**4.3 Sorption properties**

It is well known that the regenerated cellulosic fibres absorb significant quantities of water by the expansion of void spaces within the semi-crystalline morphology, forming a watercellulose two-phase structure. Dyestuffs used for cellulosic fibres are highly water soluble, with molecular structures which are designed to interact at the internal interface between cellulose and water. The uptake of dyes is often used to monitor changes in fibre properties brought about by variations in processing, hence, dyes can be considered as coloured probe molecules that provide information on the detailed internal pore structure of fibres (Ibbett et al., 2006). The adsorption study of *hydrolyzed reactive dye*, namely C.I. Reactive Red 120 (RR120), on lyocell fabric was performed (Široký et al., 2011b) to gather information of the alkali treatment effect on the thermodynamics of the dye sorption and on changes in the fabric/fibre structure. Hydrolyzed reactive dye behaves essentially like a direct dye, which provides particular advantages of assurance that covalent bonding between the dye and fibre did not interfere with sorption, the substantivity (physical adsorption) is high and therefore, relatively short dyeing times can be employed as well as being used as molecular sensors to characterize cellulose substrate properties such as pore structure.(Inglesby & Zeronian, 2002; Luo et al., 2003)

Thorough study of dye adsorption revealed that sorption observed herein may occur *via* a combination of Langmuir, typical of limited side-specific adsorption, and Freundlich, indicating unlimited adsorption at non-specific sites, isotherms, although, sorption theory teaches that the interaction between dye molecules and cellulosic fibres is typically of Freundlich isotherm and based on hydrogen bonding and van der Waals interactions. On other site, carboxylate function groups, which are formed through oxidation of cellulosic fibres following mercerisation, are capable to provide anionic side-specific sides for adsorption. Interesting relation was obtained when the observations for equilibrium dye sorption (*q*e) from aqueous solution onto a lyocell was compared with the *reacting structural fraction* (RSF) concept developed and established by Fink *et al.* (1986). The *q*e of hydrolyzed RR120 by lyocell fabrics treated with varying concentrations of NaOH showed very similar shape of the plots with a maximum at *ca.* 3.50 mol dm-3 NaOH for lyocell, assigned to transition of cellulose II to its Na-cellulose II form following break of inter-molecular hydrogen bonds. Subsequent decrease within the respective transition ranges occurred to *ca.* 4.50 mol dm-3 NaOH for cellulose II. A second maximum occurred at *ca.* 4.75 mol dm-3 for cellulose II which is caused by an additional increase in the RSF of the polymer, relative to disruption of the intra-molecular hydrogen bonds in the Na-cellulose II form. Increasing treatment concentration of NaOH causes strong lateral fibre swelling and subsequently increases accessible internal volume within the structure of the fibre. Therefore, the observed differences in sorption may be explained by the accessible pore volume (APV) of lyocell fibres to probes as a function of NaOH concentration previously studied by Özturk *et* 

25°C and 40°C, respectively. Under these treatment conditions, it was proposed therein that maximum molecular reorganisation occurs in the amorphous and quasi-crystalline phases of the cellulose II polymer, which are corresponding to the changes evidenced from evaluation of physico-chemical properties (Široký et al., 2009). Moreover, it was demonstrated that even subtle changes in ATR-FTIR spectra can substantially affect

It is well known that the regenerated cellulosic fibres absorb significant quantities of water by the expansion of void spaces within the semi-crystalline morphology, forming a watercellulose two-phase structure. Dyestuffs used for cellulosic fibres are highly water soluble, with molecular structures which are designed to interact at the internal interface between cellulose and water. The uptake of dyes is often used to monitor changes in fibre properties brought about by variations in processing, hence, dyes can be considered as coloured probe molecules that provide information on the detailed internal pore structure of fibres (Ibbett et al., 2006). The adsorption study of *hydrolyzed reactive dye*, namely C.I. Reactive Red 120 (RR120), on lyocell fabric was performed (Široký et al., 2011b) to gather information of the alkali treatment effect on the thermodynamics of the dye sorption and on changes in the fabric/fibre structure. Hydrolyzed reactive dye behaves essentially like a direct dye, which provides particular advantages of assurance that covalent bonding between the dye and fibre did not interfere with sorption, the substantivity (physical adsorption) is high and therefore, relatively short dyeing times can be employed as well as being used as molecular sensors to characterize cellulose substrate properties such as pore structure.(Inglesby &

Thorough study of dye adsorption revealed that sorption observed herein may occur *via* a combination of Langmuir, typical of limited side-specific adsorption, and Freundlich, indicating unlimited adsorption at non-specific sites, isotherms, although, sorption theory teaches that the interaction between dye molecules and cellulosic fibres is typically of Freundlich isotherm and based on hydrogen bonding and van der Waals interactions. On other site, carboxylate function groups, which are formed through oxidation of cellulosic fibres following mercerisation, are capable to provide anionic side-specific sides for adsorption. Interesting relation was obtained when the observations for equilibrium dye sorption (*q*e) from aqueous solution onto a lyocell was compared with the *reacting structural fraction* (RSF) concept developed and established by Fink *et al.* (1986). The *q*e of hydrolyzed RR120 by lyocell fabrics treated with varying concentrations of NaOH showed very similar shape of the plots with a maximum at *ca.* 3.50 mol dm-3 NaOH for lyocell, assigned to transition of cellulose II to its Na-cellulose II form following break of inter-molecular hydrogen bonds. Subsequent decrease within the respective transition ranges occurred to *ca.* 4.50 mol dm-3 NaOH for cellulose II. A second maximum occurred at *ca.* 4.75 mol dm-3 for cellulose II which is caused by an additional increase in the RSF of the polymer, relative to disruption of the intra-molecular hydrogen bonds in the Na-cellulose II form. Increasing treatment concentration of NaOH causes strong lateral fibre swelling and subsequently increases accessible internal volume within the structure of the fibre. Therefore, the observed differences in sorption may be explained by the accessible pore volume (APV) of lyocell fibres to probes as a function of NaOH concentration previously studied by Özturk *et* 

crystallinity indices.

**4.3 Sorption properties** 

Zeronian, 2002; Luo et al., 2003)

*al.* (2009) using inversion size exclusion chromatography (ISEC). It was demonstrated that for pore diameters of 19 Å and below that APV increased with increasing NaOH concentration to a maximum around 2.5 mol dm-3 NaOH, which was ca. 10% higher than for untreated lyocell. Applying molecular dynamics, the geometry and electronic properties of hydrolyzed RR120 were explored, and it was found that the dimensions of employed dye are about 30 Å (length) × 14 Å (wide) × 3.5 Å (depth) in one of the minimum energy states, providing a molecular diameter of 14 Å (end-on). It is less than 19 Å and therefore, it is likely that the maximum APV at around 2.5 mol dm-3 NaOH observed by (Özturk et al., 2009) influences the accompanying maxima in the theoretical monolayer capacity (*q*0) and adsorption energy (Δ*G*0) also known as a standard free energy of dyeing or standard affinity of the dye for the substrate, which was indeed observed from the experimental results.

The measurement of *iodine sorption* has been frequently applied in order to study an accessibility of cellulosic fibers.(Nelson et al., 1970; Schwertassek K., 1956; Široká et al., 2008; Strnad et al., 2001) This method has also been employed as a measure of fiber crystallinity.(Elesini U. S. & Cuden A. P., 2002; Hessler & Power, 1954; Zemljic et al., 2008) The computation of substrate crystallinity from iodine sorption value is based on the principle that iodine sorption in substrates is limited only to amorphous regions. However, earlier studies have shown that iodine penetrates into crystalline regions when the adsorption exceeds 11-12%, and the potassium iodide, in which the iodine is dissolved, acts as a swelling agent for cellulose.(Doppert H. L., 1967) These factors will influence results of iodine sorption. Hence, the iodine sorption may be better regarded as a general measure of overall accessibility in substrates. From the iodine sorption experiments performed on alkali treated lyocell fabrics, it was observed that the results do not follow the crystallinity results obtained by ATR-FTIR method. It seems that not only crystallinity of alkali treated lyocell fabrics swelling which is generally considered to increase accessibility of substrates, but also the construction of substrate can significantly influence the accessibility to iodine.
