting of iridium(III) complexes with no additives. This pioneering work, however, revealed several problems concerning the low stability and poor reproducibility in sensing functions due to their fragile properties.

Application of Clay Mineral-Iridium(III) Complexes Hybrid Langmuir-Blodgett Films for Photosensing 261

In case of synthetic saponite, the luminescence spectra were measured under air on an aqueous dispersion containing [Ir(III)L1] complex and various amounts of a clay. Notably quantum yield ( increased from 0.04 to ca.1.0 with the increase of an added clay even in an aqueous dispersion as shown in Fig. 1. The main cause for the increase of might lie in the elimination of water molecules in the vicinity of the [Ir(III)L1] complexes located on a clay surface. The structural fixation of a flexible ligand (dmbpy) in the [Ir(III)L1] complex could be another factor. The introduction of air had little effects on . The fact was in marked contrast with the homogeneous media, in which oxygen molecules quench the excited complexes efficiently. Thus a clay provided such a site as protected from quenching by oxygen molecules. The emission intensity continued to increase even after the equivalent amount of a clay and attained the maximum value around [clay]/[Ir(III)L1] = 10. This might reflect that the adsorbed

complexes were in an isolated state, being free from the self-quenching among them.

**Figure 1.** The effect of synthetic saponite on the luminescence spectra of [Ir(bpy)2dmbpy]+ under degassed condition. The excitation wavelength was 430 nm. The concentration of [Ir(III)L1] complex was 1 x 10-5 M and clay (A) 0.0 M, (B) 1.0x10-5 M, (C) 4.0x10-5 M and (D) 1.0x10-4 M . The lowest dotted curve

The effect of a clay on the transient behavior of excited [Ir(III)L1] complexes was studied by the lifetime measurements under various conditions. Under air, the decay profile was composed of at least two components. This suggested that there were more than two kinds of adsorption states. For example, a part of the complexes were in the interlayer space and the other on the external surface of a clay. If it was the case, the latter state was more easily quenched by oxygen molecules in correspondence to the shorter component of life time. Under argon

was an emission spectrum for the absence of a clay. A solvent was 3:1(v/v) H2O/CH3OH.

In order to overcome the above disadvantages, we attempted to construct a hybrid film of an amphiphilic iridium(III) complex with a clay (Sato et al., 2011b). In these years, the inclusion of layered materials such as layered niobates, titania and clays has been attempted to enhance the mechanical strength of a molecular film and stabilizing its sensor function (Acharya et al. 2009). When clay minerals were used, it was expected that the diversity of elemental compositions of clay sheets might enable us to tune the sensitivity and selectivity of sensing towards a wide range of target molecules. Motivated by these backgrounds, a LB film was constructed by hybridizing an amphiphilic cationic iridium(III) complex with various clays such as synthetic saponite, synthetic hectorite, and natural montmorillonite. As a result, a single layered hybrid LB film was shown to exhibit emission intense enough to study the interaction of the film with gaseous molecules (Sato et al., 2011b). This work would be a benchmark to explore a gas sensor based on cyclometalated iridium(III) complexes.

## **2. Interaction of clays with luminescent iridium(III) complexes**

#### **2.1. Metal ion sensing by luminescence**

Cationic cyclometalated iridium(III) complexes were used as an emitting adsorbate by a clay. We synthesized an iridium(III) complex, [Ir(ppy)2dmbpy]PF6 (ppyH = 2 phenylpyridine and dmbpy = 4,4'-dimethylbipyridine: Chart 1) (denoted by [Ir(III)L1] complex), according to the reported method (Lowry & Benhard, 2006). Synthetic saponite (Kunimine Ind. Co.; (Si7.20Al0.80)(Al0.03Mg5.97)O20(OH)4Na0.77 (CEC: 75 meq/100g) or sodium montmorillonite (Kunipia-P, Kunimine Ind. Co.; (Si7.70Al0.30)(Al3.12Mg0.68Fe0.19)O20(OH)4) (Na0.49Mg0.14) (CEC: 115 meq/100 g ) was used as a host material. Adsorption was carried out by mixing a solution of the [Ir(III)L1] complex with a clay suspension within 10 milliseconds by means of a stopped-flow apparatus. This procedure guaranteed the uniform adsorption of the metal complexes over clay particles particularly at low loading.


**Chart 1.** Chiral structures of [Ir(ppy)2dmbpy]+

In case of synthetic saponite, the luminescence spectra were measured under air on an aqueous dispersion containing [Ir(III)L1] complex and various amounts of a clay. Notably quantum yield ( increased from 0.04 to ca.1.0 with the increase of an added clay even in an aqueous dispersion as shown in Fig. 1. The main cause for the increase of might lie in the elimination of water molecules in the vicinity of the [Ir(III)L1] complexes located on a clay surface. The structural fixation of a flexible ligand (dmbpy) in the [Ir(III)L1] complex could be another factor. The introduction of air had little effects on . The fact was in marked contrast with the homogeneous media, in which oxygen molecules quench the excited complexes efficiently. Thus a clay provided such a site as protected from quenching by oxygen molecules. The emission intensity continued to increase even after the equivalent amount of a clay and attained the maximum value around [clay]/[Ir(III)L1] = 10. This might reflect that the adsorbed complexes were in an isolated state, being free from the self-quenching among them.

260 Clay Minerals in Nature – Their Characterization, Modification and Application

under the atmosphere of various gases. As far as our literature survey is concerned, it was the first report on the Langmuir-Blodgett films consisting of iridium(III) complexes with no additives. This pioneering work, however, revealed several problems concerning the low

In order to overcome the above disadvantages, we attempted to construct a hybrid film of an amphiphilic iridium(III) complex with a clay (Sato et al., 2011b). In these years, the inclusion of layered materials such as layered niobates, titania and clays has been attempted to enhance the mechanical strength of a molecular film and stabilizing its sensor function (Acharya et al. 2009). When clay minerals were used, it was expected that the diversity of elemental compositions of clay sheets might enable us to tune the sensitivity and selectivity of sensing towards a wide range of target molecules. Motivated by these backgrounds, a LB film was constructed by hybridizing an amphiphilic cationic iridium(III) complex with various clays such as synthetic saponite, synthetic hectorite, and natural montmorillonite. As a result, a single layered hybrid LB film was shown to exhibit emission intense enough to study the interaction of the film with gaseous molecules (Sato et al., 2011b). This work would be a

stability and poor reproducibility in sensing functions due to their fragile properties.

benchmark to explore a gas sensor based on cyclometalated iridium(III) complexes.

**2. Interaction of clays with luminescent iridium(III) complexes** 

of the metal complexes over clay particles particularly at low loading.

Cationic cyclometalated iridium(III) complexes were used as an emitting adsorbate by a clay. We synthesized an iridium(III) complex, [Ir(ppy)2dmbpy]PF6 (ppyH = 2 phenylpyridine and dmbpy = 4,4'-dimethylbipyridine: Chart 1) (denoted by [Ir(III)L1] complex), according to the reported method (Lowry & Benhard, 2006). Synthetic saponite (Kunimine Ind. Co.; (Si7.20Al0.80)(Al0.03Mg5.97)O20(OH)4Na0.77 (CEC: 75 meq/100g) or sodium montmorillonite (Kunipia-P, Kunimine Ind. Co.; (Si7.70Al0.30)(Al3.12Mg0.68Fe0.19)O20(OH)4) (Na0.49Mg0.14) (CEC: 115 meq/100 g ) was used as a host material. Adsorption was carried out by mixing a solution of the [Ir(III)L1] complex with a clay suspension within 10 milliseconds by means of a stopped-flow apparatus. This procedure guaranteed the uniform adsorption


**2.1. Metal ion sensing by luminescence** 

**Chart 1.** Chiral structures of [Ir(ppy)2dmbpy]+

2,2'-bipyridine).

**Figure 1.** The effect of synthetic saponite on the luminescence spectra of [Ir(bpy)2dmbpy]+ under degassed condition. The excitation wavelength was 430 nm. The concentration of [Ir(III)L1] complex was 1 x 10-5 M and clay (A) 0.0 M, (B) 1.0x10-5 M, (C) 4.0x10-5 M and (D) 1.0x10-4 M . The lowest dotted curve was an emission spectrum for the absence of a clay. A solvent was 3:1(v/v) H2O/CH3OH.

The effect of a clay on the transient behavior of excited [Ir(III)L1] complexes was studied by the lifetime measurements under various conditions. Under air, the decay profile was composed of at least two components. This suggested that there were more than two kinds of adsorption states. For example, a part of the complexes were in the interlayer space and the other on the external surface of a clay. If it was the case, the latter state was more easily quenched by oxygen molecules in correspondence to the shorter component of life time. Under argon

atmosphere, the decay profile for a clay dispersion changed to a single exponential curve whose lifetime was nearly equal to the longer component under air. This was reasonable since the Ir(III) complexes on an external surface were no more quenched by oxygen molecules.

Application of Clay Mineral-Iridium(III) Complexes Hybrid Langmuir-Blodgett Films for Photosensing 263

Clay minerals have been also applied as a host in the photochemical reactions involving optically active molecules (Fujita et al., 2006). It should be noted that enantioselective luminescence quenching is a dynamical recognition phenomenon (Inoue, 1992, Tsuchiya et al., 2009). The discrimination of chirality is accomplished within the short lifetime of an excited molecule. Clay may assist the emitter to orient preferably for the stereoselective attack by a quencher. If the emitting properties of the [Ir(III)L1] complexes are connected with their chiral structures, it may open a possibility for luminescent chiral sensing. Under these backgrounds, a clay mineral is used as a host to fix the orientation of an iridium(III)

The highly emitting properties of the iridium complex bound by a clay prompted us to investigate the possibility of stereoselective energy transfer. The optical resolution of a cationic iridium(III) complex, [Ir(ppy)2dmbpy]+ (Chart 1), was attempted by several ways such as anionic resolving reagent and chiral adsorbents (Chen et al., 2007). The only successful method was to use an ion-exchange adduct of a clay and chiral [Ru(phen)3]2+ (phen = 1,10-phenanthroline) as a resolving agent. As a chiral quencher, a tris(-

The emission intensity at 650 nm was compared between two systems, clay/-[Ir(III)L1] /- [Ru(acac)3] (pseudo-enantiomeric combination) and clay/-[Ir(III)L1]/-[Ru(acac)3] (pseudoracemic combination), in 3:1 (v/v) water-methanol. In both cases, the intensity of emission decreased on adding [Ru(acac)3], indicating that Ru(III) complex acted as an efficient quencher in these systems. The quenching effect was analyzed in terms of the Stern-Volmer plots (Eq. (1)). It was apparent that luminescence quenching was more efficient for the clay/-[Ir(III)L1] /-[Ru(acac)3] system than for the clay/-[Ir(III)L1] /-[Ru(acac)3] over the whole concentration range. The plots showed the tendency of leveling off at the higher concentration of the

2

(1)

(2)

*O F*

*P*

<sup>0</sup> 1 [] *<sup>q</sup>*

Here, *kq* and *kF* are the bimolecular rate constant of quenching and the unimolecular rate

0 1 2 1 1 2

[ ] 1 1 1

 

*sv o sv o*

1 12 2

in which *I0*, *I*, *f1*, *f2*, *P0, KSV1* and *KSV2* denote the emission intensities in the absence of and in the presence of a quencher, the fractions of processes 1 and 2, the concentration of a quencher and the Stern-Volmer constants for the processes 1 and 2, respectively. *Ksv0* is the

*sv sv sv*

*K fK fK*

*I K*

*I K*

*I f f I KP KP*

**2.2. Enantioselective sensing by luminescence** 

complex towards a quencher (Sato et al., 2011a).

diketonato)ruthenium(III), [Ru(acac)3] (Chart 2), was chosen.

constant of spontaneous luminescence, respectively.

overall Stern-Volmer constant.

quenchers. The curves were fitted by the two-site model as given by Eq . (2):

0

1 2

*f f*

In case of sodium montmorillonite, the emission quantum yield () of the complex decreased by adsorption on a clay particle. The behavior was ascribed to the quenching by Fe(III) ions located in a layer and partly by water molecules as shown in Fig. 2. Interestingly recovered by adding alkali or alkaline-earth metal ions to a clay suspension.The results were rationalized in terms of the model that the quenching by Fe(III) ions. The effect of metal ions on the recovery of luminescence indicated that bound metal ions diminished the quenching ability of water molecules. It was suggested that the adsorption of metal ions hydrated water molecules on the clay surface. Such hydration might deprive water molecules of quenching ability towards the [Ir(III)L1] complexes. If that is the case, the effect is thought to be critically dependent on the charge of the metal ion, because the hydration is stronger for metal ions of higher valence. The fact that alkaline earth metal ions were more effective than alkali metal ions was in accord with this view. It should be emphasized that the influence of metal ions as observed here appeared at concentrations as low as 10-5 M. No work has ever revealed such hydration effects by metal ions at such a low concentration. Highly emitting properties of the present [Ir(III)L1] complex enabled us to detect the effect under those extreme conditions. From a practical point of view, the present finding may open the possibility of developing the sensing of metal ions by use of the emission from a clay-metal complex adduct.

**Figure 2.** Luminescence spectra of an aqueous dispersion containing [Ir(ppy)2dmbpy]+ (6.5×10-6 M) and various amounts of clay ((A) 0.0 M, (B)1.5x10-5 M (C) 1.8x10-4 M). The maximum loading of [Ir(III)L1] complex was 5.4 % with respect to the CEC of clay.

#### **2.2. Enantioselective sensing by luminescence**

262 Clay Minerals in Nature – Their Characterization, Modification and Application

atmosphere, the decay profile for a clay dispersion changed to a single exponential curve whose lifetime was nearly equal to the longer component under air. This was reasonable since the Ir(III) complexes on an external surface were no more quenched by oxygen molecules.

In case of sodium montmorillonite, the emission quantum yield () of the complex decreased by adsorption on a clay particle. The behavior was ascribed to the quenching by Fe(III) ions located in a layer and partly by water molecules as shown in Fig. 2. Interestingly recovered by adding alkali or alkaline-earth metal ions to a clay suspension.The results were rationalized in terms of the model that the quenching by Fe(III) ions. The effect of metal ions on the recovery of luminescence indicated that bound metal ions diminished the quenching ability of water molecules. It was suggested that the adsorption of metal ions hydrated water molecules on the clay surface. Such hydration might deprive water molecules of quenching ability towards the [Ir(III)L1] complexes. If that is the case, the effect is thought to be critically dependent on the charge of the metal ion, because the hydration is stronger for metal ions of higher valence. The fact that alkaline earth metal ions were more effective than alkali metal ions was in accord with this view. It should be emphasized that the influence of metal ions as observed here appeared at concentrations as low as 10-5 M. No work has ever revealed such hydration effects by metal ions at such a low concentration. Highly emitting properties of the present [Ir(III)L1] complex enabled us to detect the effect under those extreme conditions. From a practical point of view, the present finding may open the possibility of developing the

sensing of metal ions by use of the emission from a clay-metal complex adduct.

**Figure 2.** Luminescence spectra of an aqueous dispersion containing [Ir(ppy)2dmbpy]+ (6.5×10-6 M) and various amounts of clay ((A) 0.0 M, (B)1.5x10-5 M (C) 1.8x10-4 M). The maximum loading of [Ir(III)L1]

complex was 5.4 % with respect to the CEC of clay.

Clay minerals have been also applied as a host in the photochemical reactions involving optically active molecules (Fujita et al., 2006). It should be noted that enantioselective luminescence quenching is a dynamical recognition phenomenon (Inoue, 1992, Tsuchiya et al., 2009). The discrimination of chirality is accomplished within the short lifetime of an excited molecule. Clay may assist the emitter to orient preferably for the stereoselective attack by a quencher. If the emitting properties of the [Ir(III)L1] complexes are connected with their chiral structures, it may open a possibility for luminescent chiral sensing. Under these backgrounds, a clay mineral is used as a host to fix the orientation of an iridium(III) complex towards a quencher (Sato et al., 2011a).

The highly emitting properties of the iridium complex bound by a clay prompted us to investigate the possibility of stereoselective energy transfer. The optical resolution of a cationic iridium(III) complex, [Ir(ppy)2dmbpy]+ (Chart 1), was attempted by several ways such as anionic resolving reagent and chiral adsorbents (Chen et al., 2007). The only successful method was to use an ion-exchange adduct of a clay and chiral [Ru(phen)3]2+ (phen = 1,10-phenanthroline) as a resolving agent. As a chiral quencher, a tris( diketonato)ruthenium(III), [Ru(acac)3] (Chart 2), was chosen.

The emission intensity at 650 nm was compared between two systems, clay/-[Ir(III)L1] /- [Ru(acac)3] (pseudo-enantiomeric combination) and clay/-[Ir(III)L1]/-[Ru(acac)3] (pseudoracemic combination), in 3:1 (v/v) water-methanol. In both cases, the intensity of emission decreased on adding [Ru(acac)3], indicating that Ru(III) complex acted as an efficient quencher in these systems. The quenching effect was analyzed in terms of the Stern-Volmer plots (Eq. (1)). It was apparent that luminescence quenching was more efficient for the clay/-[Ir(III)L1] /-[Ru(acac)3] system than for the clay/-[Ir(III)L1] /-[Ru(acac)3] over the whole concentration range. The plots showed the tendency of leveling off at the higher concentration of the quenchers. The curves were fitted by the two-site model as given by Eq . (2):

$$\frac{I\_0}{I} = 1 + \frac{K\_q}{K\_F} [P\_{O\_2}] \tag{1}$$

Here, *kq* and *kF* are the bimolecular rate constant of quenching and the unimolecular rate constant of spontaneous luminescence, respectively.

$$\begin{aligned} \frac{I\_0}{I} &= \mathbf{I} \frac{f\_1}{1 + K\_{sv1} P\_o} + \frac{f\_2}{1 + K\_{sv2} P\_o} \mathbf{J}^{-1} \\ f\_1 + f\_2 &= 1 \\ K\_{sv\_0} &= f\_1 \times K\_{sv1} + f\_2 \times K\_{sv2} \end{aligned} \tag{2}$$

in which *I0*, *I*, *f1*, *f2*, *P0, KSV1* and *KSV2* denote the emission intensities in the absence of and in the presence of a quencher, the fractions of processes 1 and 2, the concentration of a quencher and the Stern-Volmer constants for the processes 1 and 2, respectively. *Ksv0* is the overall Stern-Volmer constant.

Application of Clay Mineral-Iridium(III) Complexes Hybrid Langmuir-Blodgett Films for Photosensing 265

and an alkylammonium cation (trimethylstearylammonium) as prepared by the Langmuir-Blodgett method. (Inukai et al., 1994). For preparing such a film, the ion-exchanged adduct of a clay-alkylammonium is dispersed in chloroform and spread over the surface of pure water. According to the method, a layer-by-layer film was prepared in such a way as donor and acceptor molecules were intercalated in an alternative order. It was revealed that a single clay layer acted as an efficient barrier in the transfer of photon energy. For example, the photoinduced electron-transfer was studied from an amphiphilic polypyridyl-Ru(II) complex (electron donor) to an amphiphilic acetylacetonato-Ru(III) complex (electron acceptor). Recently the method was called as " *Clay LB Method*"(Tamura et al. , 1999, Ras et al., 2009). We have been attempting to improve the " *Clay LB Method*" in order to develop

An amphiphilic cyclometalated iridium(III) complex, [Ir(ppy)2(dc18bpy)]ClO4 (ppy = 2 phenylpyridine; dc18bpy = 4,4'-dioctadecyl-2,2'-bipyridine) (denoted by [Ir(III)L2] (Chart 3)), was prepared by refluxing [Ir(ppy)2Cl]2 with an equal amount of dc18bpy in glycerol at 170 °C for 8 hours. The compound was purified chromatographically by being eluted on an HPLC column (MG (Shiseido Inc. Ltd.)) with chloroform. The Langmuir-Blodgett (LB) method has been applied for preparing a thin clay film as shown in Scheme 2. The details of preparation is described below. A LB trough with an area of 10.0 cm × 13.0 cm is maintained at 20°C by circulating water. The clays used can be synthetic saponite or sodium montmorillonite or synthetic hectorite (Si8.00)(Mg3.50Li0.30)O20(OH)4) (Na0.70). A chloroform solution of an amphiphilic cationic iridium(III) complex, ([Ir(ppy)2(dc18bpy)]ClO4 (3.2x10-5 molL-1), is spread onto an aqueous suspension of a clay at various concentrations. As a reference, the same solution is spread over pure water. A floating monolayer is formed on the surface of a subphase. The surface pressure versus molecular area (-A) curves is obtained by compressing the monolayer. Figure 3 shows the example for -A curves with 0 mgL-1, 10 mgL-1 and 20 mgL-1 of synthetic saponite. In all cases, surface pressure levels off from zero in the region of the molecular area below 0.5 -1.5 nm2 per molecule. A critical molecular area (Sc) is obtained by extrapolating the linear portion of each -A curve to zero surface pressure. Both Sc changes significantly, when a subphase of pure water is replaced with a clay suspension. Moreover the slope of

nano-structured photodevices based on clay minerals (Sato et al., 2005).

**Chart 3.** The structure of [Ir(ppy)2(dc18bpy)]+

**Chart 2.** Chiral structures of [Ru(acac)3] as a quencher: [Ru(acac)3] left) and [Ru(acac)3] (right)

In order to confirm the existence of stereoselectivity, we performed the same experiments for the opposite emitter/quencher combinations or the clay/-[Ir(III)L1] /-[Ru(acac)3] (pseudo-enantiomeric combination) and the clay/-[Ir(III)L1] /-[Ru(acac)3] (pseudo-racemic combination). From the *Ksv0* obtained from Eq. 2, the overall selectivity factor, which is defined to be the ratio of *Ksv0*( or )/ *Ksv0*(), was obtained to be 1.84 in favor of the pseudo-enantiomeric combination. The quenching reaction was not a simple collisional process, but it might involve the process of molecular association on a clay surface. It was added that no stereoselectivity was detected in methanol for the same emitter/quencher pairs. Thus the fixation of the iridium(III) complex on a clay surface was concluded to be a crucial step for chiral recognition as shown in Scheme 1.

**Scheme 1.** A model of chiral sensing by [Ir(III)L1] complexes adsorbed on a clay surface

## **3. Preparation of thin films of clays by the Langmuir-Blodgett (LB) method**

The photochemical reactions involving clay minerals were further extended to thin film systems. In such attempts, the preparation of thin films with uniform properties is essentially important to achieve well-defined reaction systems. Yamagishi *et al.* first reported the nanometer-thick films of an ion-exchange adduct of a clay (synthetic saponite) and an alkylammonium cation (trimethylstearylammonium) as prepared by the Langmuir-Blodgett method. (Inukai et al., 1994). For preparing such a film, the ion-exchanged adduct of a clay-alkylammonium is dispersed in chloroform and spread over the surface of pure water. According to the method, a layer-by-layer film was prepared in such a way as donor and acceptor molecules were intercalated in an alternative order. It was revealed that a single clay layer acted as an efficient barrier in the transfer of photon energy. For example, the photoinduced electron-transfer was studied from an amphiphilic polypyridyl-Ru(II) complex (electron donor) to an amphiphilic acetylacetonato-Ru(III) complex (electron acceptor). Recently the method was called as " *Clay LB Method*"(Tamura et al. , 1999, Ras et al., 2009). We have been attempting to improve the " *Clay LB Method*" in order to develop nano-structured photodevices based on clay minerals (Sato et al., 2005).

**Chart 3.** The structure of [Ir(ppy)2(dc18bpy)]+

264 Clay Minerals in Nature – Their Characterization, Modification and Application

crucial step for chiral recognition as shown in Scheme 1.

**Scheme 1.** A model of chiral sensing by [Ir(III)L1] complexes adsorbed on a clay surface

**3. Preparation of thin films of clays by the Langmuir-Blodgett (LB)** 

The photochemical reactions involving clay minerals were further extended to thin film systems. In such attempts, the preparation of thin films with uniform properties is essentially important to achieve well-defined reaction systems. Yamagishi *et al.* first reported the nanometer-thick films of an ion-exchange adduct of a clay (synthetic saponite)

(right)

**method** 

**Chart 2.** Chiral structures of [Ru(acac)3] as a quencher: [Ru(acac)3] left) and [Ru(acac)3]

In order to confirm the existence of stereoselectivity, we performed the same experiments for the opposite emitter/quencher combinations or the clay/-[Ir(III)L1] /-[Ru(acac)3] (pseudo-enantiomeric combination) and the clay/-[Ir(III)L1] /-[Ru(acac)3] (pseudo-racemic combination). From the *Ksv0* obtained from Eq. 2, the overall selectivity factor, which is defined to be the ratio of *Ksv0*( or )/ *Ksv0*(), was obtained to be 1.84 in favor of the pseudo-enantiomeric combination. The quenching reaction was not a simple collisional process, but it might involve the process of molecular association on a clay surface. It was added that no stereoselectivity was detected in methanol for the same emitter/quencher pairs. Thus the fixation of the iridium(III) complex on a clay surface was concluded to be a

> An amphiphilic cyclometalated iridium(III) complex, [Ir(ppy)2(dc18bpy)]ClO4 (ppy = 2 phenylpyridine; dc18bpy = 4,4'-dioctadecyl-2,2'-bipyridine) (denoted by [Ir(III)L2] (Chart 3)), was prepared by refluxing [Ir(ppy)2Cl]2 with an equal amount of dc18bpy in glycerol at 170 °C for 8 hours. The compound was purified chromatographically by being eluted on an HPLC column (MG (Shiseido Inc. Ltd.)) with chloroform. The Langmuir-Blodgett (LB) method has been applied for preparing a thin clay film as shown in Scheme 2. The details of preparation is described below. A LB trough with an area of 10.0 cm × 13.0 cm is maintained at 20°C by circulating water. The clays used can be synthetic saponite or sodium montmorillonite or synthetic hectorite (Si8.00)(Mg3.50Li0.30)O20(OH)4) (Na0.70). A chloroform solution of an amphiphilic cationic iridium(III) complex, ([Ir(ppy)2(dc18bpy)]ClO4 (3.2x10-5 molL-1), is spread onto an aqueous suspension of a clay at various concentrations. As a reference, the same solution is spread over pure water. A floating monolayer is formed on the surface of a subphase. The surface pressure versus molecular area (-A) curves is obtained by compressing the monolayer. Figure 3 shows the example for -A curves with 0 mgL-1, 10 mgL-1 and 20 mgL-1 of synthetic saponite. In all cases, surface pressure levels off from zero in the region of the molecular area below 0.5 -1.5 nm2 per molecule. A critical molecular area (Sc) is obtained by extrapolating the linear portion of each -A curve to zero surface pressure. Both Sc changes significantly, when a subphase of pure water is replaced with a clay suspension. Moreover the slope of


Application of Clay Mineral-Iridium(III) Complexes Hybrid Langmuir-Blodgett Films for Photosensing 267

This section describes how the structure of a LB clay film is studied. After 30 min, the surface was compressed at a rate of 10 cm2 min-1 until the surface pressure reached 10 mNm-1. A floating film was transferred at 10 mNm-1 onto a hydrophilic glass plate or silicon by the vertical method at a dipping rate of 10 mm min-1. The transfer ratio was estimated to be 0.9 ± 0.1 for all cases. The AFM images of the deposited film showed the presence of particles with the characteristic shape depending on the kind of clay. They definitely demonstrated the inclusion of clay particles in the deposited films. In case of synthetic saponite, for example, the film was composed of spherical domains with the diameter of ca. 50 nm, which indicated the presence of saponite particles. [Ir(III)L2] complexes were thought to be attached uniformly by the particles. In case of synthetic hectorite, the flat regions with the height of ca. 2 nm were observed, indicating the inclusion of hectorite particles. Small domains were observed on such a flat region, which corresponded to the aggregated states of [Ir(III)L2] complexes. In case of montmorillonite, the films were covered with flat particles in various shapes. The thickness of the flat particle was estimated to be ca. 2 nm. Subtracting the thickness of one clay layer (1 nm) from this value, the height of an iridium(III) complex was estimated to be 1 nm. This was less than one-half of the molecular length of the iridium complex along the long alkyl chains. Thus these complexes were thought to be adsorbed

**5. Application of clay-iridium(III) complex LB films for photo-sensing** 

The emission behavior was investigated on a hybrid film of an [Ir(III)L2] complex and a clay as prepared by the LB method. For measurement of emission spectra from a LB film, a glass substrate was placed in a quartz cell at 45 degrees with respect to the incident light (Scheme 3(a)). A gas was introduced into the cell after it was evacuated below 0.1 m torr. The emission spectra was measured under vacuum at room temperature when these substrates

**Scheme 3.** (a) A quartz cell containing a substrate modified with a LB film; (b) Experimental set-up of

**4. Structures and properties of clay LB films** 

with their alkyl chains declined from a clay surface.

were irradiated by a light at 430 nm (Scheme 3(b)).

measuring an emission spectrum from a film in a quartz cell

**5.1. Emission properties of the deposited hybrid LB films** 

**Scheme 2.** *Clay LB* method (vertical deposition)

**Figure 3.** The surface pressure versus molecular area (-A) curves when a chloroform solution of [Ir(ppy)2(dc18bpy)]ClO4 was spread over a subphase of (A) pure water, or (B) synthetic saponite (10 mg L-1) or (C) synthetic saponite (20 mgL-1).

### **4. Structures and properties of clay LB films**

266 Clay Minerals in Nature – Their Characterization, Modification and Application

monolayer of the metal complex.

**Scheme 2.** *Clay LB* method (vertical deposition)

L-1) or (C) synthetic saponite (20 mgL-1).


**Figure 3.** The surface pressure versus molecular area (-A) curves when a chloroform solution of [Ir(ppy)2(dc18bpy)]ClO4 was spread over a subphase of (A) pure water, or (B) synthetic saponite (10 mg This section describes how the structure of a LB clay film is studied. After 30 min, the surface was compressed at a rate of 10 cm2 min-1 until the surface pressure reached 10 mNm-1. A floating film was transferred at 10 mNm-1 onto a hydrophilic glass plate or silicon by the vertical method at a dipping rate of 10 mm min-1. The transfer ratio was estimated to be 0.9 ± 0.1 for all cases. The AFM images of the deposited film showed the presence of particles with the characteristic shape depending on the kind of clay. They definitely demonstrated the inclusion of clay particles in the deposited films. In case of synthetic saponite, for example, the film was composed of spherical domains with the diameter of ca. 50 nm, which indicated the presence of saponite particles. [Ir(III)L2] complexes were thought to be attached uniformly by the particles. In case of synthetic hectorite, the flat regions with the height of ca. 2 nm were observed, indicating the inclusion of hectorite particles. Small domains were observed on such a flat region, which corresponded to the aggregated states of [Ir(III)L2] complexes. In case of montmorillonite, the films were covered with flat particles in various shapes. The thickness of the flat particle was estimated to be ca. 2 nm. Subtracting the thickness of one clay layer (1 nm) from this value, the height of an iridium(III) complex was estimated to be 1 nm. This was less than one-half of the molecular length of the iridium complex along the long alkyl chains. Thus these complexes were thought to be adsorbed with their alkyl chains declined from a clay surface.

### **5. Application of clay-iridium(III) complex LB films for photo-sensing**

#### **5.1. Emission properties of the deposited hybrid LB films**

The emission behavior was investigated on a hybrid film of an [Ir(III)L2] complex and a clay as prepared by the LB method. For measurement of emission spectra from a LB film, a glass substrate was placed in a quartz cell at 45 degrees with respect to the incident light (Scheme 3(a)). A gas was introduced into the cell after it was evacuated below 0.1 m torr. The emission spectra was measured under vacuum at room temperature when these substrates were irradiated by a light at 430 nm (Scheme 3(b)).

**Scheme 3.** (a) A quartz cell containing a substrate modified with a LB film; (b) Experimental set-up of measuring an emission spectrum from a film in a quartz cell

The emission peak was slightly dependent on the kind of hybridized clay: 550 nm for synthetic saponite, synthetic hectorite and montmorillonite, and 535 nm for without clay, respectively. The emission intensity was nearly doubled for hybridization with saponite in comparison to that of without clay. Since these films contained nearly the same amount of [Ir(III)L2] complexes as in the film without clay, the increase was thought to be caused by the interaction with a clay surface. Figure 4 shows the emission spectra of [Ir(III)L2]/synthetic saponite and [Ir(III)L2]/pure water. The complexes formed a mono-molecular layer on a clay surface, while they were coagulated to form a multi-molecular layer in the pure LB film. Thus the self-quenching by neighboring molecules would be reduced on a clay surface in comparison to the pure LB film. Among the hybrid films, no enhancement of emission intensity was observed for montmorillonite, probably because Fe(III) ions in a clay layer had an effect of quenching excited iridium(III) complexes.

Application of Clay Mineral-Iridium(III) Complexes Hybrid Langmuir-Blodgett Films for Photosensing 269

observed at the various oxygen pressures. The decrease of emission intensity was evaluated as a function of oxygen pressure. I0/I is plotted as a function of oxygen pressure ([PO2]). Here I and Io denote the luminescence intensity at 550 nm with and without a quencher, respectively. The plots were analyzed according to the equation (Stern-Volmer plots Eq. (1)) The experimental plot did not obey a linear relation but curved downwards at the higher pressure region. The effect was interpreted in terms of the presence of different types of oxygen quenching sites. Assuming that there were two sites for quenching, the curves are fitted by Eq. (2). Comparing the weighted quenching constant among four films, hybrid

**Figure 5.** Emission from an [Ir(III)L2]/montmorillonite LB film deposited on a quartz substrate in air.

**Figure 6.** Dependence of the change of emission intensity on the vapor pressure of oxygen gas for (A) [Ir(III)L2]/synthetic saponite and (B) [Ir(III)L2]/no clay. The excitation and emission wavelengths were 430 nm and 550 nm, respectively. The luminescence intensity was recorded at 535 nm for [Ir(III)L2]/no

For other gases such as water, ethanol, acetone and chloroform, similar experiments were performed on the hybrid LB films of various clays. All these gases acted as a quencher in deactivating the excited iridium(III) complexes. Since these molecules were in the singlet states in contrast to an oxygen molecule, they were assumed to relax the electronic energy of an excited iridium(III) complex non-radiatively through their vibration energy levels. The time

saponite LB film showed the highest sensitivity towards O2.

clay. Curves were fit by two-site model proposed by Eq. (2).

**Figure 4.** Emission spectra from (a) [Ir(III)L2]/synthetic saponite and (b) [Ir(III)L2]/no clay: oxygen pressure was (a) vacuum, (b) 3.7, (c) 9.0, (d) 31.4 and (e) 101.3 kPa, respectively. The films were prepared for an aqueous dispersion containing 10 mg L-1 of synthetic saponite. The excitation wavelength was 430 nm.

#### **5.2. Quenching effects by oxygen and other gaseous molecules**

In order to pursue the possibility of applying a clay hybrid films for oxygen sensing, the effect of oxygen gas was studied on the emission behavior. The emission of [Ir(III)L2]/montmorillonite in air was shown in Fig.5. An oxygen gas was introduced into a quartz cell containing a glass substrate modified with a single-layered hybrid LB film. The emission decreased rapidly until it attained the stationary value within a few seconds. It recovered to the initial value by evacuating the oxygen gas. The results implied that the electronic energy of the [Ir(III)L2] complex in the excited triplet state transferred efficiently to an oxygen molecule in the triplet ground state, leading to the formation of a singlet oxygen molecule. As shown in the Figures 4 and 6, the quenching phenomena were observed at the various oxygen pressures. The decrease of emission intensity was evaluated as a function of oxygen pressure. I0/I is plotted as a function of oxygen pressure ([PO2]). Here I and Io denote the luminescence intensity at 550 nm with and without a quencher, respectively. The plots were analyzed according to the equation (Stern-Volmer plots Eq. (1)) The experimental plot did not obey a linear relation but curved downwards at the higher pressure region. The effect was interpreted in terms of the presence of different types of oxygen quenching sites. Assuming that there were two sites for quenching, the curves are fitted by Eq. (2). Comparing the weighted quenching constant among four films, hybrid saponite LB film showed the highest sensitivity towards O2.

268 Clay Minerals in Nature – Their Characterization, Modification and Application

an effect of quenching excited iridium(III) complexes.

wavelength was 430 nm.

The emission peak was slightly dependent on the kind of hybridized clay: 550 nm for synthetic saponite, synthetic hectorite and montmorillonite, and 535 nm for without clay, respectively. The emission intensity was nearly doubled for hybridization with saponite in comparison to that of without clay. Since these films contained nearly the same amount of [Ir(III)L2] complexes as in the film without clay, the increase was thought to be caused by the interaction with a clay surface. Figure 4 shows the emission spectra of [Ir(III)L2]/synthetic saponite and [Ir(III)L2]/pure water. The complexes formed a mono-molecular layer on a clay surface, while they were coagulated to form a multi-molecular layer in the pure LB film. Thus the self-quenching by neighboring molecules would be reduced on a clay surface in comparison to the pure LB film. Among the hybrid films, no enhancement of emission intensity was observed for montmorillonite, probably because Fe(III) ions in a clay layer had

**Figure 4.** Emission spectra from (a) [Ir(III)L2]/synthetic saponite and (b) [Ir(III)L2]/no clay: oxygen pressure was (a) vacuum, (b) 3.7, (c) 9.0, (d) 31.4 and (e) 101.3 kPa, respectively. The films were prepared for an aqueous dispersion containing 10 mg L-1 of synthetic saponite. The excitation

In order to pursue the possibility of applying a clay hybrid films for oxygen sensing, the effect of oxygen gas was studied on the emission behavior. The emission of [Ir(III)L2]/montmorillonite in air was shown in Fig.5. An oxygen gas was introduced into a quartz cell containing a glass substrate modified with a single-layered hybrid LB film. The emission decreased rapidly until it attained the stationary value within a few seconds. It recovered to the initial value by evacuating the oxygen gas. The results implied that the electronic energy of the [Ir(III)L2] complex in the excited triplet state transferred efficiently to an oxygen molecule in the triplet ground state, leading to the formation of a singlet oxygen molecule. As shown in the Figures 4 and 6, the quenching phenomena were

**5.2. Quenching effects by oxygen and other gaseous molecules** 

**Figure 5.** Emission from an [Ir(III)L2]/montmorillonite LB film deposited on a quartz substrate in air.

**Figure 6.** Dependence of the change of emission intensity on the vapor pressure of oxygen gas for (A) [Ir(III)L2]/synthetic saponite and (B) [Ir(III)L2]/no clay. The excitation and emission wavelengths were 430 nm and 550 nm, respectively. The luminescence intensity was recorded at 535 nm for [Ir(III)L2]/no clay. Curves were fit by two-site model proposed by Eq. (2).

For other gases such as water, ethanol, acetone and chloroform, similar experiments were performed on the hybrid LB films of various clays. All these gases acted as a quencher in deactivating the excited iridium(III) complexes. Since these molecules were in the singlet states in contrast to an oxygen molecule, they were assumed to relax the electronic energy of an excited iridium(III) complex non-radiatively through their vibration energy levels. The time

course of the emission intensity was dependent on the kinds of clays remarkably (Fig.7). For the case of [Ir(III)L2]/montmorillonite, for example, the signal was reversible for introducing and evaporating gases. The results were consistent with the uniform adsorption of iridium(III) complexes on the clay particle as observed in the AFM image. For the case of [Ir(III)L2]/synthetic hectorite, however, methanol and acetonitrile increased emission intensity instead of acting as a quencher. The results suggested that the self-quenching among [Ir(III)L2] complexes decreased by the inclusion of the gas molecules. The disordering of the alkoxy chains might result in the decrease of quenching among the neighboring [Ir(III)L2] complexes in the film. It was noted that a small molecule with functional group such as -OH, >C=O, CN and -Cl quenched the excited [Ir(III)L2] complexes efficiently, while molecules with nofunctional group such as cyclohexane affected little the emission from the hybrid films. Thus the energy relaxation into vibration energy levels occurred exclusively through the specific interaction of the [Ir(III)L2] complexes with these functional groups.

Application of Clay Mineral-Iridium(III) Complexes Hybrid Langmuir-Blodgett Films for Photosensing 271

surface of the film. In this sense, the sensitivity for sensing gas molecules was remarkably

The hybrid Langmuir-Blodgett (LB) films of an amphiphilic iridium(III) complex, [Ir(ppy)2(dc18bpy)]+, and clays (synthetic saponite, synthetic hectorite, and sodium montmorillonite) were prepared. A glass substrate was modified with a single layered LB film and placed into a quartz cell. Luminescence was monitored under the atmosphere of various gases. An oxygen gas, for example, quenched the emission from excited iridium(III) complexes linearly in the pressure range of 0 - 30 kPa, while the quenching effect was saturated above 30 kPa, The results indicated the occurrence of adsorption saturation of oxygen molecules into the film. Other gases with functional groups also quenched the luminescence efficiently. These results demonstrated the potentiality of the present hybrid

This work has been financially supported by MEXT KAKENHI Grant-Aid-for Scientific Research (B) Number 23350069 of Japan. The part of work was financially supported by

Acharya, S., Hill, J. P., & Ariga, K. (2009). Soft Langmuir-Boldgett Technique for Hard

Chen, X., Okamoto, Y., Yano, T., & Otsuki, J. (2007). Direct Enantiomeric Separations of Tris (2-phenylpyridine) Iridium (III) Complexes on Polysaccharide Derivative-based Chiral

Fujita, S., Sato, H., Kakegawa, N., & Yamagishi, A. (2006). Enantioselective Photooxidation of a Sulfide by a Chiral Ruthenium (II) Complex Immobilized on a Montmorillonite Clay Surface: The Role of Weak Interactions in Asymmetric Induction. *J. Phys. Chem. B,*

Nippon Sheet Glass Foundation of Materials and Science and Engineering of Japan.

enhanced by constructing a LB film with nanometer thickness.

*Department of Chemistry, Graduate School of Science and Engineering,* 

**6. Conclusion** 

**Author details** 

Hisako Sato

Kenji Tamura

Akihiko Yamagishi

**7. References** 

**Acknowledgement** 

110, pp. 2533-2540.

LB films as a gas sensing device.

*Ehime University, Matsuyama, Japan* 

*National Institute of Materials Science, Tsukuba, Japan* 

*School of Medicine, Toho University, Ota-ku, Tokyo, Japan* 

Nannomaterials. *Adv. Mater.* , Vol. 21, pp.2959-2981.

Stationary Phases. *J. Sep. Sci.,* Vol. *30*, pp. 713-716.

**Figure 7.** Effects of gases on the time course of the emission intensity for the singly deposited hybrid LB films: (a) [Ir(III)L2]/synthetic saponite, (b) [Ir(III)L2]/synthetic hectorite and (c) [Ir(III)L2]/montmorillonite. The luminescence intensity was measured at 550 nm, respectively. The following gases were used: oxygen, water, methanol, ethanol, acetone, acetonitrile, chloroform and cyclohexane. 101.3 kPa of oxygen was introduced. In other gases, about one-third of their saturation vapor pressure was introduced. The films were prepared for an aqueous dispersion containing 10 mg L-1 of clay. The excitation wavelength was 430 nm.

For comparison, the effect of an oxygen gas on the luminescence was studied for the cast film of saponite ion-exchanged with the [Ir(III)L2] complex. The quenching effect was much less efficient than that for the LB films. The results were reasonable, considering the situations that only a small portion of [Ir(III)L2] complexes were located on the external surface of the film. In this sense, the sensitivity for sensing gas molecules was remarkably enhanced by constructing a LB film with nanometer thickness.

## **6. Conclusion**

270 Clay Minerals in Nature – Their Characterization, Modification and Application

interaction of the [Ir(III)L2] complexes with these functional groups.

course of the emission intensity was dependent on the kinds of clays remarkably (Fig.7). For the case of [Ir(III)L2]/montmorillonite, for example, the signal was reversible for introducing and evaporating gases. The results were consistent with the uniform adsorption of iridium(III) complexes on the clay particle as observed in the AFM image. For the case of [Ir(III)L2]/synthetic hectorite, however, methanol and acetonitrile increased emission intensity instead of acting as a quencher. The results suggested that the self-quenching among [Ir(III)L2] complexes decreased by the inclusion of the gas molecules. The disordering of the alkoxy chains might result in the decrease of quenching among the neighboring [Ir(III)L2] complexes in the film. It was noted that a small molecule with functional group such as -OH, >C=O, CN and -Cl quenched the excited [Ir(III)L2] complexes efficiently, while molecules with nofunctional group such as cyclohexane affected little the emission from the hybrid films. Thus the energy relaxation into vibration energy levels occurred exclusively through the specific

**Figure 7.** Effects of gases on the time course of the emission intensity for the singly deposited hybrid LB

For comparison, the effect of an oxygen gas on the luminescence was studied for the cast film of saponite ion-exchanged with the [Ir(III)L2] complex. The quenching effect was much less efficient than that for the LB films. The results were reasonable, considering the situations that only a small portion of [Ir(III)L2] complexes were located on the external

[Ir(III)L2]/montmorillonite. The luminescence intensity was measured at 550 nm, respectively. The following gases were used: oxygen, water, methanol, ethanol, acetone, acetonitrile, chloroform and cyclohexane. 101.3 kPa of oxygen was introduced. In other gases, about one-third of their saturation vapor pressure was introduced. The films were prepared for an aqueous dispersion containing 10 mg L-1

films: (a) [Ir(III)L2]/synthetic saponite, (b) [Ir(III)L2]/synthetic hectorite and (c)

of clay. The excitation wavelength was 430 nm.

The hybrid Langmuir-Blodgett (LB) films of an amphiphilic iridium(III) complex, [Ir(ppy)2(dc18bpy)]+, and clays (synthetic saponite, synthetic hectorite, and sodium montmorillonite) were prepared. A glass substrate was modified with a single layered LB film and placed into a quartz cell. Luminescence was monitored under the atmosphere of various gases. An oxygen gas, for example, quenched the emission from excited iridium(III) complexes linearly in the pressure range of 0 - 30 kPa, while the quenching effect was saturated above 30 kPa, The results indicated the occurrence of adsorption saturation of oxygen molecules into the film. Other gases with functional groups also quenched the luminescence efficiently. These results demonstrated the potentiality of the present hybrid LB films as a gas sensing device.

## **Author details**

Hisako Sato *Department of Chemistry, Graduate School of Science and Engineering, Ehime University, Matsuyama, Japan* 

Kenji Tamura *National Institute of Materials Science, Tsukuba, Japan* 

Akihiko Yamagishi *School of Medicine, Toho University, Ota-ku, Tokyo, Japan* 

## **Acknowledgement**

This work has been financially supported by MEXT KAKENHI Grant-Aid-for Scientific Research (B) Number 23350069 of Japan. The part of work was financially supported by Nippon Sheet Glass Foundation of Materials and Science and Engineering of Japan.

## **7. References**

	- Inoue, Y. (1992). Asymmetric Photochemical Reactions in Solution*. Chem. Rev.,* Vol. *92,* pp. 741-770.

**Chapter 14** 

© 2012 Navrátilová and Maršálek, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is

distribution, and reproduction in any medium, provided the original work is properly cited.

© 2012 Navrátilová and Maršálek, licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

**Application of Electrochemistry for Studying** 

Electrochemical methods have been used for studying clay minerals to a limited extent in comparison with X-ray diffraction, infrared spectroscopy, thermal analysis etc. However, so called "clay electrodes" have become individual and indisputable part of electrochemistry [1 - 8] since the first electrode modified with clay mineral was described in 1983 [1]. Voltammetry on clay electrodes has found its role in the research of clay minerals and their properties, especially ion-exchange and sorption. A thin layer of inorganic material – clay mineral – on the electrode surface does not possess the significant isolating properties thus charge transport proceeds on the clay electrode. The electrode covered with the clay mineral film enables to study the electrode processes and surfaces. By means of the standard electrochemical methods, transport of charge through the clay layer, the sorption and ionexchange processes in the clay minerals structure can be studied, too. Accumulation of the electroactive compounds into the clay mineral can be successfully used in electroanalysis [7,

Possibilities of electrochemistry in the study of clay minerals by the clay modified electrodes have been in detail stated in the reviews of A. Fitch and her colleagues [3 - 5]. Very interesting opinion concerning the clay mineral structure is presented in the work dealing with study of flow and transport of compounds through the clay film [5]. The clay structure due to its layers charge forms an electrically charged interphase clay – liquid, thus electric double-layer exists on the surface of the clay minerals particles. The processes taking place in the double-layer are considered to be analogous to those in the interphase electrode – solution. The transport mechanisms in the charged media can be studied by the similar way – for example electroosmosis, electromigration or conductivity. These phenomena study has a practical significance in the electrochemical renewal of the soils contaminated with metals [5], for example the technology for elimination of As, Cu, Cd, Cr, Pb, and Zn from the

**Sorption Properties of Montmorillonite** 

Zuzana Navrátilová and Roman Maršálek

http://dx.doi.org/10.5772/48227

**1. Introduction** 

9].

Additional information is available at the end of the chapter

properly cited.


## **Application of Electrochemistry for Studying Sorption Properties of Montmorillonite**

Zuzana Navrátilová and Roman Maršálek

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/48227

## **1. Introduction**

272 Clay Minerals in Nature – Their Characterization, Modification and Application

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Iridium (III) Complex. *Chem Lett.*, Vol. 38, pp.14-15.

Colloidal Saponite Clay. *Chem. Lett.*, Vol. 40, pp.63-65.

Sensing. *New J. Chem.*, Vol. 34, pp.617-622.

Interface. *New J. Chem.*, Vol. 35, pp.394-399.

Iridium(III). *Eur. J. Inorg. Chem.,* pp. *2104-2109*.

*Chem. B*, Vol*.* 109*,* pp.18935-18941.

741-770.

pp.265-287.

pp.7970-7977.

932.

Vol. 95, pp. 399-438.

*Rev*., Vol. 8, pp.67-84.

21, pp.4418-4441.

Inoue, Y. (1992). Asymmetric Photochemical Reactions in Solution*. Chem. Rev.,* Vol. *92,* pp.

Inukai, K. , Hotta, Y., Taniguchi, M., Tomura, S., & Yamagishi, A. (1994). Formation of a Clay Monolayer at an Air-Water Interface. *J. Chem. SOC. Chem. Commun.*, pp.959-960. Lo, K. K.-W., Li, S.P.-Y., & Zhnag, K. Y. (2011). Development of Luminescent Iridium(III) Polypyridine Complexes as Chemical and Biological Probes. *New J. Chem.,* Vol. 35,

Lowry, M. S., & Bernhard, S. (2006). Synthetically Tailored Excited States: Phosphorescent, Cyclometalated Iridium(III) Complexes and Their Applications. *Chem. Eur. J*. , Vol. 12,

Ogawa, M., & Kuroda, K. (1995). Photofunctions of Intercalation Compunds. *Chem. Rev.*,

Ras, R. H. A. , Umemura, Y. , Johnston, C. T. , Yamagishi, A. , & Schoonheydt, R. A. (2007).Ultrathin Hybrid Films of Clay Minerals. *Phys. Chem. Chem. Phys.*, Vol. 9, pp. 918-

Sajoto, T., Djurovich, P. I., Tamayo, A. B., Oxgaard, J., Goddard III., W. A. & Thompson, M. E. (2009). Temperature Dependence of Blue Phosphorescent Cyclometalated Ir(III)

Sato, H., Hiroe, Y., Tamura, K., & Yamagishi, A. (2005). Orientation Tuning of a Polypyridyl Ru(II) Complex Immobilized on a Clay Surface toward Chiral Discrimination. *J. Phys.* 

Sato, H., & Yamagishi, A. (2007). Application of the ΔΛ Isomerism of Octahedral Metal Complexes as a Chiral Source in Photochemistry. *J. Photochem.Photobiol. C; Photochem.* 

Sato, H., Tamura,K., Taniguchi, M., & Yamagishi, A. (2009). Metal Ion Sensing by Luminescence from an Ion -exchange Adduct of Clay and Cationic Cyclometalated

Sato, H., Tamura, K., Taniguchi , M., & Yamagishi, A. (2010). Highly Luminescent Langmuir-Blodgett Films of Amphiphilic Ir(III) Complexes for Application in Gas

Sato, H., Tamura, K., Aoki, R., Kato, M., & Yamagishi, A. (2011a). Enantioselective Sensing by Luminescence from Cyclometalated Iridium (III) Complexes Adsorbed on a

Sato, H., Tamura, K., Ohara, K., Nagaoka, S. , & Yamagishi, A. (2011b). Hybridization of Clay Minerals with the Floating Film of a Cationic Ir(III) Complex at an Air-water

Tamura, K., Setsuda, H., Taniguchi, M., Yamagishi, A. (1999). A Clay-Metal Complex Ultrathin Film as Prepared by the Langmuir-Blodgett Technique. *Chem. Lett. ,* pp*.*121-122. Tsuchiya, K., Ito, E., Yagai, S., Kitamura, A., & Karatsu, T. (2009). Chirality in the Photochemical mer->fac Geometrical Isomerization of Tris(1-Phenyloytrazolato, N, C2')

Ulbricht, C., Beyer, B., Friebe, C., Winter, A., & Schubert, U. S. (2009). Recent Developments in the Application of Phosphorescent Iridium (III) Complex Systems. *Adv. Mater.*, Vol. Electrochemical methods have been used for studying clay minerals to a limited extent in comparison with X-ray diffraction, infrared spectroscopy, thermal analysis etc. However, so called "clay electrodes" have become individual and indisputable part of electrochemistry [1 - 8] since the first electrode modified with clay mineral was described in 1983 [1]. Voltammetry on clay electrodes has found its role in the research of clay minerals and their properties, especially ion-exchange and sorption. A thin layer of inorganic material – clay mineral – on the electrode surface does not possess the significant isolating properties thus charge transport proceeds on the clay electrode. The electrode covered with the clay mineral film enables to study the electrode processes and surfaces. By means of the standard electrochemical methods, transport of charge through the clay layer, the sorption and ionexchange processes in the clay minerals structure can be studied, too. Accumulation of the electroactive compounds into the clay mineral can be successfully used in electroanalysis [7, 9].

Possibilities of electrochemistry in the study of clay minerals by the clay modified electrodes have been in detail stated in the reviews of A. Fitch and her colleagues [3 - 5]. Very interesting opinion concerning the clay mineral structure is presented in the work dealing with study of flow and transport of compounds through the clay film [5]. The clay structure due to its layers charge forms an electrically charged interphase clay – liquid, thus electric double-layer exists on the surface of the clay minerals particles. The processes taking place in the double-layer are considered to be analogous to those in the interphase electrode – solution. The transport mechanisms in the charged media can be studied by the similar way – for example electroosmosis, electromigration or conductivity. These phenomena study has a practical significance in the electrochemical renewal of the soils contaminated with metals [5], for example the technology for elimination of As, Cu, Cd, Cr, Pb, and Zn from the

© 2012 Navrátilová and Maršálek, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Navrátilová and Maršálek, licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

localities impacted with the dangerous wastes. Of course, the technology is suitable in the case of the metals ions able to participate in the ions reactions and to migrate. An advantage of the electrochemical removing of metals consists in the low financial costs and the insignificant environmental impact in comparison with the technologies based on extraction vaporization or exhaustion.

Application of Electrochemistry for Studying Sorption Properties of Montmorillonite 275

Clay minerals represent the significant natural matrix in the soil medium, which participate in many geochemical processes both natural and those connected with transport and behaviour of the anthropogenic compounds in the soils. Interactions of clay minerals with metal and organic compounds influence their activity, transport, and biological availability. The electrodes modified with clay minerals can serve as a model suitable to study some soil processes connected with clay minerals. The carbon paste electrode modified with vermiculite was used as a model of soil fraction to study the binding interactions of Cu(II) with vermiculite [19]. The selected pesticides and their influence on sorption of Cu(II) on vermiculite were studied. The noncomplexing ligands such as fenamiphos, fenmedipham, and atrazine did not exhibited any influence on the Cu(II) ions sorption. The compounds such as desethylatrazine, desizopropylatrazine, and desethyldesizopropylatrazine do not bind on vermiculite, but they decrease the Cu(II) sorption due to formation the coordination compounds with Cu(II). Apart from the determination of these influences on the metal sorption on clay mineral kinetic and thermodynamic aspects of the sorption processes can be characterized by this way. A soil organo-clay complex – clay humate – is formed by interaction of natural organic matter based on humic and fulvic acids with the clay particles surfaces. The humic adsorbates significantly change properties of clay minerals which influenced their reactions with both natural and anthropogenic substances. The carbon paste electrode modified with the prepared clay humates was used for characterization of the clay minerals reactions with Cu(II) in comparison with the origin clay minerals [20]. Cyclic voltammetry on these electrodes distinguished various types of the clay humates, the obtained results were proved by X-ray diffraction study of the clay humate structures.

Organo-clay modified electrodes represent a new type of clay modified electrodes similar to those with clays grafted with the suitable organic function groups [18, 21, 22]. Similarly as the above mentioned "tunning charge selectivity", cation-exchange ability of clay can be changed to anion-exchange ability due to the cationic surfactants adsorbed onto the clay structure [23]. Clay minerals intercalated with alkylammonium cations (cationic surfactants) exhibit the higher affinity to organic compounds. For example, montmorillonite intercalated with hexadecyltrimethylammonium as a modifier in the carbon paste electrode was able to adsorb pesticides isoproturon, carbendiazim, and methyl parathion [24], which showed to be suitable for stripping voltammetric determination of these pesticides in soil and water. Preconcentration of phenol on glassy carbon electrode modified with film of hydrotalcitelike clay containing surfactant sodium octyl sulfate, sodium dodecyl sulfate, or sodium dodecylbenzenesulfonate [25] as well as octylphenoxypolyethoxyethanol or cetylpyridinium bromide [26] was studied. The electrodes exhibited good sensitivity and reproducibility of phenol determination [25]. Carbon paste electrode modified with montmorillonite exchanged with hexadecyltrimethylammonium bromide was successfully used to determine 4-chlorphenol in water samples [27]. Sorption of Hg, Cd, Pb, Cu, and Zn on montmorillonite intercalated with hexadecyltrimethylammonium cations resulted in use of this organomontmorillonite as a carbon paste modifier [28]. This organo-montmorillonite loaded with 1,3,4-thiadiazole-2,5-dithiol exhibited an excellent selectivity for Hg(II) ions in presence of other ions. The carbon paste electrode modified with these 1,3,4-thiadiazole-2,5-dithiol-

organo-montmorillonite provides a selective sensor for the mercury determination.

Cyclic voltammetry on the clay modified electrodes has been used to study the sorption properties of clay minerals. Repetitive (multisweep) cyclic voltammetry on the clay modified electrode exhibited dependences similar to the sorption isotherms [8]. A consecutive occupation of the ion-exchange sites in the structure of clay mineral by an appropriate compound results in a potential shift in comparison with the unmodified electrode. With increasing concentration of sorbate the potential shift exhibits curves in the shape of the sorption isotherms which can be used to evaluation of an extent of the ionexchange or sorption process. Sorption of metals cations on montmorillonite, vermiculite and kaolinite was studied by means of multisweep cyclic voltammetry on the carbon paste electrodes modified with these clays [10]. Similarly to [8] the current response dependences on the cycling time exhibited the same course as the sorption isotherms. The dependences enable to distinguish an extent of sorption of the individual cation and an ability of clay mineral to adsorb the given cation, of course only in the first approximation. For example, the highest sorption of copper was found in the case of montmorillonite, which was used for determination of Cu [11]. The current vs. time dependences obtained by multisweep cyclic voltammetry on the montmorillonite modified carbon paste electrodes were used for determination of the Cu(II) adsorption kinetics [12]. Adsorption of Cu(II) on the various types of montmorillonite was found to be in accordance with the second order model, the experimental values of the maximum current correlate to those calculated from the supposed equation of the kinetics. Cation exchange of Ag(I) and Ca(II) studied on the carbon paste electrode modified with vermiculite showed to be a dominant process of the cations sorption; the simplified model was worked out and equilibrium constant of the Ag(I) ion exchange was determined [13]. The equilibrium constant value was in a good agreement with the constant determined by other method.

In spite of the lower anion exchange capacity of clay minerals in comparison with the cation exchange capacity the exchange of the complex anions [Hg(ac)4]2-, [HgCl4]2-, and [HgCl3]- (ac – acetate) was proved on the carbon paste electrodes modified with montmorillonite and vermiculite [14] and it was used for determination of Hg [15]. The same mechanism was found in the case of [Au(Cl)4] on the montmorillonite modified carbon paste electrode [16], which was also used in the electroanalysis [16, 17]. The lower anion exchange ability of clay minerals is caused by presence of the negative charge of layer. It is supposed, that the anion forms of compounds are "repelled" and they are not gripped in the interlayer [4]. A suitable chemical modification of clay minerals can enhance their affinity to anions. This so called "tunning charge selectivity" has been applied in the field of clay electrodes [18]. For example, smectite with bound propylamine groups exhibited the higher ability to accumulate anion [Fe(CN)6]3- due to protonization of amine groups. The originally cationexchange smectite was "tunned" to anion-exchange.

Clay minerals represent the significant natural matrix in the soil medium, which participate in many geochemical processes both natural and those connected with transport and behaviour of the anthropogenic compounds in the soils. Interactions of clay minerals with metal and organic compounds influence their activity, transport, and biological availability. The electrodes modified with clay minerals can serve as a model suitable to study some soil processes connected with clay minerals. The carbon paste electrode modified with vermiculite was used as a model of soil fraction to study the binding interactions of Cu(II) with vermiculite [19]. The selected pesticides and their influence on sorption of Cu(II) on vermiculite were studied. The noncomplexing ligands such as fenamiphos, fenmedipham, and atrazine did not exhibited any influence on the Cu(II) ions sorption. The compounds such as desethylatrazine, desizopropylatrazine, and desethyldesizopropylatrazine do not bind on vermiculite, but they decrease the Cu(II) sorption due to formation the coordination compounds with Cu(II). Apart from the determination of these influences on the metal sorption on clay mineral kinetic and thermodynamic aspects of the sorption processes can be characterized by this way. A soil organo-clay complex – clay humate – is formed by interaction of natural organic matter based on humic and fulvic acids with the clay particles surfaces. The humic adsorbates significantly change properties of clay minerals which influenced their reactions with both natural and anthropogenic substances. The carbon paste electrode modified with the prepared clay humates was used for characterization of the clay minerals reactions with Cu(II) in comparison with the origin clay minerals [20]. Cyclic voltammetry on these electrodes distinguished various types of the clay humates, the obtained results were proved by X-ray diffraction study of the clay humate structures.

274 Clay Minerals in Nature – Their Characterization, Modification and Application

vaporization or exhaustion.

with the constant determined by other method.

exchange smectite was "tunned" to anion-exchange.

found in the case of [Au(Cl)4]-

localities impacted with the dangerous wastes. Of course, the technology is suitable in the case of the metals ions able to participate in the ions reactions and to migrate. An advantage of the electrochemical removing of metals consists in the low financial costs and the insignificant environmental impact in comparison with the technologies based on extraction

Cyclic voltammetry on the clay modified electrodes has been used to study the sorption properties of clay minerals. Repetitive (multisweep) cyclic voltammetry on the clay modified electrode exhibited dependences similar to the sorption isotherms [8]. A consecutive occupation of the ion-exchange sites in the structure of clay mineral by an appropriate compound results in a potential shift in comparison with the unmodified electrode. With increasing concentration of sorbate the potential shift exhibits curves in the shape of the sorption isotherms which can be used to evaluation of an extent of the ionexchange or sorption process. Sorption of metals cations on montmorillonite, vermiculite and kaolinite was studied by means of multisweep cyclic voltammetry on the carbon paste electrodes modified with these clays [10]. Similarly to [8] the current response dependences on the cycling time exhibited the same course as the sorption isotherms. The dependences enable to distinguish an extent of sorption of the individual cation and an ability of clay mineral to adsorb the given cation, of course only in the first approximation. For example, the highest sorption of copper was found in the case of montmorillonite, which was used for determination of Cu [11]. The current vs. time dependences obtained by multisweep cyclic voltammetry on the montmorillonite modified carbon paste electrodes were used for determination of the Cu(II) adsorption kinetics [12]. Adsorption of Cu(II) on the various types of montmorillonite was found to be in accordance with the second order model, the experimental values of the maximum current correlate to those calculated from the supposed equation of the kinetics. Cation exchange of Ag(I) and Ca(II) studied on the carbon paste electrode modified with vermiculite showed to be a dominant process of the cations sorption; the simplified model was worked out and equilibrium constant of the Ag(I) ion exchange was determined [13]. The equilibrium constant value was in a good agreement

In spite of the lower anion exchange capacity of clay minerals in comparison with the cation exchange capacity the exchange of the complex anions [Hg(ac)4]2-, [HgCl4]2-, and [HgCl3]-

– acetate) was proved on the carbon paste electrodes modified with montmorillonite and vermiculite [14] and it was used for determination of Hg [15]. The same mechanism was

which was also used in the electroanalysis [16, 17]. The lower anion exchange ability of clay minerals is caused by presence of the negative charge of layer. It is supposed, that the anion forms of compounds are "repelled" and they are not gripped in the interlayer [4]. A suitable chemical modification of clay minerals can enhance their affinity to anions. This so called "tunning charge selectivity" has been applied in the field of clay electrodes [18]. For example, smectite with bound propylamine groups exhibited the higher ability to accumulate anion [Fe(CN)6]3- due to protonization of amine groups. The originally cation-

on the montmorillonite modified carbon paste electrode [16],

(ac

Organo-clay modified electrodes represent a new type of clay modified electrodes similar to those with clays grafted with the suitable organic function groups [18, 21, 22]. Similarly as the above mentioned "tunning charge selectivity", cation-exchange ability of clay can be changed to anion-exchange ability due to the cationic surfactants adsorbed onto the clay structure [23]. Clay minerals intercalated with alkylammonium cations (cationic surfactants) exhibit the higher affinity to organic compounds. For example, montmorillonite intercalated with hexadecyltrimethylammonium as a modifier in the carbon paste electrode was able to adsorb pesticides isoproturon, carbendiazim, and methyl parathion [24], which showed to be suitable for stripping voltammetric determination of these pesticides in soil and water. Preconcentration of phenol on glassy carbon electrode modified with film of hydrotalcitelike clay containing surfactant sodium octyl sulfate, sodium dodecyl sulfate, or sodium dodecylbenzenesulfonate [25] as well as octylphenoxypolyethoxyethanol or cetylpyridinium bromide [26] was studied. The electrodes exhibited good sensitivity and reproducibility of phenol determination [25]. Carbon paste electrode modified with montmorillonite exchanged with hexadecyltrimethylammonium bromide was successfully used to determine 4-chlorphenol in water samples [27]. Sorption of Hg, Cd, Pb, Cu, and Zn on montmorillonite intercalated with hexadecyltrimethylammonium cations resulted in use of this organomontmorillonite as a carbon paste modifier [28]. This organo-montmorillonite loaded with 1,3,4-thiadiazole-2,5-dithiol exhibited an excellent selectivity for Hg(II) ions in presence of other ions. The carbon paste electrode modified with these 1,3,4-thiadiazole-2,5-dithiolorgano-montmorillonite provides a selective sensor for the mercury determination.

The examples mentioned above have at least one common denominator: processes are accompanied by changes of charge on or inside materials. The measurement of zeta potential is one of the methods which provide to obtain imagination about character of the particle surface itself and then also about the processes running on this surface (e.g. adsorption, ion exchange, modification). The experiments connected to the zeta potential measuring represent a factor helping to explain the principles of interactions between surface and its surroundings. As an example heavy metals adsorption on clay minerals (heavy metals removing) or surfactant adsorption on carbonaceous materials (flotation of coals) can be mentioned. The zeta potential knowledge can be also applied in the field of oxidative catalysts, pigments, waste slurries, etc. [29 – 31].

Application of Electrochemistry for Studying Sorption Properties of Montmorillonite 277

and montmorillonite SWy-2 - and their organo-derivatives containing alkylammonium cations - hexadecyltrimethylammonium, benzyldimethylhexadecylammonium, and hexadecylpyridinium. The zeta potential measurement was used to characterize the Cu(II)

Montmorillonites of two types – montmorillonite SAz-1 (MMT,SAz-1) (Apache County, USA) and montmorillonite SWy-2 – (MMT,SWy-2) (Crook County, USA) were provided from The Clay Minerals Society, Source Clays Repository (USA). The fraction used for all experiments consisted of 80 % of particles below 5 μm (Fritsch Particle Sizer Analysette 22, Fritsch GmbH, Idar-Oberstein, Germany). Another sample of montmorillonite MMT,Wy (deposit Wyoming) was obtained from an older collection of colleagues from Institute of

Cation exchange capacity (CEC) was calculated as the sum of cations exchanged with NH4+ ions during leaching per gram of montmorillonite [14]. The CEC values were 56 cmol(+)/kg and 76 cmol(+)/kg for MMT,SAz-1, resp. MMT,SWy-2. Mineralogical characterisation was performed by infrared spectrometry and X-ray diffraction [37]. The montmorillonite samples were classified as pure montmorillonites without any admixture of other minerals

Organo-montmorillonites were prepared by intercalation of three alkylammonium cations. Hexadecyltrimethylammonium bromide (HDTMABr) (Sigma-Aldrich), benzyldimethyl hexadecylammonium chloride (BDHDACl) (Fluka), and hexadecylpyridinium bromide (HDPBr) (Sigma-Aldrich) of analytical reagent grade were used to prepare the modified montmorillonites MMT,SAz-1–HDTMA, MMT,SAz-1-BDHDA, MMT,SAz-1-HDP),

CH3

CH3 N+ Br-

Cl-

3CH

3CH

HDPBr

CH3

MMT,SWy-2–HDTMA, MMT,SWy-2-BDHDA, MMT,SWy-2-HDP.

**Figure 1.** Structures of the used alkylammonium cations

HDTMABr

3CH N+

BDHDACl

and hexadecyltrimethylammonium cation sorption on montmorillonite.

**2. Experimental part** 

Geonics, CAS Ostrava.

including quartz.

3CH

**2.1. Materials and chemicals** 

Clay minerals are very often characterized by measurement of the zeta potential. One of the most common measurements is monitoring the zeta potential changes with changing the pH value. This monitoring is performed by titration of montmorillonite, illite, and chlorite by hydrochloric acid and sodium hydroxide. The dependence of the zeta potential on pH exhibited a typical form. The acid addition led to increase of the zeta potential, on the contrary increase in pH (addition of base) caused decrease of the zeta potential. The individual clay minerals exhibited differences arising from the structure and chemical composition of the studied samples. The most significant change of pH came up in the case of chlorite and it was also the only one clay mineral where the isoelectric point was determined (pH=5) [32]. Knowledge of the zeta potential value at the given pH is necessary for understanding the processes running on the surfaces.

The adsorption of heavy metals and metal oxides on the surface of clay minerals plays an important role as well. Sorption of iron and aluminium on the surface of illite, montmorillonite and kaolinite led to reduction of the negative charges on the particle surface so that the isoelectric point of these minerals was shifted to the higher values of pH [33].

The zeta potential of zeolites was examined in connection to the sorption of heavy metals on these adsorption materials. Dependence of the zeta potential on pH was influenced by concentration of a bulk electrolyte (NaNO3). As the concentration of NaNO3 was increasing the value of the zeta potential was increasing as well. That fact is explained by change of thickness of double-layer caused by ionic strength of solution. These changes consequently influenced the adsorption of heavy metals (Pb, Cu, Cd, and Zn). The highest adsorption capacity was found in water [34].

The surfactant molecules generally adsorb in the interfaces between two bulk phases such as solid-liquid or electrode-solution [35]. When adsorbing on solid an ionic surfactant exhibits the surface charge. Zeta potential is one of few effective techniques for characterization of the surface charge as well as the surface chemical properties of solids in solution and for understanding the changes on the solid surfaces. The zeta potential values correspond to the quantity and quality of functional groups on the surface [36].

The work deals with use of the montmorillonite modified carbon paste electrodes for studying of the Cu(II) sorption on two types of montmorillonite - montmorillonite SAz-1 and montmorillonite SWy-2 - and their organo-derivatives containing alkylammonium cations - hexadecyltrimethylammonium, benzyldimethylhexadecylammonium, and hexadecylpyridinium. The zeta potential measurement was used to characterize the Cu(II) and hexadecyltrimethylammonium cation sorption on montmorillonite.

## **2. Experimental part**

276 Clay Minerals in Nature – Their Characterization, Modification and Application

oxidative catalysts, pigments, waste slurries, etc. [29 – 31].

for understanding the processes running on the surfaces.

quantity and quality of functional groups on the surface [36].

[33].

capacity was found in water [34].

The examples mentioned above have at least one common denominator: processes are accompanied by changes of charge on or inside materials. The measurement of zeta potential is one of the methods which provide to obtain imagination about character of the particle surface itself and then also about the processes running on this surface (e.g. adsorption, ion exchange, modification). The experiments connected to the zeta potential measuring represent a factor helping to explain the principles of interactions between surface and its surroundings. As an example heavy metals adsorption on clay minerals (heavy metals removing) or surfactant adsorption on carbonaceous materials (flotation of coals) can be mentioned. The zeta potential knowledge can be also applied in the field of

Clay minerals are very often characterized by measurement of the zeta potential. One of the most common measurements is monitoring the zeta potential changes with changing the pH value. This monitoring is performed by titration of montmorillonite, illite, and chlorite by hydrochloric acid and sodium hydroxide. The dependence of the zeta potential on pH exhibited a typical form. The acid addition led to increase of the zeta potential, on the contrary increase in pH (addition of base) caused decrease of the zeta potential. The individual clay minerals exhibited differences arising from the structure and chemical composition of the studied samples. The most significant change of pH came up in the case of chlorite and it was also the only one clay mineral where the isoelectric point was determined (pH=5) [32]. Knowledge of the zeta potential value at the given pH is necessary

The adsorption of heavy metals and metal oxides on the surface of clay minerals plays an important role as well. Sorption of iron and aluminium on the surface of illite, montmorillonite and kaolinite led to reduction of the negative charges on the particle surface so that the isoelectric point of these minerals was shifted to the higher values of pH

The zeta potential of zeolites was examined in connection to the sorption of heavy metals on these adsorption materials. Dependence of the zeta potential on pH was influenced by concentration of a bulk electrolyte (NaNO3). As the concentration of NaNO3 was increasing the value of the zeta potential was increasing as well. That fact is explained by change of thickness of double-layer caused by ionic strength of solution. These changes consequently influenced the adsorption of heavy metals (Pb, Cu, Cd, and Zn). The highest adsorption

The surfactant molecules generally adsorb in the interfaces between two bulk phases such as solid-liquid or electrode-solution [35]. When adsorbing on solid an ionic surfactant exhibits the surface charge. Zeta potential is one of few effective techniques for characterization of the surface charge as well as the surface chemical properties of solids in solution and for understanding the changes on the solid surfaces. The zeta potential values correspond to the

The work deals with use of the montmorillonite modified carbon paste electrodes for studying of the Cu(II) sorption on two types of montmorillonite - montmorillonite SAz-1

#### **2.1. Materials and chemicals**

Montmorillonites of two types – montmorillonite SAz-1 (MMT,SAz-1) (Apache County, USA) and montmorillonite SWy-2 – (MMT,SWy-2) (Crook County, USA) were provided from The Clay Minerals Society, Source Clays Repository (USA). The fraction used for all experiments consisted of 80 % of particles below 5 μm (Fritsch Particle Sizer Analysette 22, Fritsch GmbH, Idar-Oberstein, Germany). Another sample of montmorillonite MMT,Wy (deposit Wyoming) was obtained from an older collection of colleagues from Institute of Geonics, CAS Ostrava.

Cation exchange capacity (CEC) was calculated as the sum of cations exchanged with NH4+ ions during leaching per gram of montmorillonite [14]. The CEC values were 56 cmol(+)/kg and 76 cmol(+)/kg for MMT,SAz-1, resp. MMT,SWy-2. Mineralogical characterisation was performed by infrared spectrometry and X-ray diffraction [37]. The montmorillonite samples were classified as pure montmorillonites without any admixture of other minerals including quartz.

Organo-montmorillonites were prepared by intercalation of three alkylammonium cations. Hexadecyltrimethylammonium bromide (HDTMABr) (Sigma-Aldrich), benzyldimethyl hexadecylammonium chloride (BDHDACl) (Fluka), and hexadecylpyridinium bromide (HDPBr) (Sigma-Aldrich) of analytical reagent grade were used to prepare the modified montmorillonites MMT,SAz-1–HDTMA, MMT,SAz-1-BDHDA, MMT,SAz-1-HDP), MMT,SWy-2–HDTMA, MMT,SWy-2-BDHDA, MMT,SWy-2-HDP.

**Figure 1.** Structures of the used alkylammonium cations

All the chemicals used (sodium acetate and acetic acid for preparation of the background electrolyte as well as sodium hydroxide and hydrochloric acid for measurement of zeta potential) were of analytical grade (Merck, Darmstadt, Germany). The sorption solutions of copper were prepared from Cu(NO3)2·3H2O (Lachema Neratovice). The Cu standard for AAS (Cu) (Fluka) was used for AAS analysis. Stock standard solutions of Cu for voltammetry were prepared from Titrisol standards (Merck, Darmstadt, Germany).

Application of Electrochemistry for Studying Sorption Properties of Montmorillonite 279

equipped with a carbon paste electrode (CPE) (working), an Ag/AgCl (saturated KCl) reference electrode, and a Pt wire auxiliary electrode. MCV at a scan rate of 20 mV s–1 was

An appropriate amount of montmorillonite or its organo-derivative and a volume of the Cu(II) solution (concentrations in the range 0,5 – 10 mmol . l-1) was inserted into an Erlenmeyer flask (ratio solid : liquid = 1 : 100). The suspension was shaken at the laboratory temperature for 24 h. The amount of the adsorbed Cu(II) was determined as a difference

The Coulter Delsa 440 SX (Coulter Electronic, USA) instrument was used to measure the zeta potential. Delsa 440 SX uses the scattering effect of Doppler light to determine the electrophoretic mobility. The zeta potential was obtained from the electrophoretic mobility

> .

*ζ* is the zeta potential (V), *η* represents dynamic viscosity (Pa.s), and *ε* stands for the dielectric constant. The fixed conditions of measuring were the following ones: temperature (298 K), electric field (15 V), frequency (500 Hz), and the properties of the samples – viscosity (0.0089 kg.m-1.s-1), refraction index (1.333), and dielectric constant (78.36). The samples were sonicated for 1 minute before zeta potential analysis. All zeta potential measurements were at least duplicated; the mean relative standard deviation of the values reported usually did not exceed 5 %. All the solutions were made in distilled water. Analytical grade chemicals were used. Zeta potential measurements consisted from three

At first a dependence of zeta potential on pH was measured. An amount of 0.1g montmorillonite MMT,Wy was added to the flask with 50 ml of distilled water. The pH value of each suspension was adjusted by adding either NaOH or HCl; pH of the solution was measured using the combination single-junction pH electrode with Ag/AgCl reference

The second and the third step of the zeta potential measurements were in principle the same. The zeta potential changes were monitored after adsorption of Cu(II) and HDTMA on MMT,Wy. The clay fraction with the particle size below 5 μm was used for the adsorption experiments. The clay amount of 0.1 g of was weighed in the flask and 100 ml of the Cu(II) or HDTMA solution of a known concentration was added. The suspensions were inserted into a thermostatic bath (25oC) and flasks were permanently shaken. As it was found in the previous experiments, a 24 hours´ period is needed to reach equilibrium. The zeta potential

(1)

between its concentration before and after the sorption (equation 2 below).

applied with a potential range from –0.6 V to +0.2 V.

**3.3. Sorption of copper** 

**3.4. Zeta potential measurement** 

by the Smoluchowski equation:

cell (LP Prague, model MS 22 pH meter).

steps.

## **3. Procedures**

## **3.1. Preparation of carbon paste electrodes**

Carbon paste electrodes (CPEs) modified with either montmorillonites or their organoderivatives were prepared by the standard procedure [7]. Flake graphite and paraffin oil (Nujol) were thoroughly mixed; in the case of the modified CPEs, an appropriate amount of modifier was added to the graphite before mixing it with oil. A ratio of graphite or the admixture of graphite–modifier to oil was 2.5. The modifier content in the prepared carbon pastes was 10 %, (w/w). The electrodes modifier was before mixing to the carbon paste previously saturated with water vapour, which ensured that the modifier was sufficiently wet but without excess water. No activation and regeneration of the electrode surface prepared in such a way was necessary. The surface was easily renewed by extruding a very small amount of paste and by polishing it on a plastic sheet or scratchboard.

The following carbon paste electrodes were prepared:


#### **3.2. Cyclic voltammetry**

Multisweep cyclic voltammetry (MCV) on the modified CPEs was performed on EKO-TRIBO-Polarograph (EKOTREND, Prague, Czech Republic). A three-electrode cell was equipped with a carbon paste electrode (CPE) (working), an Ag/AgCl (saturated KCl) reference electrode, and a Pt wire auxiliary electrode. MCV at a scan rate of 20 mV s–1 was applied with a potential range from –0.6 V to +0.2 V.

#### **3.3. Sorption of copper**

278 Clay Minerals in Nature – Their Characterization, Modification and Application

**3.1. Preparation of carbon paste electrodes** 

The following carbon paste electrodes were prepared:


of HDTMA-montmorillonite SAz-1

BDHDA-montmorillonite SAz-1

HDP-montmorillonite SAz-1

montmorillonite MMT,SWy-2

of HDTMA-montmorillonite SWy-2

BDHDA-montmorillonite SWY-2

HDP-montmorillonite SWy-2

**3.2. Cyclic voltammetry** 

montmorillonite SAz-1

**3. Procedures** 

All the chemicals used (sodium acetate and acetic acid for preparation of the background electrolyte as well as sodium hydroxide and hydrochloric acid for measurement of zeta potential) were of analytical grade (Merck, Darmstadt, Germany). The sorption solutions of copper were prepared from Cu(NO3)2·3H2O (Lachema Neratovice). The Cu standard for AAS (Cu) (Fluka) was used for AAS analysis. Stock standard solutions of Cu for

Carbon paste electrodes (CPEs) modified with either montmorillonites or their organoderivatives were prepared by the standard procedure [7]. Flake graphite and paraffin oil (Nujol) were thoroughly mixed; in the case of the modified CPEs, an appropriate amount of modifier was added to the graphite before mixing it with oil. A ratio of graphite or the admixture of graphite–modifier to oil was 2.5. The modifier content in the prepared carbon pastes was 10 %, (w/w). The electrodes modifier was before mixing to the carbon paste previously saturated with water vapour, which ensured that the modifier was sufficiently wet but without excess water. No activation and regeneration of the electrode surface prepared in such a way was necessary. The surface was easily renewed by extruding a very









Multisweep cyclic voltammetry (MCV) on the modified CPEs was performed on EKO-TRIBO-Polarograph (EKOTREND, Prague, Czech Republic). A three-electrode cell was

voltammetry were prepared from Titrisol standards (Merck, Darmstadt, Germany).

small amount of paste and by polishing it on a plastic sheet or scratchboard.

An appropriate amount of montmorillonite or its organo-derivative and a volume of the Cu(II) solution (concentrations in the range 0,5 – 10 mmol . l-1) was inserted into an Erlenmeyer flask (ratio solid : liquid = 1 : 100). The suspension was shaken at the laboratory temperature for 24 h. The amount of the adsorbed Cu(II) was determined as a difference between its concentration before and after the sorption (equation 2 below).

#### **3.4. Zeta potential measurement**

The Coulter Delsa 440 SX (Coulter Electronic, USA) instrument was used to measure the zeta potential. Delsa 440 SX uses the scattering effect of Doppler light to determine the electrophoretic mobility. The zeta potential was obtained from the electrophoretic mobility by the Smoluchowski equation:

$$
\zeta' = \frac{\mu \,\eta}{\varepsilon} \tag{1}
$$

*ζ* is the zeta potential (V), *η* represents dynamic viscosity (Pa.s), and *ε* stands for the dielectric constant. The fixed conditions of measuring were the following ones: temperature (298 K), electric field (15 V), frequency (500 Hz), and the properties of the samples – viscosity (0.0089 kg.m-1.s-1), refraction index (1.333), and dielectric constant (78.36). The samples were sonicated for 1 minute before zeta potential analysis. All zeta potential measurements were at least duplicated; the mean relative standard deviation of the values reported usually did not exceed 5 %. All the solutions were made in distilled water. Analytical grade chemicals were used. Zeta potential measurements consisted from three steps.

At first a dependence of zeta potential on pH was measured. An amount of 0.1g montmorillonite MMT,Wy was added to the flask with 50 ml of distilled water. The pH value of each suspension was adjusted by adding either NaOH or HCl; pH of the solution was measured using the combination single-junction pH electrode with Ag/AgCl reference cell (LP Prague, model MS 22 pH meter).

The second and the third step of the zeta potential measurements were in principle the same. The zeta potential changes were monitored after adsorption of Cu(II) and HDTMA on MMT,Wy. The clay fraction with the particle size below 5 μm was used for the adsorption experiments. The clay amount of 0.1 g of was weighed in the flask and 100 ml of the Cu(II) or HDTMA solution of a known concentration was added. The suspensions were inserted into a thermostatic bath (25oC) and flasks were permanently shaken. As it was found in the previous experiments, a 24 hours´ period is needed to reach equilibrium. The zeta potential of the adsorption suspensions was measured. Then, the clay sample was separated by filtration with paper filter.

The amount of Cu(II) and HDTMA adsorbed (*a*) was determined from the change in the solution concentration before and after equilibrium, according to:

$$a = \frac{(c\_0 - c\_e)V}{m} \tag{2}$$

Application of Electrochemistry for Studying Sorption Properties of Montmorillonite 281

The metal amount in the supernatant after sorption was found by means of atomic absorption spectrometry (AA240FS Varian, USA) by flame atomization air-acetylene (flow

The HDTMA concentrations in the supernatant after sorption were determined by UV-VIS spectrophotometry (ALS laboratory group, CZ\_SOP\_D06\_07\_N03, Ostrava, Czech Republic). The BDHDA and HDP concentrations in the supernatant after sorption were determined by UV VIS spectrophotometry (Varian Cary 50) at 264 nm (BDHDA) and 260 nm (HDP). The amount of the adsorbed metal was determined as a difference between its concentration before and after the sorption. The amount of the adsorbed alkylammonium cation was determined as a difference between its concentration before and after the

The obtained XRD patterns of the organo-montmorillonites were analysed for d-values of the basal spacing (001) and compared with those of the original montmorillonites (Table 1).

The organo-montmorillonites exhibited an evident increase of the basal spacing in comparison with those of the unmodified which indicates an intercalation of alkylammonium cations in their interlayer [38 - 40]. The obtained values about 1.7 nm can be judged to bilayer arrangement of cations, the higher values about 2.2 – 2.4 nm probably correspond to a paraffin-type of arrangement – the ammonium groups are attached to the

The infrared spectra of the original montmorillonites have been already studied [37] and interpreted according to [43]. The infrared spectra of the organo-montmorillonites were interpreted with help of the spectra of the pure alkylammonium salts to distinguish new absorption bands in the organo-montmorillonites. All infrared spectra exhibited characteristic absorption bands of the presented alkylammonium cations. The absorption bands at 2920 cm-1 correspond to antisymmetric stretching vibrations and the bands at 2850

silicate layer, the nonopolar chains are oriented under the tilt angle [41, 42].

montmorillonite d(001) [nm] montmorillonite d(001) [nm] MMT,SAz-1 1.47 MMT,SWy-2 1.36 MMT,SAz-1–HDTMA 1.65 MMT,SWy-2–HDTMA 1.68 – 2.40 MMT,SAz-1–BDHDA 1.78 – 2.40 MMT,SWy-2–BDHDA 1.79 MMT,SAz-1–HDP 1.69 – 2.20 MMT,SWy-2–HDP 1.72

rate 13.5 l min-1, Cu 249.2 nm, and slit width 0.5 nm for Cu.

**4.1. Characterization of prepared organo-montmorillonites** 

**3.10. Analysis of alkylammonium cations** 

**3.9. Analysis of metals** 

sorption (equation 2).

**Table 1.** Basal spacing values d(001)

**4. Results** 

where *c0* is the initial concentration of the HDTMA solution, *ce* the concentration of the HDTMA solution at the adsorption equilibrium, *V* the volume of the HDTMA solution and *m* the mass of the clay.

The HDTMA concentration of the filtered solutions was determined by UV/VIS spectrophotometry (ALS laboratory group, CZ\_SOP\_D06\_07\_N03).

#### **3.5. Preparation of organo-montmorillonites**

An amount of 1 g of montmorillonite and 100 ml 7.5 mmol . l-1 solution of alkylammonium cation was shaken at the laboratory temperature for 2.5 h (the time was found in the previous experiments). After the sorption the suspension was centrifuged (9000 rev min-1) for 10 min. The supernatant was removed, the solid was washed out with 5 ml solution of ethanol : water = 2 : 1 and the suspension was again centrifuged at the same rate. The washing out was repeated with 5 ml of ethanol and the preparative of organomontmorillonite was air dried after centrifugation.

#### **3.6. X-ray diffraction**

X-Ray diffraction (XRD) was carried out at Nanotechnology Centre, VSB-Technical University Ostrava. XRD patterns of the tested samples were measured by diffractometer INEL equipped with Cu anode, generator (2000 sec, 35 kV, 20 mA) and detector CPSD 120, samples were measured in a flat rotation holder.

#### **3.7. Infrared spectroscopy**

Infrared spectra were recorded on Nicolet Avatar 320 FTIR spectrometer (ThermoNicolet, USA) equipped with the DTGS/KBr detector for the middle IR range. The KBr pressed-disc (13 mm diameter) technique (1 mg of sample and 200 mg of KBr) was used. The spectra were measured in the spectral range from 4000 to 400 cm-1 (64 scans, 4 cm-1 resolutions).

#### **3.8. Thermal analysis**

Thermal analysis was carried out using multimodular thermal analyser SETSYS 12- SETARAM equipped with a measurement head TG/DTA rod (Institute of Geonics, CAS, Ostrava). The TG/DTA curves were recorded under an air environment from 25 to 1200 °C, the heating rate was 10 K min− 1.

## **3.9. Analysis of metals**

280 Clay Minerals in Nature – Their Characterization, Modification and Application

solution concentration before and after equilibrium, according to:

spectrophotometry (ALS laboratory group, CZ\_SOP\_D06\_07\_N03).

**3.5. Preparation of organo-montmorillonites** 

montmorillonite was air dried after centrifugation.

samples were measured in a flat rotation holder.

filtration with paper filter.

*m* the mass of the clay.

**3.6. X-ray diffraction** 

**3.7. Infrared spectroscopy** 

**3.8. Thermal analysis** 

the heating rate was 10 K min− 1.

of the adsorption suspensions was measured. Then, the clay sample was separated by

The amount of Cu(II) and HDTMA adsorbed (*a*) was determined from the change in the

<sup>0</sup> ( )*<sup>e</sup> c cV*

(2)

*m*

where *c0* is the initial concentration of the HDTMA solution, *ce* the concentration of the HDTMA solution at the adsorption equilibrium, *V* the volume of the HDTMA solution and

The HDTMA concentration of the filtered solutions was determined by UV/VIS

An amount of 1 g of montmorillonite and 100 ml 7.5 mmol . l-1 solution of alkylammonium cation was shaken at the laboratory temperature for 2.5 h (the time was found in the previous experiments). After the sorption the suspension was centrifuged (9000 rev min-1) for 10 min. The supernatant was removed, the solid was washed out with 5 ml solution of ethanol : water = 2 : 1 and the suspension was again centrifuged at the same rate. The washing out was repeated with 5 ml of ethanol and the preparative of organo-

X-Ray diffraction (XRD) was carried out at Nanotechnology Centre, VSB-Technical University Ostrava. XRD patterns of the tested samples were measured by diffractometer INEL equipped with Cu anode, generator (2000 sec, 35 kV, 20 mA) and detector CPSD 120,

Infrared spectra were recorded on Nicolet Avatar 320 FTIR spectrometer (ThermoNicolet, USA) equipped with the DTGS/KBr detector for the middle IR range. The KBr pressed-disc (13 mm diameter) technique (1 mg of sample and 200 mg of KBr) was used. The spectra were measured in the spectral range from 4000 to 400 cm-1 (64 scans, 4 cm-1 resolutions).

Thermal analysis was carried out using multimodular thermal analyser SETSYS 12- SETARAM equipped with a measurement head TG/DTA rod (Institute of Geonics, CAS, Ostrava). The TG/DTA curves were recorded under an air environment from 25 to 1200 °C,

*a*

The metal amount in the supernatant after sorption was found by means of atomic absorption spectrometry (AA240FS Varian, USA) by flame atomization air-acetylene (flow rate 13.5 l min-1, Cu 249.2 nm, and slit width 0.5 nm for Cu.

## **3.10. Analysis of alkylammonium cations**

The HDTMA concentrations in the supernatant after sorption were determined by UV-VIS spectrophotometry (ALS laboratory group, CZ\_SOP\_D06\_07\_N03, Ostrava, Czech Republic). The BDHDA and HDP concentrations in the supernatant after sorption were determined by UV VIS spectrophotometry (Varian Cary 50) at 264 nm (BDHDA) and 260 nm (HDP). The amount of the adsorbed metal was determined as a difference between its concentration before and after the sorption. The amount of the adsorbed alkylammonium cation was determined as a difference between its concentration before and after the sorption (equation 2).

## **4. Results**

### **4.1. Characterization of prepared organo-montmorillonites**

The obtained XRD patterns of the organo-montmorillonites were analysed for d-values of the basal spacing (001) and compared with those of the original montmorillonites (Table 1).


**Table 1.** Basal spacing values d(001)

The organo-montmorillonites exhibited an evident increase of the basal spacing in comparison with those of the unmodified which indicates an intercalation of alkylammonium cations in their interlayer [38 - 40]. The obtained values about 1.7 nm can be judged to bilayer arrangement of cations, the higher values about 2.2 – 2.4 nm probably correspond to a paraffin-type of arrangement – the ammonium groups are attached to the silicate layer, the nonopolar chains are oriented under the tilt angle [41, 42].

The infrared spectra of the original montmorillonites have been already studied [37] and interpreted according to [43]. The infrared spectra of the organo-montmorillonites were interpreted with help of the spectra of the pure alkylammonium salts to distinguish new absorption bands in the organo-montmorillonites. All infrared spectra exhibited characteristic absorption bands of the presented alkylammonium cations. The absorption bands at 2920 cm-1 correspond to antisymmetric stretching vibrations and the bands at 2850 cm-1 to symmetric stretching vibrations of C – H bounds reflecting alkyl chains of alkylammonium cations. The presence of benzene ring in BDHDA and HDP is confirmed by symmetric stretching vibration of C – H bounds in aromates at 3050 cm-1 and by symmetric stretching vibration of C – C bounds of conjugated system at 1620 and 1471 cm-1. The absorption band at 1487 cm-1 corresponds to bending vibration of the N – H bounds of ammonium groups. The measured infrared spectra of MMT,SWy-2 and its derivatives with HDTMA and BDHDA are shown at Figure 2. The other organo-montmorillonites exhibited the similar infrared spectra. The presence of all cations in the organo-montmorillonites is evident due to either intercalation or adsorption process.

Application of Electrochemistry for Studying Sorption Properties of Montmorillonite 283

increasing amount of the added surfactant. In addition, the organo-montmorillonites are subjected to a thermal effect at the temperature interval 270–450 °C corresponding to the alkylammonium decomposition [44]. As expected, the mass loss in the whole temperature

The results obtained by X-ray diffraction, infrared spectroscopy and thermal analysis proved the presence of the alkylammonium cations in the organo-montmorillonites. As it was already stated [38, 45] intercalation of alkylammonium cations takes place due to both ion-exchange and induced and π-π interactions forming a double-layer in the interlayer space of montmorillonites. Although the previous work [38] supposed the higher intercalation due to a content of benzene ring in the case of BDHDA, the values of the

The most important factor that affects the zeta potential is pH. The zeta potential value on its own without a stated pH is only a virtually meaningless number. Generally, the zeta potential versus pH curve will be positive at low pH and lower or negative at high pH. The point where the plot passes through the zero value of the zeta potential is called the isoelectric point and it is very important from a practical consideration. It is normally the

The following figure shows a typical curve for the zeta potential value on the pH value in

1 2 3 4 5 6 7 8 9 10 11 12 13 14

**pH**

interval 25-1200 °C increased with increasing amount of the alkylammonium cations.

**4.2. Characterization of montmorillonite by zeta potential measurement** 

*4.2.1. The pH influence on zeta potential of montmorillonite suspension* 

parameter d(001) (Table 1) do not prove this suggestion.

point where the colloidal system is stable to a lesser extent.

**Figure 3.** The influence of pH on zeta potential of the MMT,Wy particles

the case of the montmorillonite particles.





**zeta potential (mV)**



**Figure 2.** Infrared spectra of montmorillonite SWy-2 and its organo-derivatives

Thermogravimetric (TG) and differential thermal (DTA) curves of the original montmorillonites were found in the curve library of Clay Minerals (Institute of Geonics, CAS, Ostrava). The peak temperatures for montmorillonites obtained from DTA curve are: 166 °C and 234 °C for dehydration, 673 °C and 884 °C for dehydroxilation/melting and 1027 °C for recrystallization/transformation. In the case of the organo-montmorillonites, the shape of TG/DTA curves corresponded to the original montmorillonites, but the thermal effects exhibited the slightly different temperatures and intensities. The organomontmorillonites exhibited the higher values of temperatures related to the total melting. The temperatures of exothermic effects connected to recrystallization and transformation increased with the increasing amount of the alkylammonium cations. The temperature and intensity of the first two peaks related to the dehydration process decreased with the increasing amount of the added surfactant. In addition, the organo-montmorillonites are subjected to a thermal effect at the temperature interval 270–450 °C corresponding to the alkylammonium decomposition [44]. As expected, the mass loss in the whole temperature interval 25-1200 °C increased with increasing amount of the alkylammonium cations.

The results obtained by X-ray diffraction, infrared spectroscopy and thermal analysis proved the presence of the alkylammonium cations in the organo-montmorillonites. As it was already stated [38, 45] intercalation of alkylammonium cations takes place due to both ion-exchange and induced and π-π interactions forming a double-layer in the interlayer space of montmorillonites. Although the previous work [38] supposed the higher intercalation due to a content of benzene ring in the case of BDHDA, the values of the parameter d(001) (Table 1) do not prove this suggestion.

## **4.2. Characterization of montmorillonite by zeta potential measurement**

### *4.2.1. The pH influence on zeta potential of montmorillonite suspension*

282 Clay Minerals in Nature – Their Characterization, Modification and Application

evident due to either intercalation or adsorption process.

**Figure 2.** Infrared spectra of montmorillonite SWy-2 and its organo-derivatives

Thermogravimetric (TG) and differential thermal (DTA) curves of the original montmorillonites were found in the curve library of Clay Minerals (Institute of Geonics, CAS, Ostrava). The peak temperatures for montmorillonites obtained from DTA curve are: 166 °C and 234 °C for dehydration, 673 °C and 884 °C for dehydroxilation/melting and 1027 °C for recrystallization/transformation. In the case of the organo-montmorillonites, the shape of TG/DTA curves corresponded to the original montmorillonites, but the thermal effects exhibited the slightly different temperatures and intensities. The organomontmorillonites exhibited the higher values of temperatures related to the total melting. The temperatures of exothermic effects connected to recrystallization and transformation increased with the increasing amount of the alkylammonium cations. The temperature and intensity of the first two peaks related to the dehydration process decreased with the

cm-1 to symmetric stretching vibrations of C – H bounds reflecting alkyl chains of alkylammonium cations. The presence of benzene ring in BDHDA and HDP is confirmed by symmetric stretching vibration of C – H bounds in aromates at 3050 cm-1 and by symmetric stretching vibration of C – C bounds of conjugated system at 1620 and 1471 cm-1. The absorption band at 1487 cm-1 corresponds to bending vibration of the N – H bounds of ammonium groups. The measured infrared spectra of MMT,SWy-2 and its derivatives with HDTMA and BDHDA are shown at Figure 2. The other organo-montmorillonites exhibited the similar infrared spectra. The presence of all cations in the organo-montmorillonites is

> The most important factor that affects the zeta potential is pH. The zeta potential value on its own without a stated pH is only a virtually meaningless number. Generally, the zeta potential versus pH curve will be positive at low pH and lower or negative at high pH. The point where the plot passes through the zero value of the zeta potential is called the isoelectric point and it is very important from a practical consideration. It is normally the point where the colloidal system is stable to a lesser extent.

> The following figure shows a typical curve for the zeta potential value on the pH value in the case of the montmorillonite particles.

**Figure 3.** The influence of pH on zeta potential of the MMT,Wy particles

The zeta potential of the montmorillonite particles in distilled water (pH 6) reaches approximately -24 mV, the zeta potential is negative. With increasing addition of alkali to the suspension of pH 6 the particles tends to acquire a more negative charge and with increasing addition of acid a charge is negative to a lesser extent.

Application of Electrochemistry for Studying Sorption Properties of Montmorillonite 285


0

50

100

zeta potential/mV

150

200

of the Cu(II) adsorption isotherm with the zeta potential dependence on the Cu(II) concentration in the sorption solutions demonstrated on Figure 4 proved this conclusion.

*4.2.2. Sorption of alkylammonium cations – coherence of classical batch experiments and* 

The following research has been performed in order to show the above mentioned possibility of zeta potential for evaluation of the sorption processes. The adsorption of HDTMA on the montmorillonite SAz-1 was studied by conventionally measured adsorption isotherms and the zeta potential was measured simultaneously in the sorption suspensions. Figure 5 demonstrates the typical adsorption isotherm which shape indicates the adsorption

**Figure 5.** Dependence of zeta potential (blank symbols) and adsorption of HDTM (full symbols) on

012345

*c*e/(mmol dm–3)

Subsequently, the HDTMA adsorption proceeds via van der Waals interactions [45].

Figure 5 shows the remarkable same course of the adsorption isotherm and the changes of zeta potential of the adsorption system indicating a change of the surface charge due to the HDTMA adsorption. In comparison to other studied sorbent (e.g. coal), the zeta potential is influenced by adsorption to a lesser extent on montmorillonite (about 100 mV). However, an amount of the adsorbed HDTMA is much more higher on montmorillonite and it even exceeds its cation exchange capacity, which proves the concept of the double- or triple-layer arrangement of the adsorbed alkylammonium cations. Thus, adsorption of HDTMA on montmorillonite probably takes place by cation exchange into its interlayer space as well on the external surface.

MMT,SAz-1 on the equilibrium concentration of the HDTMA solution (*c*e) [45]

isotherm of Langmuir model of monolayer coverage of an adsorbent.

*zeta potential measurement* 

0

1

2

*a*/(mmol g–1)

3

4

At least two main results can be mentioned from the previous picture:


Next, the zeta potential of the montmorillonite particles in the copper solutions was determined during the Cu(II) sorption. The values of the zeta potential before and after the Cu(II) adsorption on the montmorillonite were compared (Figure 4).

**Figure 4.** Figure 4. Dependence of zeta potential and adsorption of Cu(II) on MMT,Wy on the Cu(II) equilibrium concentration (*c*e)

The adsorption of the copper ions caused the change of the zeta potential of the clay particles. The zeta potential became more positive. On the contrary to Figure 3 where the isoelectric point was not reached with increasing H+, in the case of addition of the cooper ions the zeta potential was very close to 0 mV. It just confirmed well known fact that the valencies of the ions have great impact on the electrokinetic behaviour of the suspensions.

The zeta potential changes during the Cu(II) sorption can be used as an additional parameter for characterization of the sorption on montmorillonite. The excellent correlation of the Cu(II) adsorption isotherm with the zeta potential dependence on the Cu(II) concentration in the sorption solutions demonstrated on Figure 4 proved this conclusion.

284 Clay Minerals in Nature – Their Characterization, Modification and Application

increasing addition of acid a charge is negative to a lesser extent.

was not reached.

equilibrium concentration (*c*e)

0

0.1

0.2

0.3

a/mmol g-1

0.4

0.5

0.6

0.7

At least two main results can be mentioned from the previous picture:

range 4 - 9. In this range particles have tendency to coagulate.

Cu(II) adsorption on the montmorillonite were compared (Figure 4).

The zeta potential of the montmorillonite particles in distilled water (pH 6) reaches approximately -24 mV, the zeta potential is negative. With increasing addition of alkali to the suspension of pH 6 the particles tends to acquire a more negative charge and with



Next, the zeta potential of the montmorillonite particles in the copper solutions was determined during the Cu(II) sorption. The values of the zeta potential before and after the

**Figure 4.** Figure 4. Dependence of zeta potential and adsorption of Cu(II) on MMT,Wy on the Cu(II)

0 2 4 6 8 10 12

ce/mmol dm-3





adsorption zeta potential zeta potential/mV



0

5

The adsorption of the copper ions caused the change of the zeta potential of the clay particles. The zeta potential became more positive. On the contrary to Figure 3 where the isoelectric point was not reached with increasing H+, in the case of addition of the cooper ions the zeta potential was very close to 0 mV. It just confirmed well known fact that the valencies of the ions have great impact on the electrokinetic behaviour of the suspensions.

The zeta potential changes during the Cu(II) sorption can be used as an additional parameter for characterization of the sorption on montmorillonite. The excellent correlation

#### *4.2.2. Sorption of alkylammonium cations – coherence of classical batch experiments and zeta potential measurement*

The following research has been performed in order to show the above mentioned possibility of zeta potential for evaluation of the sorption processes. The adsorption of HDTMA on the montmorillonite SAz-1 was studied by conventionally measured adsorption isotherms and the zeta potential was measured simultaneously in the sorption suspensions. Figure 5 demonstrates the typical adsorption isotherm which shape indicates the adsorption isotherm of Langmuir model of monolayer coverage of an adsorbent.

**Figure 5.** Dependence of zeta potential (blank symbols) and adsorption of HDTM (full symbols) on MMT,SAz-1 on the equilibrium concentration of the HDTMA solution (*c*e) [45]

Figure 5 shows the remarkable same course of the adsorption isotherm and the changes of zeta potential of the adsorption system indicating a change of the surface charge due to the HDTMA adsorption. In comparison to other studied sorbent (e.g. coal), the zeta potential is influenced by adsorption to a lesser extent on montmorillonite (about 100 mV). However, an amount of the adsorbed HDTMA is much more higher on montmorillonite and it even exceeds its cation exchange capacity, which proves the concept of the double- or triple-layer arrangement of the adsorbed alkylammonium cations. Thus, adsorption of HDTMA on montmorillonite probably takes place by cation exchange into its interlayer space as well on the external surface. Subsequently, the HDTMA adsorption proceeds via van der Waals interactions [45].

## **4.3. Comparison of Cu(II) sorption on montmorillonites and their organoderivatives**

Application of Electrochemistry for Studying Sorption Properties of Montmorillonite 287

The linearized forms of the adsorption isotherms proved that all obtained adsorption isotherms exhibited the Langmuir model of sorption. The linearized forms of the adsorption isotherms were used to calculate a maximum adsorbed amount of Cu(II) named aCu (Table 2). The values of parameter aCu indicate no significant differences between the studied organo-montmorillonites. This fact corresponds to the found changes of the parameter d(001) that indicated very similar increase of the basal spacing in all prepared organo-

Montmorillonite aCu [mmol . g-1] montmorillonite aCu [mmol . g-1]

**Table 2.** Maximum absorbed amount of Cu(II) on montmorillonites and their organo-derivatives

Multisweep cyclic voltammetry (MCV) represents a suitable technique to study adsorption of metals onto a modifier in the carbon paste. In the case of the montmorillonite modifier the obtained current increases with successive occupation of the ion-exchange sites of its structure until a constant, maximum value of current (steady state current) is achieved. The obtained dependences of the current response on a number of cycling (on time) can be used as a characteristic feature for the metals sorption on montmorillonites. The typical multisweep cyclic voltammograms are shown on Figure 8 that depicts MCV of Cu(II) performed on the carbon paste electrode modified with MMT,SAz-1. The successive occupation of the ion-exchange sites of montmorillonite with increasing number of cycling (time) caused the current increase corresponded to the adsorbed amount of Cu(II) [8,10] (the

The Cu(II) sorption on two types of montmorillonite – SWy-2 and SAz-1 - was studied by means of multisweep cyclic voltammetry. The obtained current responses on the carbon paste electrodes CPE(MMT,SWy-2) and CPE(MMT,SAz-1) exhibited the time dependences that correspond to the Cu(II) sorption on the montmorillonites (Figure 9). These dependences enable to distinguish the Cu(II) sorption on the various types of montmorillonite. It is seen, that the higher sorption capacity was found in the case of MMT,SWy-2. These finding closely corresponds to the results obtained by the batch

technique (Table 2) that proved the slightly higher sorption on the MMT,SWy-2, too.

1 decreased the Cu(II) sorption approximately to 65 % [48].

The multisweep voltammetric study of the Cu(II) sorption on the montmorillonite and its organo-derivative has already demonstrated that the organo-derivative MMT,SAz-1**-**HDTM exhibited the lower steady state current due to a lower sorption of Cu(II) (Figure 10). The cation exchange sites of the MMT,SAz-1**-**HDTMA are occupied with the HDTMA cations, which inhibits sorption of the cationic forms Cu2+ and [Cu(ac)]+ (ac – acetate) in comparison with the unmodified montmorillonite. HDTM incorporated into the interlayer of MMT,SAz-

MMT,SAz-1 0.34 MMT,Swy-2 0.38 MMT,SAz-1–HDTMA 0.17 MMT,Swy-2–HDTMA 0.17 MMT,SAz-1–BDHDA 0.17 MMT,Swy-2–BDHDA 0.17 MMT,SAz-1–HDP 0.18 MMT,Swy-2–HDP 0.16

montmorillonites (Table 1).

*4.3.2. Cyclic voltammetry study* 

underneath voltammetric peak on Figure 8).

#### *4.3.1. Batch technique study*

The adsorption isotherms of Cu(II) measured by the batch technique on montmorillonites and their alkylammonium-derivatives are demonstrated on Figures 6 and 7. It is evident that the presence of all three alkylammonium cations in the montmorillonites caused a decrease of the Cu(II) sorption. No significant differences were found in the case of individual alkylammonium cation, the sorption was decreased on about 50 % in comparison with the original montmorillonites. The same decrease of sorption was also found in the case of HDTMA [46] and tetrabutylammonium cations [47]. The sorption sites of clay mineral are occupied by a relatively great alkylammonium cation, which inhibits sorption of the metal cations.

**Figure 6.** Adsorption isotherms of Cu(II) on MMTA,SAz-1 and its organo-derivatives

**Figure 7.** Adsorption isotherms of Cu(II) on MMTA,SWy-2 and its organo-derivatives

The linearized forms of the adsorption isotherms proved that all obtained adsorption isotherms exhibited the Langmuir model of sorption. The linearized forms of the adsorption isotherms were used to calculate a maximum adsorbed amount of Cu(II) named aCu (Table 2). The values of parameter aCu indicate no significant differences between the studied organo-montmorillonites. This fact corresponds to the found changes of the parameter d(001) that indicated very similar increase of the basal spacing in all prepared organomontmorillonites (Table 1).


**Table 2.** Maximum absorbed amount of Cu(II) on montmorillonites and their organo-derivatives

### *4.3.2. Cyclic voltammetry study*

286 Clay Minerals in Nature – Their Characterization, Modification and Application

great alkylammonium cation, which inhibits sorption of the metal cations.

**Figure 6.** Adsorption isotherms of Cu(II) on MMTA,SAz-1 and its organo-derivatives

0 2 4 6 810 c [mmol.l-1]

**Figure 7.** Adsorption isotherms of Cu(II) on MMTA,SWy-2 and its organo-derivatives

0 2 4 6 810

c [mmol.l-1]

**derivatives** 

*4.3.1. Batch technique study* 

0,00 0,05 0,10 0,15 0,20 0,25 0,30 0,35

0,00 0,05 0,10 0,15 0,20 0,25 0,30 0,35 0,40

am [mmol.g-1]

am [mmol.g-1]

**4.3. Comparison of Cu(II) sorption on montmorillonites and their organo-**

The adsorption isotherms of Cu(II) measured by the batch technique on montmorillonites and their alkylammonium-derivatives are demonstrated on Figures 6 and 7. It is evident that the presence of all three alkylammonium cations in the montmorillonites caused a decrease of the Cu(II) sorption. No significant differences were found in the case of individual alkylammonium cation, the sorption was decreased on about 50 % in comparison with the original montmorillonites. The same decrease of sorption was also found in the case of HDTMA [46] and tetrabutylammonium cations [47]. The sorption sites of clay mineral are occupied by a relatively

MMT,SAz-1

MMT,SAz-1-HDP

MMT,SAz-1-BDHDA

MMT,SAz-1-HDTMA

MMT,SWy-2

MMT,SWy-2-HDP

MMT,Swy-2-BDHDA

MMT,Swy-2-HDTMA

Multisweep cyclic voltammetry (MCV) represents a suitable technique to study adsorption of metals onto a modifier in the carbon paste. In the case of the montmorillonite modifier the obtained current increases with successive occupation of the ion-exchange sites of its structure until a constant, maximum value of current (steady state current) is achieved. The obtained dependences of the current response on a number of cycling (on time) can be used as a characteristic feature for the metals sorption on montmorillonites. The typical multisweep cyclic voltammograms are shown on Figure 8 that depicts MCV of Cu(II) performed on the carbon paste electrode modified with MMT,SAz-1. The successive occupation of the ion-exchange sites of montmorillonite with increasing number of cycling (time) caused the current increase corresponded to the adsorbed amount of Cu(II) [8,10] (the underneath voltammetric peak on Figure 8).

The Cu(II) sorption on two types of montmorillonite – SWy-2 and SAz-1 - was studied by means of multisweep cyclic voltammetry. The obtained current responses on the carbon paste electrodes CPE(MMT,SWy-2) and CPE(MMT,SAz-1) exhibited the time dependences that correspond to the Cu(II) sorption on the montmorillonites (Figure 9). These dependences enable to distinguish the Cu(II) sorption on the various types of montmorillonite. It is seen, that the higher sorption capacity was found in the case of MMT,SWy-2. These finding closely corresponds to the results obtained by the batch technique (Table 2) that proved the slightly higher sorption on the MMT,SWy-2, too.

The multisweep voltammetric study of the Cu(II) sorption on the montmorillonite and its organo-derivative has already demonstrated that the organo-derivative MMT,SAz-1**-**HDTM exhibited the lower steady state current due to a lower sorption of Cu(II) (Figure 10). The cation exchange sites of the MMT,SAz-1**-**HDTMA are occupied with the HDTMA cations, which inhibits sorption of the cationic forms Cu2+ and [Cu(ac)]+ (ac – acetate) in comparison with the unmodified montmorillonite. HDTM incorporated into the interlayer of MMT,SAz-1 decreased the Cu(II) sorption approximately to 65 % [48].

Application of Electrochemistry for Studying Sorption Properties of Montmorillonite 289

CPE(MMT,SAz-1)

CPE(MMT,SAz-1-HDTM)

CPE(MMT,SAz-1-BDHDA)

CPE(MMT,SAz-1-HDP)

dependence of the maximum current response (steady state current) on the cycling time. The typical dependences in comparison to the above mentioned results [48] are shown on Figure 10. The dependences also indicate the decrease of the steady state current on all

**Figure 10.** Current vs. t dependences for MCV of Cu(II) (2.5 x 10-5 mol . l-1) in acetate buffer pH 4.0 on

The steady state current decrease corresponds to the decrease of the Cu(II) sorption onto the modifier in the carbon paste which is caused by the presence of the alkylammonium cations in the interlayer structure or on the surface of the montmorillonite modifier in the carbon paste electrode. It is seen form Figure 10 that the Cu(II) sorption is decreased on about 80 %

Although the demonstrated dependences (Figure 9 and 10) cannot be considered as the classical sorption isotherms, they indicates the same characteristics of the Cu(II) sorption:




and 63 % in the case of MMT,SAz-1-HDP, resp. MMT,SAz-BDHDA.

0 5 10 15 20 25 t [min]

carbon paste electrodes modified with montmorillonites

CPEs modifed with organo-montmorillonites

0

100

200

300

400

i [nA]

500

600

700

with the original montmorillonite

organo-montmorillonites.

organo-montmorillonites in comparison to the original montmorillonite.

**Figure 8.** Multisweep cyclic voltammetry of Cu(II) (2.5 x 10-5 mol . l-1) in acetate buffer pH 3.6 on CPE(MMT**,**SAz-1)

**Figure 9.** Multisweep cyclic voltammetry of Cu(II) (2.5 x 10-5 mol .l-1) in acetate buffer pH 3,6 on CPE(MMT,SAz-1) and CPE(MMT,SWy-2)

In this study MCV of the Cu(II) was performed by the same procedure [48] on the carbon paste electrodes modified with the prepared organo-montmorillonites MMT,SAz-1- BDHDA, MMT,SAz-1-HDP, MMT,SWy-2-BDHDA, and MMT,SWy-2-HDP. The MCV voltammograms of Cu(II) performed on the CPEs modified with the organomontmorillonites in the medium of acetate buffer pH 3.6 – 5.2 were used to construct the dependence of the maximum current response (steady state current) on the cycling time. The typical dependences in comparison to the above mentioned results [48] are shown on Figure 10. The dependences also indicate the decrease of the steady state current on all organo-montmorillonites in comparison to the original montmorillonite.

288 Clay Minerals in Nature – Their Characterization, Modification and Application

CPE(MMT**,**SAz-1)

i [nA]

CPE(MMT,SAz-1) and CPE(MMT,SWy-2)

**Figure 8.** Multisweep cyclic voltammetry of Cu(II) (2.5 x 10-5 mol . l-1) in acetate buffer pH 3.6 on

**Figure 9.** Multisweep cyclic voltammetry of Cu(II) (2.5 x 10-5 mol .l-1) in acetate buffer pH 3,6 on

0 5 10 15 20 25 t [min]

In this study MCV of the Cu(II) was performed by the same procedure [48] on the carbon paste electrodes modified with the prepared organo-montmorillonites MMT,SAz-1- BDHDA, MMT,SAz-1-HDP, MMT,SWy-2-BDHDA, and MMT,SWy-2-HDP. The MCV voltammograms of Cu(II) performed on the CPEs modified with the organomontmorillonites in the medium of acetate buffer pH 3.6 – 5.2 were used to construct the

MMT,Swy-2 MMT,SAz-1

**Figure 10.** Current vs. t dependences for MCV of Cu(II) (2.5 x 10-5 mol . l-1) in acetate buffer pH 4.0 on CPEs modifed with organo-montmorillonites

The steady state current decrease corresponds to the decrease of the Cu(II) sorption onto the modifier in the carbon paste which is caused by the presence of the alkylammonium cations in the interlayer structure or on the surface of the montmorillonite modifier in the carbon paste electrode. It is seen form Figure 10 that the Cu(II) sorption is decreased on about 80 % and 63 % in the case of MMT,SAz-1-HDP, resp. MMT,SAz-BDHDA.

Although the demonstrated dependences (Figure 9 and 10) cannot be considered as the classical sorption isotherms, they indicates the same characteristics of the Cu(II) sorption:


## **5. Conclusions**

The applied electrochemical techniques – measurement of zeta potential and multisweep cyclic voltammetry – offer possibility to study and characterize properties of clay minerals connected with the sorption processes on their surfaces.

Application of Electrochemistry for Studying Sorption Properties of Montmorillonite 291

Innovations Operational Programme financed by Structural Founds of Europe Union and

The authors are also grateful to The Specific university research Ostrava University, project

[1] Ghosh P K, Bard A J (1983) Clay-modified Electrodes. J. am. chem. soc. 105: 5691 - 5693. [2] Bard A J, Mallouk T (1992) Electrodes Modified with Clays, Zeolites, and Related Microporous Solids. In: Murray R W, editor. Molecular Design of Electrode Surfaces.

[3] Fitch A (1990) Clay-modified Electrodes: A Review. Clay. clay miner. 38: 391 - 400.

[4] Macha S M, Fitch A (1998) Clays as aAchitectural Units at Modified Electrodes.

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[12] Navrátilová Z, Hranicka Z (2008) Montmorillonite Modified Electrodes for Study of Cu

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[14] Navratilova Z, Kula P (2000) Cation and Anion Exchange on Clay Modified Electrodes.

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**6. References** 

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695.

The zeta potential measurement is a method suitable for characterization of the clay minerals particles from the point of view of their surface charge that is one of the significant parameters influencing sorption on clay minerals. As it was shown the measurement of zeta potential enable to determine the pH range where the montmorillonite particles did not change significantly the surface charge and where they tend to coagulate. The zeta potential measurement during sorption of Cu(II) exhibited the excellent agreement of the zeta potential dependence on the Cu(II) equilibrium concentration with the adsorption isotherm measured by the classical sorption experiments (batch techniques). Analogously, the same course of the adsorption isotherm and the zeta potential changes in the sorption system of montmorillonite – hexadecyltrimethylammonium cation was found. The zeta potential changes can be compared with its changes on other sorbents (coals). The alkylammonium adsorption way can be evaluated from the zeta potential changes [45].

Multisweep cyclic voltammetry represents another method to characterize the metals sorption using the electrodes modified with a studied adsorbent – clay mineral. The current response dependences on time give the typical curves suitable to describe the sorption. The following general conclusions can be obtained by this method:


The advantage of multisweep cyclic voltammetry consists in relatively fast performance providing the first idea about sorption. For example, measurement on carbon paste electrode takes about 20 – 30 min in comparison with the time-consuming batch sorption experiments – minimum 24 hours. On the other hand, multisweep cyclic voltammetry enable only a semi-quantitative evaluation of sorption.

The described electrochemical methods can be successfully used as the additional methods of study and characterization of clay minerals.

## **Author details**

Zuzana Navrátilová and Roman Maršálek *Department of Chemistry, Faculty of Science, University of Ostrava, Ostrava, Czech Republic* 

## **Acknowledgement**

The contribution has been done in connection with the project Institute of Environmental Technologies, reg. no. CZ.1.05/2.1.00/03.0100 supported by Research and Development for Innovations Operational Programme financed by Structural Founds of Europe Union and from the means of state budget of the Czech Republic.

The authors are also grateful to The Specific university research Ostrava University, project No. sgs18/PrF/2011.

#### **6. References**

290 Clay Minerals in Nature – Their Characterization, Modification and Application

connected with the sorption processes on their surfaces.

adsorption way can be evaluated from the zeta potential changes [45].

following general conclusions can be obtained by this method:

voltammetry enable only a semi-quantitative evaluation of sorption.

other cations, temperature

**Author details** 

**Acknowledgement** 

of study and characterization of clay minerals.

Zuzana Navrátilová and Roman Maršálek

The applied electrochemical techniques – measurement of zeta potential and multisweep cyclic voltammetry – offer possibility to study and characterize properties of clay minerals

The zeta potential measurement is a method suitable for characterization of the clay minerals particles from the point of view of their surface charge that is one of the significant parameters influencing sorption on clay minerals. As it was shown the measurement of zeta potential enable to determine the pH range where the montmorillonite particles did not change significantly the surface charge and where they tend to coagulate. The zeta potential measurement during sorption of Cu(II) exhibited the excellent agreement of the zeta potential dependence on the Cu(II) equilibrium concentration with the adsorption isotherm measured by the classical sorption experiments (batch techniques). Analogously, the same course of the adsorption isotherm and the zeta potential changes in the sorption system of montmorillonite – hexadecyltrimethylammonium cation was found. The zeta potential changes can be compared with its changes on other sorbents (coals). The alkylammonium

Multisweep cyclic voltammetry represents another method to characterize the metals sorption using the electrodes modified with a studied adsorbent – clay mineral. The current response dependences on time give the typical curves suitable to describe the sorption. The

The advantage of multisweep cyclic voltammetry consists in relatively fast performance providing the first idea about sorption. For example, measurement on carbon paste electrode takes about 20 – 30 min in comparison with the time-consuming batch sorption experiments – minimum 24 hours. On the other hand, multisweep cyclic

The described electrochemical methods can be successfully used as the additional methods

*Department of Chemistry, Faculty of Science, University of Ostrava, Ostrava, Czech Republic* 

The contribution has been done in connection with the project Institute of Environmental Technologies, reg. no. CZ.1.05/2.1.00/03.0100 supported by Research and Development for


**5. Conclusions** 

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[34] Lv L., Tsoi G., Zhao X.S. (2004). Uptake Equilibria and Mechanisms of Heavy Metal Ions

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[36] Wu S F, Yanagisawa K., Nishizawa T (2001) Zeta Potential on Carbons and Carbides.

[37] Navratilova Z, Vaculikova L (2006) Electrodeposition of Mercury Film on Electrodes

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[39] He H, Ma Y, Zhu J, Yuan P, Qing Y (2010) Organoclays Prepared from Montmorillonites with Different Cation Exchange Capacity and Surfactant Configuration. Applied clay

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[41] Betega de Paiva L, Morales A R, Valenzuela Díaz F R (2008) Organoclays: Properties,

[42] Kooli F, Liu Y, Alshahateet S F, Messali M, Bergaya F (2009) Reaction of Acid Activated Montmorillonites with Hexadecyltrimethylammonium Bromide Solution. Applied clay

[43] Madejova J (2003) FTIR Techniques in Clay Mineral Studies. Vibrational Spectroscopy.

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[18] Tonle I K, Ngameni E, Walcarius A (2004) From Clay- to Organoclay-film Modified Electrodes: tuning chargé selectivity in Ion Exchange Voltammetry. Electrochim. Acta.

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[21] Tonle I K, Ngameni E, Njopwouo D, Carteret C, Walcarius A (2003) Functionalization of Natural Smectite-type Clays by Grafting with Organosilanes: Physico-chemical Characterization and Application to Mercury(II) Upteke. Phys. chem. chem. phys. 5:

[22] Tonle I K, Ngameni E, Walcarius A (2005) Preconcentration and Voltammetric Analysis of Mercury(II) at a Carbon Paste Electrode Modified with Natural Smectite-type Clays grafted with Organic Chelating Groups. Sensors and actuators B – chemical. 110: 195 -

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**Chapter 15** 

© 2012 Vasić et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2012 Vasić et al., licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**Methods of Determination for Effective** 

Miloš Vasić, Željko Grbavčić and Zagorka Radojević

Additional information is available at the end of the chapter

**of Clay Products** 

http://dx.doi.org/10.5772/48217

**1. Introduction** 

**Diffusion Coefficient During Convective Drying** 

Drying research is an outstanding example of a very complex field where it is necessary to look comprehensively on simultaneous energy and mass transfer process that takes place within and on the surface of the material. In order to get the full view of drying process, beside previously mentioned, researchers have to incorporate and deal with highly non linear physical phenomena inside drying clay products, non-homogenous distribution of temperature and humidity inside dryers, equipment selection, design, control and final product quality [1]. That is the reason why a unique theoretical setting of drying has to be determined through the balance of the heat flow, temperature changes and moisture flow. Simultaneous heat and mass process are related, regarding to the fact that all phases have to remain in thermodynamic balance established on a local temperature value [2]. In the economy that is becoming increasingly global, laboratory drying process analyses should ensure enough data which are necessary for optimal drying regime establishment. In order to find optimal drying regime it is necessary to understand transport mechanisms which takes place within and on the surface of the clay product. The drying process is characterized by the existence of several internal transport mechanisms such as pure diffusion, surface diffusion, Knudsen diffusion, capillary flow, evaporation and condensation, thermo-diffusion, *etc.* Moisture diffusivity, viewed as a transport of matter due to the random motion of molecules, is the most important mass transport mechanisms, essential for the calculation and modelling of various clay processing operations. Moisture transfer within the solid clay body at a certain temperature is realized due to the different moisture content in the interior and on the surface of a solid body. The mass transfer rate by pure diffusion is therefore proportional to the concentration gradient of the moisture content, with the diffusion coefficient being the proportionality factor. Determination of the


## **Methods of Determination for Effective Diffusion Coefficient During Convective Drying of Clay Products**

Miloš Vasić, Željko Grbavčić and Zagorka Radojević

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/48217

## **1. Introduction**

294 Clay Minerals in Nature – Their Characterization, Modification and Application

and interface science. 295: 21 – 32.

Bituminous Coal and Clay Mineral. Chemical Papers. 65: 77 – 84.

Alkylammonium-clay Composite. Sensing in elecroanalysis 4: 39 – 46.

[45] Marsalek R, Navratilova Y (2011) Comparative Study of CTAB Adsorption on

[46] Zeng Z, Jiang J (2005) Effects of the Type and Structure of Modified Clays on Adsorption Performance. International journal of environmental studies. 62: 403 – 414. [47] Gupta S S, Bhattacharyya K G (2006) Adsorption of Ni (II) on Clays. Journal of colloid

[48] Navratilova Z, Hranicka Z (2009) Carbon Paste Electrode Modified with

Drying research is an outstanding example of a very complex field where it is necessary to look comprehensively on simultaneous energy and mass transfer process that takes place within and on the surface of the material. In order to get the full view of drying process, beside previously mentioned, researchers have to incorporate and deal with highly non linear physical phenomena inside drying clay products, non-homogenous distribution of temperature and humidity inside dryers, equipment selection, design, control and final product quality [1]. That is the reason why a unique theoretical setting of drying has to be determined through the balance of the heat flow, temperature changes and moisture flow. Simultaneous heat and mass process are related, regarding to the fact that all phases have to remain in thermodynamic balance established on a local temperature value [2]. In the economy that is becoming increasingly global, laboratory drying process analyses should ensure enough data which are necessary for optimal drying regime establishment. In order to find optimal drying regime it is necessary to understand transport mechanisms which takes place within and on the surface of the clay product. The drying process is characterized by the existence of several internal transport mechanisms such as pure diffusion, surface diffusion, Knudsen diffusion, capillary flow, evaporation and condensation, thermo-diffusion, *etc.* Moisture diffusivity, viewed as a transport of matter due to the random motion of molecules, is the most important mass transport mechanisms, essential for the calculation and modelling of various clay processing operations. Moisture transfer within the solid clay body at a certain temperature is realized due to the different moisture content in the interior and on the surface of a solid body. The mass transfer rate by pure diffusion is therefore proportional to the concentration gradient of the moisture content, with the diffusion coefficient being the proportionality factor. Determination of the

© 2012 Vasić et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Vasić et al., licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

diffusion coefficient is essential for a credible description of the mass transfer process, described by the Fick's equation [3]. It is a common practice to describe complete mass transfer with same equations as pure diffusion and to take the correction, for all secondary types of mass transfer into account simply by replacing the pure diffusion coefficient with an effective diffusion coefficient.

Methods of Determination for Effective Diffusion Coefficient During Convective Drying of Clay Products 297

*n l*

22 2

In equation (1) *X0, X* and *Xeq,* represent respectively, the initial, current and equilibrium moisture content, kg moisture/kg of dry material, *D*eff is the effective diffusion coefficient, m2/s, *l* is the half plate thickness, m and *t* is time, s. MR represents moisture ratio and has no unit. There is a large body of literature comparing predicted results of drying models that considered as well as neglect shrinkage [16]. Most published drying models do not take shrinkage into account in the balance equations. The drying model equations are typically borrowed from corresponding non-shrinking models, frequently without appropriate physical and mathematical consideration, and are applied to a shrinkage medium. A few studies, describing the sample dimensional correction, can be found in literature. Some data can be found in the papers [17-20]. Silva [21] presented, a way of solving the diffusion equation for the case of spherical samples. Since clay products show dimensional change during drying it was necessary to develop a model that would take this phenomenon into account. By introducing into equation (1) the expression *l*(t), which represents the experimental dependence of the thickness of the tiles in time, equation (1) is corrected. It should be kept in mind that this type of correction is not mathematically one hundred percent accurate because the resulting equation (1) was obtained using the assumption of unchangeable sample thickness. Formally speaking, a mathematically accurate correction can be obtained by entering the expression *l*(t) into the equation for the case of constant

In order to solve the equations (1) it is necessary to dispose with the experimental results and to have the experimentally determined dependence *MReks - t*. *MReks* represents the experimentally determined value of *MR* obtained by calculation from the experimentally

2 2 22 2 2

22 2

and equation (2) is transformed into equation (4). The value of N used in equation (3) can be

*<sup>N</sup> eff*

*n l*

= 0.05 is accepted for the further calculations in this paper. When *t=0*, *MR*=1,

equation (2), the value of *N* can be determined and equation (2) is transformed form an

8 1 (2 1) (2 1) 4

*n xn x*

 

 (2)

is defined as the relative error of neglecting terms higher then *N* in

8 1 (2 1) 8 1 (2 1) exp (2 1) 4 4 (2 1)

2 2 2 2

2

(3)

*n D t*

*n n D t D t*

*<sup>N</sup> eff eff*

 

8 1 (2 1) (2 1) 4

0

*n*

2 2

*n D t*

*eff*

(1)

 *MR* =

sample thickness, after an integration step.

*2.2.1. Model A1 - The case when shrinkage is not included* 

measured data *X0, X* and *Xeq*. Equation (1) can be converted into the form:

1 0

*n N n*

infinite sum into a finite sum of *N* terms given by equation (3):

 *MR* = <sup>2</sup>

0

*n*

**2.2. Description of Model A** 

determined from equation (4):

 *MR* =

If the value of

The value of

Relatively small number of research papers that describe the drying process of ceramic and especially clay materials are available. Some data can be found in the papers of Efremov [4] (bricks), Vasić [5] (heavy clay tiles) Chemkhi [6], Zagrouba [7, 8] (clays), Skansi [9, 10] (brick, hollow brick, heavy clay tiles, tiles) and others. In his paper Efrem [4] gave an analytical solution of diffusion differential equation with boundary conditions in the form of flux. Relying on these studies M. Vasić and colleagues [11] have developed a drying model based on the modification of Efremov's equation and the computer program for determining the effective diffusion coefficient.

Chemki and Zagrouba [6] have estimated the coefficient of moisture diffusivity from drying curve. F. Zagrouba and colleagues [7, 8] have developed a mathematical model of transfer phenomena which has involved at the same time heat, mass and momentum transfer during the convective drying of clay tiles. In their study a method for determination of the heat transfer coefficient and effective diffusion coefficient is presented. Zanden and Kerkhof [12] have performed extensive research on isothermal mass transport mechanisms during the convective drying of clay products. They presented a model which describes moisture transport inside a porous clay material during drying.

M. Vasić and colleagues [5] have developed two computer programs for determination of effective diffusion coefficient, based on mathematical calculation of the second Fick's law and Cranck diffusion equation. Skansi and colleagues [9, 10] were investigating the kinetics of conventional drying of flat tiles in experimental and industrial tunnel dryer. They presented several thin layer models such as exponential one which correlates the kinetics of the whole tile-drying process well and has physical significance. They also presented a method for determining heat transfer coefficient, effective diffusion coefficient and drying constant.

## **2. Materials and methods**

#### **2.1. Theoretical development**

In drying studies performed on clay materials, diffusion is generally accepted as the main mechanism of moisture transport from the material interior to its surface. The restriction to one-dimensional diffusion gives a good approximation in many practical systems. Analytical solution of Fick's equation is given for various geometrical shapes, assuming that the transport of moisture occurs by diffusion, that sample shrinkage is neglected and that diffusion coefficient and temperature have constant values. For the case of "thin plate" geometry, a solution is given by Cranck [3] which is represented by the expression:

Methods of Determination for Effective Diffusion Coefficient During Convective Drying of Clay Products 297

$$MR = \frac{8}{\pi^2} \sum\_{n=0}^{n} \frac{1}{(2n+1)^2} \left( -\frac{(2n+1)^2}{4} \pi^2 \frac{D\_{\ell\ell}t}{l^2} \right) \tag{1}$$

In equation (1) *X0, X* and *Xeq,* represent respectively, the initial, current and equilibrium moisture content, kg moisture/kg of dry material, *D*eff is the effective diffusion coefficient, m2/s, *l* is the half plate thickness, m and *t* is time, s. MR represents moisture ratio and has no unit. There is a large body of literature comparing predicted results of drying models that considered as well as neglect shrinkage [16]. Most published drying models do not take shrinkage into account in the balance equations. The drying model equations are typically borrowed from corresponding non-shrinking models, frequently without appropriate physical and mathematical consideration, and are applied to a shrinkage medium. A few studies, describing the sample dimensional correction, can be found in literature. Some data can be found in the papers [17-20]. Silva [21] presented, a way of solving the diffusion equation for the case of spherical samples. Since clay products show dimensional change during drying it was necessary to develop a model that would take this phenomenon into account. By introducing into equation (1) the expression *l*(t), which represents the experimental dependence of the thickness of the tiles in time, equation (1) is corrected. It should be kept in mind that this type of correction is not mathematically one hundred percent accurate because the resulting equation (1) was obtained using the assumption of unchangeable sample thickness. Formally speaking, a mathematically accurate correction can be obtained by entering the expression *l*(t) into the equation for the case of constant sample thickness, after an integration step.

#### **2.2. Description of Model A**

296 Clay Minerals in Nature – Their Characterization, Modification and Application

an effective diffusion coefficient.

effective diffusion coefficient.

constant.

**2. Materials and methods** 

**2.1. Theoretical development** 

transport inside a porous clay material during drying.

diffusion coefficient is essential for a credible description of the mass transfer process, described by the Fick's equation [3]. It is a common practice to describe complete mass transfer with same equations as pure diffusion and to take the correction, for all secondary types of mass transfer into account simply by replacing the pure diffusion coefficient with

Relatively small number of research papers that describe the drying process of ceramic and especially clay materials are available. Some data can be found in the papers of Efremov [4] (bricks), Vasić [5] (heavy clay tiles) Chemkhi [6], Zagrouba [7, 8] (clays), Skansi [9, 10] (brick, hollow brick, heavy clay tiles, tiles) and others. In his paper Efrem [4] gave an analytical solution of diffusion differential equation with boundary conditions in the form of flux. Relying on these studies M. Vasić and colleagues [11] have developed a drying model based on the modification of Efremov's equation and the computer program for determining the

Chemki and Zagrouba [6] have estimated the coefficient of moisture diffusivity from drying curve. F. Zagrouba and colleagues [7, 8] have developed a mathematical model of transfer phenomena which has involved at the same time heat, mass and momentum transfer during the convective drying of clay tiles. In their study a method for determination of the heat transfer coefficient and effective diffusion coefficient is presented. Zanden and Kerkhof [12] have performed extensive research on isothermal mass transport mechanisms during the convective drying of clay products. They presented a model which describes moisture

M. Vasić and colleagues [5] have developed two computer programs for determination of effective diffusion coefficient, based on mathematical calculation of the second Fick's law and Cranck diffusion equation. Skansi and colleagues [9, 10] were investigating the kinetics of conventional drying of flat tiles in experimental and industrial tunnel dryer. They presented several thin layer models such as exponential one which correlates the kinetics of the whole tile-drying process well and has physical significance. They also presented a method for determining heat transfer coefficient, effective diffusion coefficient and drying

In drying studies performed on clay materials, diffusion is generally accepted as the main mechanism of moisture transport from the material interior to its surface. The restriction to one-dimensional diffusion gives a good approximation in many practical systems. Analytical solution of Fick's equation is given for various geometrical shapes, assuming that the transport of moisture occurs by diffusion, that sample shrinkage is neglected and that diffusion coefficient and temperature have constant values. For the case of "thin plate"

geometry, a solution is given by Cranck [3] which is represented by the expression:

#### *2.2.1. Model A1 - The case when shrinkage is not included*

In order to solve the equations (1) it is necessary to dispose with the experimental results and to have the experimentally determined dependence *MReks - t*. *MReks* represents the experimentally determined value of *MR* obtained by calculation from the experimentally measured data *X0, X* and *Xeq*. Equation (1) can be converted into the form:

$$MR = \frac{8}{\pi^2} \sum\_{n=N+1}^{\upsilon} \frac{1}{\left(2n+1\right)^2} \exp\left(-\frac{\left(2n+1\right)^2}{4}\pi^2 \frac{D\_{\varepsilon\sharp}t}{\varkappa^2}\right) + \frac{8}{\pi^2} \sum\_{n=0}^{N} \frac{1}{\left(2n+1\right)^2} \left(-\frac{\left(2n+1\right)^2}{4}\pi^2 \frac{D\_{\varepsilon\sharp}t}{\varkappa^2}\right) \tag{2}$$

If the value of is defined as the relative error of neglecting terms higher then *N* in equation (2), the value of *N* can be determined and equation (2) is transformed form an infinite sum into a finite sum of *N* terms given by equation (3):

$$MR = \frac{8}{\pi^2} \sum\_{n=0}^{N} \frac{1}{\left(2n+1\right)^2} \left(-\frac{\left(2n+1\right)^2}{4} \pi^2 \frac{D\_{eff}t}{l^2}\right) \tag{3}$$

The value of = 0.05 is accepted for the further calculations in this paper. When *t=0*, *MR*=1, and equation (2) is transformed into equation (4). The value of N used in equation (3) can be determined from equation (4):

$$11 = \frac{8}{\pi^2} \sum\_{n=0}^{N} \frac{1}{\left(2n+1\right)^2} + 0.05\tag{4}$$

Methods of Determination for Effective Diffusion Coefficient During Convective Drying of Clay Products 299

2 *eff* 2 *X X <sup>D</sup> t x*

(7)

(6)

(8)

(9)

(10)

If parameters of drying medium are kept constant during convective drying of solid bodies, moisture transfer could be treated on macro level as quasi diffusion with appropriate effective diffusion coefficient Deff. The general expression for mass conductivity (Fick's

The exact solution for drying kinetics can be obtained by applying Laplace transform method in time t for equation of isotropic diffusion with boundary conditions in a form of mass flux J. This flux is proportional to the difference between an equilibrium concentration in the pores of the material Xeq and the current concentration X on the material surface.

> *eff x* <sup>0</sup> *eq <sup>X</sup> J D kX X x*

Kinetic desorption coefficient k (m/s) in equation (7) can be calculated as a ratio l

By applying Laplace transform method to equation (7) Efremov in his PhD thesis [13]

2

exp 2 2

*eff eff eq eff eff eff X X l kk t l erf l t erfc k X X D t DD D D t* 

2

 

*eq eff eff*

2 2

*n n*

   

exp *eq*

function of the argument in equation (9), thus the drying equation becomes (10).

*X X k t MR t erfc k XX D D*

Equation (8) was obtained for the process of molecular diffusion. If we analyze equation (9) at the beginning of the drying process (t=0; X=X0) MR=1 and for long times (t→∞; X=Xeq) MR=0 will see that it has a real physical meaning. In order to get the drying equation which is valid for convective mass transport processes it is necessary to introduce the power

1 1

*eq eff eff*

*X X l l erfc X X tD t D* 

*X0, X* and *Xeq,* represent respectively, the initial, current and equilibrium moisture content, kg moisture/kg of dry material, *D*eff is the effective diffusion coefficient, m2/s, and *t* is time, s. The mass flux on the material surface (x=0) can be calculated through the use of the

second law) can be presented as a partial differential equation of diffusivity.

( ) *eff <sup>X</sup> div D gradX <sup>t</sup>* ; <sup>→</sup>

**2.3. Description of model B** 

*2.3.1. Model B1 - The case when shrinkage is not included* 

(characteristic thickness value) and time *<sup>l</sup> <sup>k</sup> <sup>t</sup>* .

concentration ratio which is given in equation (9)

0

*eq*

0

exp

presented the solution given by equation (8).

0

*eq*

*MRan* represents the analytically determined value calculated from equation (3). It is necessary to introduce the concept of a numerical counter *i*, which can have only an integer value. The numerical counter *i* is defined for each value of the experimental pairs (*MReks, t*). It starts form the value zero and increases by one until it reaches a final value which is related to the last experimental pairs (*MReks, t*). This concept enables the number of experimental pairs (*MReks, t*) from its first to its last value to be countered. In order to work properly, the program requires the initial value of the effective diffusion coefficient *Deff*, and the value to be entered. Let the initial value of the effective diffusion coefficient *Deff* be given the value of 1·10-20 /m2/s. Then, for each numerical counter value *i*, the program calculates the value <sup>2</sup> from equation (5);

$$\mathcal{X}^2 = \sum\_{1}^{i} \left( MR\_{eksi} - MR\_{av\_i} \right)^2 \tag{5}$$

In the first cycle, *MRan i* is calculated according to equation (3) using the previously determined value of *N* and the initial value of *Deff*. In the next cycle the value of *Deff* is doubled giving a new value for *MRan i* which is now used to calculate a new <sup>2</sup> according to equation (5). The program then compares the value <sup>2</sup> obtained in the first cycle and the newly obtained <sup>2</sup> value. If the statement 2 2 sec < *first ond* is satisfied, the program will continue previously described cycle, otherwise the program will temporarily stop.

Note: χ2first and χ2second refer to the last and the penultimate value of the cycle in which <sup>2</sup> is determined.

The last three values for *Deff* and <sup>2</sup> are then recorded. Then, the recorded *Deff* interval is divided into 100 parts. A hundredth part of this interval is defined as a step s. The program commences a cycle again using the initial value for *Deff* as *Deff* third from end + s. The cycle is repeated until the statement 2 2 sec < *first ond* <1·10-10 is satisfied. In other words, the cycle is interrupted when the difference 2 2 sec - *ond first* reaches 1·10-10. The final *Deff* value is then recorded. This value represents the finally calculated effective diffusion coefficient in m2/s.

#### *2.2.2. Model A2 - The case when there shrinkage is included*

For materials which shows shrinkage during drying equation (3) needs to be changed by the introduction of the expression *l (t)* into it. This expression represents the experimentally determined time dependence of the sample thickness. When this correction is entered, the previously described optimizing concept for the determination of the effective diffusion coefficient is applied.

#### **2.3. Description of model B**

298 Clay Minerals in Nature – Their Characterization, Modification and Application

the 

calculates the value <sup>2</sup>

newly obtained <sup>2</sup>

coefficient in m2/s.

coefficient is applied.

determined.

The last three values for *Deff* and <sup>2</sup>

cycle is repeated until the statement 2 2

the cycle is interrupted when the difference 2 2

*2.2.2. Model A2 - The case when there shrinkage is included* 

from equation (5);

2

equation (5). The program then compares the value <sup>2</sup>

*i*

doubled giving a new value for *MRan i* which is now used to calculate a new <sup>2</sup>

value. If the statement 2 2

continue previously described cycle, otherwise the program will temporarily stop.

2 2 0 8 1 <sup>1</sup> 0.05 (2 1)

*MRan* represents the analytically determined value calculated from equation (3). It is necessary to introduce the concept of a numerical counter *i*, which can have only an integer value. The numerical counter *i* is defined for each value of the experimental pairs (*MReks, t*). It starts form the value zero and increases by one until it reaches a final value which is related to the last experimental pairs (*MReks, t*). This concept enables the number of experimental pairs (*MReks, t*) from its first to its last value to be countered. In order to work properly, the program requires the initial value of the effective diffusion coefficient *Deff*, and

 value to be entered. Let the initial value of the effective diffusion coefficient *Deff* be given the value of 1·10-20 /m2/s. Then, for each numerical counter value *i*, the program

*MR MR eksi an*

<sup>1</sup> *<sup>i</sup>*

In the first cycle, *MRan i* is calculated according to equation (3) using the previously determined value of *N* and the initial value of *Deff*. In the next cycle the value of *Deff* is

Note: χ2first and χ2second refer to the last and the penultimate value of the cycle in which <sup>2</sup>

divided into 100 parts. A hundredth part of this interval is defined as a step s. The program commences a cycle again using the initial value for *Deff* as *Deff* third from end + s. The

sec < *first ond*

 

value is then recorded. This value represents the finally calculated effective diffusion

For materials which shows shrinkage during drying equation (3) needs to be changed by the introduction of the expression *l (t)* into it. This expression represents the experimentally determined time dependence of the sample thickness. When this correction is entered, the previously described optimizing concept for the determination of the effective diffusion

2

sec < *first ond*

sec - *ond first*

 

 

(5)

are then recorded. Then, the recorded *Deff* interval is

<1·10-10 is satisfied. In other words,

is satisfied, the program will

reaches 1·10-10. The final *Deff*

obtained in the first cycle and the

according to

is

(4)

 *<sup>n</sup> <sup>n</sup>* 

*N*

#### *2.3.1. Model B1 - The case when shrinkage is not included*

If parameters of drying medium are kept constant during convective drying of solid bodies, moisture transfer could be treated on macro level as quasi diffusion with appropriate effective diffusion coefficient Deff. The general expression for mass conductivity (Fick's second law) can be presented as a partial differential equation of diffusivity.

$$\frac{\partial \mathbf{X}}{\partial t} = \operatorname{div} (D\_{\rm eff} \cdot \operatorname{grad} \mathbf{X}) ; \quad \to \quad \frac{\partial \mathbf{X}}{\partial t} = D\_{\rm eff} \frac{\partial^2 \mathbf{X}}{\partial \mathbf{x}^2} \tag{6}$$

The exact solution for drying kinetics can be obtained by applying Laplace transform method in time t for equation of isotropic diffusion with boundary conditions in a form of mass flux J. This flux is proportional to the difference between an equilibrium concentration in the pores of the material Xeq and the current concentration X on the material surface.

$$\left. \right|\_{\mathcal{X}} = -D\_{e\mathcal{Y}} \cdot \frac{\partial X}{\partial \mathbf{x}} \Big|\_{x=0} = k \cdot \left( X\_{e\eta} - X \right) \tag{7}$$

Kinetic desorption coefficient k (m/s) in equation (7) can be calculated as a ratio l (characteristic thickness value) and time *<sup>l</sup> <sup>k</sup> <sup>t</sup>* .

By applying Laplace transform method to equation (7) Efremov in his PhD thesis [13] presented the solution given by equation (8).

$$\frac{X - X\_{eq}}{X\_0 - X\_{eq}} = \text{erf}\left(\frac{l}{2\sqrt{D\_{eff}t}}\right) + \exp\left(\frac{k}{D\_{eff}} \frac{l + \frac{k^2}{2D\_{eff}} \cdot t}{D\_{eff}}\right) \cdot \text{erfc}\left(k\sqrt{\frac{t}{D\_{eff}}} + \frac{l}{2\sqrt{D\_{eff}t}}\right) \tag{8}$$

*X0, X* and *Xeq,* represent respectively, the initial, current and equilibrium moisture content, kg moisture/kg of dry material, *D*eff is the effective diffusion coefficient, m2/s, and *t* is time, s. The mass flux on the material surface (x=0) can be calculated through the use of the concentration ratio which is given in equation (9)

$$MR = \frac{X - X\_{eq}}{X\_0 - X\_{eq}} = \exp\left(\frac{k^2}{D\_{eff}} \cdot t\right) \cdot \text{erfc}\left(k\sqrt{\frac{t}{D\_{eff}}}\right) \tag{9}$$

Equation (8) was obtained for the process of molecular diffusion. If we analyze equation (9) at the beginning of the drying process (t=0; X=X0) MR=1 and for long times (t→∞; X=Xeq) MR=0 will see that it has a real physical meaning. In order to get the drying equation which is valid for convective mass transport processes it is necessary to introduce the power function of the argument in equation (9), thus the drying equation becomes (10).

$$\frac{X - X\_{eq}}{X\_0 - X\_{eq}} = \exp\left(\frac{1}{\pi} \left(\frac{\pi \cdot l^2}{t \cdot D\_{eff}}\right)^n\right) \cdot \text{erfc}\left(\sqrt{\frac{1}{\pi} \left(\frac{\pi \cdot l^2}{t \cdot D\_{eff}}\right)^n}\right) \tag{10}$$

Simple approximation formula for function erf (A) is defined by equation (11) and can be found in Sergei Winitzki [14, 15] papers. The relative precision of this approximation is higher than 4·10-3, uniformly for all real A.

$$\text{erf}(A) = \left[1 - \exp\left(-A^2 \frac{1.27 + 0.14A^2}{1 + 0.14A^2}\right)\right]^{1/2} \tag{11}$$

Methods of Determination for Effective Diffusion Coefficient During Convective Drying of Clay Products 301

will be determined using equation (5)

sec - *ond first*

  .

is satisfied, the program will continue previously

are then recorded. Then, the recorded *Deff* interval

<1·10-10 is satisfied. In other words,

reaches 1·10-10

6. For each value from the database using equation *(3) in case of A1* or *(12) in case of B1*

8. In next cycle step starting value *Deff* is doubled and a new value MRan will be

is divided into 100 parts. A hundredth part of this interval is defined as a step s. The program commences a cycle again using the initial value for *Deff* as *Deff* third from end + s. The

sec < *first ond*

 

Program algorithm for models A2 and B2, which includes shrinkage, is obtained from

6. In equation *(3) for the case A2* or *(12) for the case B2* l is a function of time; l is provided from database where values of l were determined by experimental measuring of thin

4. Enter the characteristic dimension l (samples half thickness, m) 5. Enter the value n form equation (12) (n=1.95) *(Exists only in case of B1)*

described cycle, otherwise the program will temporarily stop.

12. Result will be saved as database: time (s), MReks, MRan, and value of average *Deff*.

previously presented algorithm after a few modifications: in steps 1, and 6 are made.

1. Read the values from database: the time (s), MReks, and characteristic l (m).

determined and initially used for determination of new <sup>2</sup>

sec < *first ond*

the cycle is interrupted when the difference 2 2

13. On the base of this database a graphical view can be displayed

For long drying times, equation (1) is transformed into equation (13).

**3.1. Clay characterization and sample preparation** 

From the equation (13) slope *Deff* coefficient can be calculated.

2 2 2 <sup>8</sup> exp( )

or

*D t eff l* 

Three raw masonry clays from the locality Banatski Karlovac (I), Ćirilkovac (II) and Orlovat (III) were analyzed. Characterization of raw masonry clays has included chemical, mineralogical, granulometrical, XRD, DTA and TGA examination. Results of chemical analysis are presented in table 1, while granulometric analyze is presented at

2

<sup>2</sup> ln( ) <sup>8</sup> *MR D t eff*

2

*l*

(13)

 

cycle is repeated until the statement 2 2

MRan will be determined. 7. For each value from database <sup>2</sup>

9. If the statement 2 2

10. The last three values for *Deff* and <sup>2</sup>

11. The final *Deff* value is then recorded.

plate sample shrinkage vs. time.

**2.5. Description of the slope model** 

 *MR* =

**3. Results and discussion** 

fig. 1 and 2.

After some mathematical manipulation, knowing that erfc (A) = 1 – erf (A), the final drying kinetic equation (12) is obtained.

$$\text{MR} = \exp(\frac{1}{\pi} \left( \frac{\pi \cdot l^2}{tD\_{\text{eff}}} \right)^n \cdot \left[ 1 - \left[ 1 - \exp\left( -\frac{1}{\pi} \left( \frac{\pi \cdot l^2}{tD\_{\text{eff}}} \right)^n \cdot \frac{1.27 + 0.14 \frac{1}{\pi} \left( \frac{\pi \cdot l^2}{tD\_{\text{eff}}} \right)^n}{1 + 0.14 \frac{1}{\pi} \left( \frac{\pi \cdot l^2}{tD\_{\text{eff}}} \right)^n} \right) \right]^{1/2} \right] \tag{12}$$

Efremov [4] has calculated the power function of the argument n for clay materials as 1.95. In order to calculate *Deff* optimization concept is applied. Drying equation (3) is replaced by equation (12). When this correction is entered, the previously described optimizing concept for the determination of the effective diffusion coefficient can be applied.

#### *2.3.2. Model B2 - The case when shrinkage is included*

For materials which shows shrinkage during drying equation (12) needs to be changed by the introduction of the expression *l (t)* into it. This expression represents the experimentally determined time dependence of the sample thickness. When this correction is entered, the previously described optimizing concept for the determination of the effective diffusion coefficient is applied.

#### **2.4. Program algorithm**

The algorithm presented below is the same for any software program. Program named "Drying calculator" was written in the Borland C program language on a standard Pentium IV computer (AMD 1200 MHz, 80GB HDD, 256 MB ram memory). Program has the ability to calculate effective diffusion coefficient using the calculation method A1, A2, B1 and B2.

Program algorithm for models A1 and B1, which neglects shrinkage, contains the following steps:


Methods of Determination for Effective Diffusion Coefficient During Convective Drying of Clay Products 301


300 Clay Minerals in Nature – Their Characterization, Modification and Application

higher than 4·10-3, uniformly for all real A.

<sup>2</sup> <sup>1</sup> exp( )

 

*tD* 

*eff*

*n*

*2.3.2. Model B2 - The case when shrinkage is included* 

1. Read the values from database: the time (s), MReks.

3. Enter the initial value of *Deff* (*Deff* =1·10-20)

2. Enter number ε (Usually ε =0.05). *(Exists only in case of A1)*

kinetic equation (12) is obtained.

*<sup>l</sup> MR*

coefficient is applied.

B1 and B2.

steps:

**2.4. Program algorithm** 

Simple approximation formula for function erf (A) is defined by equation (11) and can be found in Sergei Winitzki [14, 15] papers. The relative precision of this approximation is

2

After some mathematical manipulation, knowing that erfc (A) = 1 – erf (A), the final drying

1.27 0.14 ( ) 1 exp 1 0.14 *<sup>A</sup> erf A <sup>A</sup>*

<sup>1</sup> 1 1 exp

for the determination of the effective diffusion coefficient can be applied.

1/2 <sup>2</sup>

<sup>1</sup> 1.27 0.14

*l tD*

<sup>1</sup> 1 0.14

*tD <sup>l</sup>*

*<sup>n</sup> eff*

(11)

(12)

1/2

2

*l*

*eff*

*n*

2

*eff*

*tD*

2

*A*

2

Efremov [4] has calculated the power function of the argument n for clay materials as 1.95. In order to calculate *Deff* optimization concept is applied. Drying equation (3) is replaced by equation (12). When this correction is entered, the previously described optimizing concept

For materials which shows shrinkage during drying equation (12) needs to be changed by the introduction of the expression *l (t)* into it. This expression represents the experimentally determined time dependence of the sample thickness. When this correction is entered, the previously described optimizing concept for the determination of the effective diffusion

The algorithm presented below is the same for any software program. Program named "Drying calculator" was written in the Borland C program language on a standard Pentium IV computer (AMD 1200 MHz, 80GB HDD, 256 MB ram memory). Program has the ability to calculate effective diffusion coefficient using the calculation method A1, A2,

Program algorithm for models A1 and B1, which neglects shrinkage, contains the following

*n*


Program algorithm for models A2 and B2, which includes shrinkage, is obtained from previously presented algorithm after a few modifications: in steps 1, and 6 are made.


#### **2.5. Description of the slope model**

For long drying times, equation (1) is transformed into equation (13).

$$MR = \frac{8}{\pi^2} \exp(-\pi^2 \frac{D\_{eff}t}{l^2}) \quad \text{or} \quad \ln(\frac{\pi^2 MR}{8}) = -\pi^2 \frac{D\_{eff}t}{l^2} \tag{13}$$

From the equation (13) slope *Deff* coefficient can be calculated.

#### **3. Results and discussion**

#### **3.1. Clay characterization and sample preparation**

Three raw masonry clays from the locality Banatski Karlovac (I), Ćirilkovac (II) and Orlovat (III) were analyzed. Characterization of raw masonry clays has included chemical, mineralogical, granulometrical, XRD, DTA and TGA examination. Results of chemical analysis are presented in table 1, while granulometric analyze is presented at fig. 1 and 2.



Methods of Determination for Effective Diffusion Coefficient During Convective Drying of Clay Products 303

XRD examinations far all three clays were recorded on the Belgrade faculty of mining and geology, using the device PHILIPS PW 1710. DTA and TGA examinations for clay (I) was recorded in Belgrade ITNMS institute, using the device SDT Q600 (TA Instruments), while for clay (II) and clay (III) these examinations were done on the Belgrade chemistry faculty,

Clay I

Clay II

Clay III

**Figure 3.** XRD diagrams

using the device DERIVATOGRAPH-C (MOM Budapest) and DUPON.

**Table 1.** Results of chemical analysis

**Figure 1.** Granulometric test results for clay (I)

**Figure 2.** Granulometric test results for Clay (II) and (III)

XRD examinations far all three clays were recorded on the Belgrade faculty of mining and geology, using the device PHILIPS PW 1710. DTA and TGA examinations for clay (I) was recorded in Belgrade ITNMS institute, using the device SDT Q600 (TA Instruments), while for clay (II) and clay (III) these examinations were done on the Belgrade chemistry faculty, using the device DERIVATOGRAPH-C (MOM Budapest) and DUPON.

**Figure 3.** XRD diagrams

302 Clay Minerals in Nature – Their Characterization, Modification and Application

**Table 1.** Results of chemical analysis

**Figure 1.** Granulometric test results for clay (I)

Composition Clay (I) Clay (II) Clay (III)

SiO2 53.23 53.08 54.49 Al2O3 13.64 16.73 13.91 Fe2O3 5.34 7.10 5.09 CaO 7.50 6.69 8.05 MgO 3.59 1.41 3.70 SO3 0.00 0.08 0.07 S2- 0.00 0.00 0.01 Na2O 1.24 0.48 1.14 K2O 3.42 1.70 1.70 MnO 0.091 0.15 0.08 TiO2 0.60 0.71 0.46 Summary: 100.36 100.40 100.19

Loss ignition on 10000C 11.71 7.15 6.71

Clay II Clay III

**Figure 2.** Granulometric test results for Clay (II) and (III)

% % %

Methods of Determination for Effective Diffusion Coefficient During Convective Drying of Clay Products 305

include in its structure the drying air parameters. Mancuhan [3] was studying industrial drying of bricks in a tunnel dryer in order to find optimal drying air parameters which were necessary for rationalization an optimization of the drying process. On the base of previously mentioned studies and along with the years of industrial production experience range of drying air parameters: temperature (40-700C), humidity (40-80%) and velocity (1-3 m/s) has been set up in this study as the boundaries of the planned drying experiment.

Drying kinetic curves were recorded, under the experimental conditions presented in Table 2, on the prepared heavy clay tiles (samples), by monitoring and recording the changes in weight and linear shrinkage of the clay tiles in a laboratory dryer, especially created for this purpose. Schematic view of the laboratory recirculation dryer is presented in Scheme 1.

regulation of the relative humidity of the drying air within 20-100%, with an accuracy of

monitoring and recording of the weight of the drying samples within 0-2000 g, with an

monitoring and recording the linear shrinkage within 0-23 mm with an accuracy of 0.2 mm;

1 1 40 40 2 3 40 40 3 1 40 80 4 3 40 80 5 1 70 40 6 3 70 40 7 1 70 80 8 3 70 80

Data acquisition, continuous time monitoring and recording of the temperature and relative humidity of the drying medium and the linear shrinkage of the drying samples were

realized automatically, using PLC controllers and a standard Pentium IV computer.

Air temperature, *T* / 0C

Air humidity, *V* / %

regulation of the drying air temperature within 0-125C, with accuracy 0.2C;

velocity regulation of the drying air within 0-3.5 m/s, with an accuracy of 1%;

*3.2.1. Laboratory recirculation dryer* 

0.2%;

and

accuracy of 0.01 g;

The laboratory recirculation dryer provides:

continuous time monitoring during drying.

Experiment Air velocity,

**Table 2.** Experimental conditions

*W* / m/s

**Figure 4.** DTA/TG diagrams

Based on chemical test results it can be concluded that all three clays are representing usual masonry raw materials, with a relatively low content of aluminium oxide, a relatively small content of clay minerals and feldspars and increased carbonate content. Clay (III) has the highest SiO2, and carbonate content. From fig. 1 and fig. 2 it can be seen that Clay (I) has the largest clay content of 30.18%, while in the case of Clay (II) and Clay (III) clay content were respectively 16.75%and 11.11%. From fig. 3 and fig. 4 it can be seen that the most common mineral in all three clays is quartz. Carbonate minerals: calcite and dolomite were present in all three clays too. Beside previously mentioned minerals clay (I) is consisted of mica, chlorite and a small amount of smectites, clay (II) is consisted of muscovite, montmorillonite, chlorite, and illite in traces, while clay (III) is consisted of illite, chlorite, and a small amount of smectites.

After initial clay characterization, the raw materials were subjected to further classical preparation. The raw material samples were first dried at 600C and then milled down in a laboratory perforated rolls mill. After that, the clays were moisturized and milled in a laboratory differential mill, first at a gap of 3 mm and then of 1 mm. Laboratory samples of size 120x50x14 mm were formed in a laboratory extruder "Hendle" type 4, under a vacuum of 0.8 bar. These samples were used in further experimental work.

#### **3.2. Drying experiments**

Moropoulou [22] was investigating the influence of drying air, temperature (20 - 400C), humidity (30-80%) and velocity (1 - 8 m/s) in order to develop a drying model which will include in its structure the drying air parameters. Mancuhan [3] was studying industrial drying of bricks in a tunnel dryer in order to find optimal drying air parameters which were necessary for rationalization an optimization of the drying process. On the base of previously mentioned studies and along with the years of industrial production experience range of drying air parameters: temperature (40-700C), humidity (40-80%) and velocity (1-3 m/s) has been set up in this study as the boundaries of the planned drying experiment.

Drying kinetic curves were recorded, under the experimental conditions presented in Table 2, on the prepared heavy clay tiles (samples), by monitoring and recording the changes in weight and linear shrinkage of the clay tiles in a laboratory dryer, especially created for this purpose. Schematic view of the laboratory recirculation dryer is presented in Scheme 1.

### *3.2.1. Laboratory recirculation dryer*

304 Clay Minerals in Nature – Their Characterization, Modification and Application

Clay I Clay II

while clay (III) is consisted of illite, chlorite, and a small amount of smectites.

of 0.8 bar. These samples were used in further experimental work.

**Figure 4.** DTA/TG diagrams

**3.2. Drying experiments** 

Clay III

Based on chemical test results it can be concluded that all three clays are representing usual masonry raw materials, with a relatively low content of aluminium oxide, a relatively small content of clay minerals and feldspars and increased carbonate content. Clay (III) has the highest SiO2, and carbonate content. From fig. 1 and fig. 2 it can be seen that Clay (I) has the largest clay content of 30.18%, while in the case of Clay (II) and Clay (III) clay content were respectively 16.75%and 11.11%. From fig. 3 and fig. 4 it can be seen that the most common mineral in all three clays is quartz. Carbonate minerals: calcite and dolomite were present in all three clays too. Beside previously mentioned minerals clay (I) is consisted of mica, chlorite and a small amount of smectites, clay (II) is consisted of muscovite, montmorillonite, chlorite, and illite in traces,

After initial clay characterization, the raw materials were subjected to further classical preparation. The raw material samples were first dried at 600C and then milled down in a laboratory perforated rolls mill. After that, the clays were moisturized and milled in a laboratory differential mill, first at a gap of 3 mm and then of 1 mm. Laboratory samples of size 120x50x14 mm were formed in a laboratory extruder "Hendle" type 4, under a vacuum

Moropoulou [22] was investigating the influence of drying air, temperature (20 - 400C), humidity (30-80%) and velocity (1 - 8 m/s) in order to develop a drying model which will The laboratory recirculation dryer provides:

regulation of the drying air temperature within 0-125C, with accuracy 0.2C;

regulation of the relative humidity of the drying air within 20-100%, with an accuracy of 0.2%;

velocity regulation of the drying air within 0-3.5 m/s, with an accuracy of 1%;

monitoring and recording of the weight of the drying samples within 0-2000 g, with an accuracy of 0.01 g;

monitoring and recording the linear shrinkage within 0-23 mm with an accuracy of 0.2 mm; and

continuous time monitoring during drying.


#### **Table 2.** Experimental conditions

Data acquisition, continuous time monitoring and recording of the temperature and relative humidity of the drying medium and the linear shrinkage of the drying samples were realized automatically, using PLC controllers and a standard Pentium IV computer.

**Scheme 1.** Laboratory recirculation dryer

#### *3.2.2. Interpretation*

Four models for predicting the drying behavior (*MRan–t* dependence) were obtained from previously described programs. Models A1 and B1 did not include shrinkage, while Models A2 and B2 did. Drying models results are presented in a form of table 3. Deviation of the predicted drying models form experimentally recorded data, presented in table 3, were given as a root mean square error (RMSE) calculated from equation (14). Typical graphical views of the experimental and predicted drying behaviour are presented in Fig. 5.

$$RMSE = \left[ \frac{1}{N} \sum\_{i=1}^{N} (MR\_{\text{exp},i} - MR\_{\text{pred},i})^2 \right]^{1/2} \tag{14}$$

Methods of Determination for Effective Diffusion Coefficient During Convective Drying of Clay Products 307

In all experiments value of RMSE had the lowest value for model B2, which means that

a b

c d

e f

**Figure 5.** Experimental and calculated moisture ratio vs. drying time

model B2 had less deviation from experimental results than all other drying models.

Lower value of RSME is representing better agreement between model predicted and experimental drying behaviour.


**Table 3.** Calculated RSME parameters

In all experiments value of RMSE had the lowest value for model B2, which means that model B2 had less deviation from experimental results than all other drying models.

306 Clay Minerals in Nature – Their Characterization, Modification and Application

Four models for predicting the drying behavior (*MRan–t* dependence) were obtained from previously described programs. Models A1 and B1 did not include shrinkage, while Models A2 and B2 did. Drying models results are presented in a form of table 3. Deviation of the predicted drying models form experimentally recorded data, presented in table 3, were given as a root mean square error (RMSE) calculated from equation (14). Typical graphical

exp, ,

*i pred i*

<sup>1</sup> ( )

RSME Clay (I) Clay (II) Clay (III) Model Model Model B2 A2 B1 A1 B2 A2 B1 A1 B2 A2 B1 A1 1 0.0281 0.0351 0.0517 0.0624 0.0241 0.0286 0.0586 0.0714 0.0166 0.0836 0.1079 0.1191 2 0.0111 0.0219 0.0694 0.0926 0.0194 0.0225 0.0523 0.0689 0.0186 0.0756 0.0995 0.1125 3 0.0143 0.0415 0.0731 0.0838 0.0174 0.0458 0.1104 0.1445 0.0524 0.0675 0.1159 0.1296 4 0.0128 0.0142 0.0461 0.0570 0.0219 0.0460 0.1002 0.1165 0.0378 0.0603 0.0944 0.1130 5 0.0121 0.0295 0.0611 0.0506 0.0212 0.0282 0.0856 0.1029 0.0474 0.0705 0.0935 0.1090 6 0.0073 0.0344 0.0689 0.0636 0.0232 0.0176 0.0532 0.0672 0.0173 0.0235 0.0841 0.1009 7 0.0125 0.0355 0.0467 0.0433 0.0188 0.0562 0.0996 0.1126 0.0171 0.0216 0.0709 0.0835 8 0.0201 0.0263 0.0730 0.0866 0.0247 0.0472 0.0942 0.1160 0.0242 0.0356 0.0820 0.0986

Lower value of RSME is representing better agreement between model predicted and

1/2 2

(14)

views of the experimental and predicted drying behaviour are presented in Fig. 5.

*N*

1

*i RMSE MR MR N*

**Scheme 1.** Laboratory recirculation dryer

experimental drying behaviour.

**Table 3.** Calculated RSME parameters

Exp.

*3.2.2. Interpretation* 

**Figure 5.** Experimental and calculated moisture ratio vs. drying time


The *Deff* values obtained through the use of the described programs and from slope of equation (13) are presented in Table 4.

Methods of Determination for Effective Diffusion Coefficient During Convective Drying of Clay Products 309

the dryer. The intersection point of the experimental drying curves and modelled drying curves is characterized as the critical point. Critical point is a characteristic kinetic parameter which is important because it determines the moment after which the products

From Table 4 it can be concluded that value of effective diffusion coefficient *Deff*  determined using the models which included the sample shrinkage correction is lower than the corresponding value determined using the models which neglected sample shrinkage or the slope model. The determined values of data of the *Deff* from the slope model were higher than the data determined by other models. This result is in agreement with the *Deff* determination and is representing additional proof that the models which included the shrinkage effect during drying have given more precise *Deff* values. Effective diffusion coefficients for masonry clay products are in range of 10-7 up to 10-12 m2/s according to references [6,10]. This relatively large range for the *Deff* values is connected with the different nature of the heavy clay and the different methods employed for their determination. The *Deff* values presented in Tables 4 are lying below the previously

Calculation methods and computer programs specially designed for calculation of effective diffusion coefficient were developed. First calculation method was based on the mathematical calculation of the second Fick's law and Cranck diffusion equation. Second calculation method was based on the analytical solution of the Efremov differential diffusion equation with a boundary condition in the form of the flux. In both calculation methods, two program variations were designed to compute the effective diffusion coefficient. First program variation did not include shrinkage effect during drying into the computation algorithm while the second one has included it. Four models (A1, A2, B1 and B2) for predicting the drying behaviour were obtained as the result of cited program. This was the first time in the mathematical modelling of the drying of masonry clay that a shrinkage correction was entered into the calculation step. Drying diagram analysis have showed that irrespective of the nature the initial mineralogical composition of the clay, the drying curves representing the models which neglects the shrinkage effect (A1 and B1) did not fully follow the configuration of the experimentally determined kinetic curves, while in the case of the models which include shrinkage (A2 and B2), the resulting curves follows the experimental ones. From Figs. 1 - 6 it can be seen that the introduction of the shrinkage correction into equations (3) and (12) was entirely justified. Drying model B2 had less deviation from experimental results than drying model A2, and all other drying models, in all experiments and for all three clays. It can be concluded that whatever the initial mineralogical composition of the clay model B2 is the best drying model. The determined values of the effective diffusion coefficient were lower than the value that could be found in the literature. The values of the effective diffusion coefficient

no longer shrink.

mentioned range.

**4. Conclusions** 

**Table 4.** Calculated values of effective diffusion coefficient

In all experiments, the value of effective diffusion coefficient *Deff* determined by models, which has included the shrinkage correction in the calculation step (A2 and B2), was lower then the value of the same coefficient determined by other models. On analyzing experiments it can be seen that by increasing the drying air velocity from 1 to 3 m/s, the value of the effective diffusion coefficient also increases up to 38% for clay (II), 45% and 60% for clay (III). Diagrams have showed that the kinetic curves representing the models which neglect the shrinkage effect (A1 and B1) do not completely follow the configuration of the experimentally determined drying curves. Drying model B1 had less deviation from experimental results than drying model A1, in all experiments and for all three clays. It can be concluded that whatever the initial mineralogical composition of the clay model B1 is better than model A1. Deviations of these models from the experimental drying curves are higher at the beginning of the drying process and after some time in most case the deviations disappear. The moment of disappearance matches the moment from when the sample continues to dry further without shrinkage.

Drying kinetic curves of the model which includes shrinkage (A2 and B2) follow the configuration of the experimentally determined curves and their matching can be more than 98% as can be seen in experiments 3, 8 in case of clay (I) or in experiments 2, 6 in case of clay (II) or in experiments 1, 7 in case of clay (III). Drying model B2 had less deviation from experimental results than drying model A2, and all other drying models, in all experiments and for all three clays. It can be concluded that whatever the initial mineralogical composition of the clay model B2 is the best drying model. In the case of model B2, if minor deviations exist, they are at the beginning of the drying process and are most probably caused by the time interval which has to pass before stationary experimental conditions are fulfilled and the products are heated up to the temperature in the dryer. The intersection point of the experimental drying curves and modelled drying curves is characterized as the critical point. Critical point is a characteristic kinetic parameter which is important because it determines the moment after which the products no longer shrink.

From Table 4 it can be concluded that value of effective diffusion coefficient *Deff*  determined using the models which included the sample shrinkage correction is lower than the corresponding value determined using the models which neglected sample shrinkage or the slope model. The determined values of data of the *Deff* from the slope model were higher than the data determined by other models. This result is in agreement with the *Deff* determination and is representing additional proof that the models which included the shrinkage effect during drying have given more precise *Deff* values. Effective diffusion coefficients for masonry clay products are in range of 10-7 up to 10-12 m2/s according to references [6,10]. This relatively large range for the *Deff* values is connected with the different nature of the heavy clay and the different methods employed for their determination. The *Deff* values presented in Tables 4 are lying below the previously mentioned range.

#### **4. Conclusions**

308 Clay Minerals in Nature – Their Characterization, Modification and Application

**Table 4.** Calculated values of effective diffusion coefficient

equation (13) are presented in Table 4.

Exp.

without shrinkage.

The *Deff* values obtained through the use of the described programs and from slope of

In all experiments, the value of effective diffusion coefficient *Deff* determined by models, which has included the shrinkage correction in the calculation step (A2 and B2), was lower then the value of the same coefficient determined by other models. On analyzing experiments it can be seen that by increasing the drying air velocity from 1 to 3 m/s, the value of the effective diffusion coefficient also increases up to 38% for clay (II), 45% and 60% for clay (III). Diagrams have showed that the kinetic curves representing the models which neglect the shrinkage effect (A1 and B1) do not completely follow the configuration of the experimentally determined drying curves. Drying model B1 had less deviation from experimental results than drying model A1, in all experiments and for all three clays. It can be concluded that whatever the initial mineralogical composition of the clay model B1 is better than model A1. Deviations of these models from the experimental drying curves are higher at the beginning of the drying process and after some time in most case the deviations disappear. The moment of disappearance matches the moment from when the sample continues to dry further

Drying kinetic curves of the model which includes shrinkage (A2 and B2) follow the configuration of the experimentally determined curves and their matching can be more than 98% as can be seen in experiments 3, 8 in case of clay (I) or in experiments 2, 6 in case of clay (II) or in experiments 1, 7 in case of clay (III). Drying model B2 had less deviation from experimental results than drying model A2, and all other drying models, in all experiments and for all three clays. It can be concluded that whatever the initial mineralogical composition of the clay model B2 is the best drying model. In the case of model B2, if minor deviations exist, they are at the beginning of the drying process and are most probably caused by the time interval which has to pass before stationary experimental conditions are fulfilled and the products are heated up to the temperature in

*Deff* · 1010 / m2/s Clay (I) Clay (II) Clay (III) Model Model Model A1 A2 B1 B2 SL A1 A2 B1 B2 SL A1 A2 B1 B2 SL 1 4.50 2.30 4.00 1.80 19.00 2.90 0.50 1.70 0.35 10.5 0.34 0.17 0.27 0.13 15.80 2 7.20 2.20 6.10 1.20 23.50 4.20 0.90 2.30 0.83 19.3 0.58 0.34 0.45 0.28 25.30 3 8.10 3.20 7.30 2.30 24.70 0.98 0.19 0.75 0.12 4.7 0.09 0.05 0.003 0.001 3.90 4 10.10 3.40 8.80 2.50 30.50 1.61 0.31 1.17 0.24 6.7 0.19 0.09 0.13 0.005 6.30 5 2.40 0.70 1.00 0.10 9.50 3.81 0.94 3.13 0.79 17.2 0.53 0.29 0.49 0.15 24.80 6 3.30 0.90 2.60 0.30 11.70 5.83 1.25 5.14 1.05 27.6 0.86 0.49 0.75 0.41 44.60 7 4.80 1.80 4.20 1.70 19.50 1.20 0.15 1.05 0.08 4.2 0.22 0.11 0.13 0.008 9,30 8 7.40 3.20 6.30 2.30 24.10 2.19 0,52 1.95 0.37 9.6 0.65 0.42 0.59 0.37 35.30

> Calculation methods and computer programs specially designed for calculation of effective diffusion coefficient were developed. First calculation method was based on the mathematical calculation of the second Fick's law and Cranck diffusion equation. Second calculation method was based on the analytical solution of the Efremov differential diffusion equation with a boundary condition in the form of the flux. In both calculation methods, two program variations were designed to compute the effective diffusion coefficient. First program variation did not include shrinkage effect during drying into the computation algorithm while the second one has included it. Four models (A1, A2, B1 and B2) for predicting the drying behaviour were obtained as the result of cited program. This was the first time in the mathematical modelling of the drying of masonry clay that a shrinkage correction was entered into the calculation step. Drying diagram analysis have showed that irrespective of the nature the initial mineralogical composition of the clay, the drying curves representing the models which neglects the shrinkage effect (A1 and B1) did not fully follow the configuration of the experimentally determined kinetic curves, while in the case of the models which include shrinkage (A2 and B2), the resulting curves follows the experimental ones. From Figs. 1 - 6 it can be seen that the introduction of the shrinkage correction into equations (3) and (12) was entirely justified. Drying model B2 had less deviation from experimental results than drying model A2, and all other drying models, in all experiments and for all three clays. It can be concluded that whatever the initial mineralogical composition of the clay model B2 is the best drying model. The determined values of the effective diffusion coefficient were lower than the value that could be found in the literature. The values of the effective diffusion coefficient

determined using the models which includes shrinkage were less than the values determined using the models which neglects shrinkage or the values obtained using the slope method. The intersection point of the experimental drying curves and the modelled drying curves is characterized as the critical point.

Methods of Determination for Effective Diffusion Coefficient During Convective Drying of Clay Products 311

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## **Author details**

Miloš Vasić and Zagorka Radojević *Institute for Testing Materials - IMS Institute Belgrade, Serbia* 

Željko Grbavčić *Faculty of Technology and Metallurgy of the University of Belgrade, Belgrade, Serbia* 

## **Acknowledgement**

This paper was realized under the project III 45008, which was financed by the Ministry of Science and Technological Development of the Republic of Serbia.

### **5. References**


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310 Clay Minerals in Nature – Their Characterization, Modification and Application

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**Author details** 

Željko Grbavčić

**5. References** 

0104-6632

London

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**Acknowledgement** 

Miloš Vasić and Zagorka Radojević *Institute for Testing Materials - IMS Institute Belgrade, Serbia* 

*Faculty of Technology and Metallurgy of the University of Belgrade, Belgrade, Serbia* 

determined using the models which includes shrinkage were less than the values determined using the models which neglects shrinkage or the values obtained using the slope method. The intersection point of the experimental drying curves and the

This paper was realized under the project III 45008, which was financed by the Ministry of

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[23] Mancuhan E. (2009). Analzsis and Optimization of Drying of Green Brick in a Tunnel

## *Edited by Marta Valaškova and Gražyna Simha Martynková*

Clay is an abundant raw material which has a variety of uses and properties depending on their structure and composition. Clay minerals are inexpensive and environmentally friendly naturally occurring nanomaterials, thanks to their 1 nm thick silicate layers, in all types of sediments and sedimentary rocks. The book chapters have been classified according to their characteristics in topics and applications. Therefore, in the first section five chapters is dedicated to the characterization and utilization of clay minerals in deposits. The second section includes four chapters about the significance of clay minerals in soils. Third section is devoted to different aspects of clay minerals research, especially to the characterization of structure and modifications for their application.

Photo by meaothai / Shutterstock

Clay Minerals in Nature - Their Characterization, Modification and Application

Clay Minerals in Nature

Their Characterization,

Modification and Application

*Edited by Marta Valaškova* 

*and Gražyna Simha Martynková*