Abstract

An analytical isotherm equation that describes the multilayer adsorption on fractal surfaces with adsorbate-adsorbate interactions (measured in terms of free energy) different from that of bulk liquid was developed. Assuming mathematical functionalities for the variation of the free energy, it is possible to evaluate the influence of the adsorbate-adsorbate interactions on the adsorption capacity of solids of high degree of surface irregularity. For those surfaces with relatively low degree of irregularity, it results that the free energy variation with the layer number in the multilayer region affects considerably the sorption capacity of the adsorbent, even for water activities lower than those corresponding to the monolayer moisture content. The energy interactions between adjacent adsorbate layers become less important as the fractal dimension of the adsorbent increases. For a fractal surface, the growing of the multilayer seems to mainly controlled by the degree of surface roughness characteristic of microporous adsorbents, where the volume and pore dimension are the true limitants to the sorption capacity. The isotherm equations obtained were tested fitting published experimental equilibrium data of various water vapor-biopolymer systems.

Keywords: isotherm, roughness, multilayer, fractal, adsorption, free energy

### 1. Introduction

It is well known that the knowledge and understanding of water adsorption isotherms is of great importance in food technology. This knowledge is highly important for the design and optimization of drying equipment, packaging of foods, prediction of quality, and stability during storage.

In order to describe the overall sorption over the whole region of relative pressures of water, an isotherm for multilayer sorption must be used.

In 1938, Brunauer, Emmett, and Teller (BET) [1] extended Langmuir's monolayer theory [2–5] to multilayer adsorption. The BET equation derived was applied to a wide variety of gases on surfaces as well as to the sorption of water vapor by food materials [6–8].

But, the simple BET equation gives a good agreement with experimental data only at relative pressures lower than 0.35 of adsorbate. A great number of

researchers have been analyzed this worrying fact, and numerous modifications have been proposed to the BET model to amend this problem [9–11].

ai P P‡ s ∗ <sup>i</sup>‐<sup>1</sup> <sup>¼</sup> bis ∗

Kinetic Approach to Multilayer Sorption: Equations of Isotherm and Applications

P <sup>P</sup>‡ <sup>¼</sup> <sup>s</sup><sup>∗</sup> i s∗ i‐1

certain amount, it can be written in general that:

Being s<sup>∗</sup>

where ΔG<sup>e</sup>

where

Defining

the relation:

As s<sup>∗</sup>

67

<sup>i</sup>‐<sup>1</sup> and s<sup>∗</sup>

written in a more convenient form:

DOI: http://dx.doi.org/10.5772/intechopen.82669

<sup>i</sup> exp – Ei

<sup>i</sup> the surfaces at the top of the respective i�1 and i layers.

ΔGi

Considering that R ln bð Þ <sup>i</sup>=ai is the sorption entropy of the i-layer, Eq. (2) can be

exp –

where ΔGi is the sorption free energy of the i-layer. This development differs from the classical BET model in that ΔGi for all layers above the first is not considered equal to free energy of bulk liquid adsorbate, ΔGL. Assuming that the free energy of sorption for the i-layer differs from the free energy of bulk liquid by a

<sup>Δ</sup>Gi <sup>¼</sup> <sup>Δ</sup>GL <sup>þ</sup> <sup>Δ</sup>G<sup>e</sup>

ΔGL RT � � exp

molecules in the pure liquid. Substituting Eq. (4) in Eq. (3), it results:

<sup>P</sup>‡ exp

<sup>ω</sup><sup>i</sup> <sup>¼</sup> <sup>P</sup>

and given that P0 <sup>¼</sup> <sup>P</sup>‡ exp ð Þ �ΔGL=RT , it results:

Combining Eqs. (8) and (9), we have

s ∗ <sup>i</sup> <sup>¼</sup> h1s0x<sup>i</sup>

<sup>1</sup> ¼ ω1s0, it results:

where C <sup>¼</sup> h1 <sup>¼</sup> exp <sup>Δ</sup>G<sup>e</sup>

s ∗ <sup>i</sup> ¼ ωis ∗

hi ¼ exp

<sup>ω</sup><sup>i</sup> <sup>¼</sup> <sup>P</sup> P0

> s ∗ <sup>i</sup> ¼ s ∗ 1 Y i

s ∗ <sup>i</sup> ¼ s ∗ 1 Y i

<sup>i</sup> differentiates the state of the adsorbed molecules from that of the

ΔG<sup>e</sup> i

being x ¼ P=P0. The fraction of surface occupied by 1st, 2nd, …, ith layer follows

j¼2

xhj

hj <sup>¼</sup> C s0x<sup>i</sup>

<sup>1</sup>=RT � � is the constant C of BET theory.

Y i

j¼2

j¼2

Y i

j¼2

ΔG<sup>e</sup> i

RT � � (2)

RT � � (3)

<sup>i</sup> (4)

<sup>i</sup>‐<sup>1</sup> (5)

RT � � (7)

hi ¼ xhi (8)

ω<sup>j</sup> (9)

� � (10)

hj (11)

RT � � (6)

Among these, the three parameters of GAB equation [12–15], introduce a modification to the BET sorption model. The GAB model is basically similar to BET ones in its assumptions. These authors propose that the state of the adsorbed molecules beyond the first layer is the same but different from that in the liquid state. This equation describes satisfactorily the sorption of water vapor in foods up to water activities of 0.8–0.9 [16–19]. The main advantage of the GAB equation is that its parameters have physical meaning. This equation has been adopted by West European Food Researchers [20].

For water activities higher than 0.8–0.9, most of the food materials show values of moisture content larger than that predicted by the GAB model. This flaw indicates that state of the adsorbed molecules beyond the first layer introduced by the GAB model is limited to a certain number of sorption layers. Then turn up as plausible to assume a third stage for the water molecules in the outer zone with true liquid-like properties, as postulated by the original BET model.

A three-zone model for the structure of water near water/solid interfaces was proposed by Drost-Hansen [21]; in this model, beyond the monolayer, a zone of ordered molecular structures of water is expected to exist adjacent to a surface, the ordering extending into the bulk liquid. This is a transition region over which one structure decays into another. At sufficiently large distances from the surface, bulk water structure exists.

The BET model and its modifications were developed for an energetically homogeneous flat surface without lateral interaction and are not suitable for highly rough surfaces [22].

This roughness plays a significant role in the determination of the adsorption characteristics [23–25], since the shape of the adsorbent surface influences the accessibility of the adsorbate to the active adsorption sites. In this chapter, their fractal dimension will characterize the roughness of the adsorbing surfaces. In addition, taking into account the model of the three zones, the derivation of an equation is presented for BET type multilayer isotherms on rough surfaces. This equation takes into account the influence of the adsorbate-adsorbent interaction of all the adsorbed layers.

It is shown that under certain conditions, this equation is reduced to the known classical forms. The capacity of the different isothermal equations to adjust the equilibrium moisture in the food is analyzed.

### 2. Mathematical model

Brunauer, Emmett, and Teller proposed an adsorption surface divided into n segments, having 1, 2, 3, …, i number of layers of adsorbed molecules. According to this model, adsorption and desorption occur at the top of these segments. So, the equilibrium between the uncovered surface so and the first layer s<sup>∗</sup> <sup>1</sup> is:

$$\mathbf{a}\_1 \frac{\mathbf{P}}{\mathbf{P}^\sharp} \ s\_0 = \mathbf{b}\_1 \mathbf{s}\_1^\* \exp\left(-\frac{\mathbf{E}\_1}{\mathbf{R}\mathbf{T}}\right) \tag{1}$$

where a1 and b1 are adsorption and desorption coefficients, the same meaning as in BET theory, E1 is the heat of adsorption of the first layer, R is the gas constant, T is the temperature, and P is the vapor pressure of adsorbate. Between any successive layers, the equilibrium can be expressed as:

Kinetic Approach to Multilayer Sorption: Equations of Isotherm and Applications DOI: http://dx.doi.org/10.5772/intechopen.82669

$$\mathbf{a}\_{\text{i}} \frac{\mathbf{P}}{\mathbf{P}^{\ddagger}} \mathbf{s}\_{\text{i-1}}^{\*} = \mathbf{b}\_{\text{i}} \mathbf{s}\_{\text{i}}^{\*} \exp\left(-\frac{\mathbf{E}\_{\text{i}}}{\mathbf{R} \mathbf{T}}\right) \tag{2}$$

Being s<sup>∗</sup> <sup>i</sup>‐<sup>1</sup> and s<sup>∗</sup> <sup>i</sup> the surfaces at the top of the respective i�1 and i layers. Considering that R ln bð Þ <sup>i</sup>=ai is the sorption entropy of the i-layer, Eq. (2) can be written in a more convenient form:

$$\frac{\mathbf{P}}{\mathbf{P}^\ddagger} = \frac{\mathbf{s}\_i^\*}{\mathbf{s}\_{i\cdot 1}^\*} \exp\left(-\frac{\Delta \mathbf{G}\_i}{\mathbf{R}\mathbf{T}}\right) \tag{3}$$

where ΔGi is the sorption free energy of the i-layer. This development differs from the classical BET model in that ΔGi for all layers above the first is not considered equal to free energy of bulk liquid adsorbate, ΔGL. Assuming that the free energy of sorption for the i-layer differs from the free energy of bulk liquid by a certain amount, it can be written in general that:

$$
\Delta \mathbf{G}\_{\rm i} = \Delta \mathbf{G}\_{\rm L} + \Delta \mathbf{G}\_{\rm i}^{\rm e} \tag{4}
$$

where ΔG<sup>e</sup> <sup>i</sup> differentiates the state of the adsorbed molecules from that of the molecules in the pure liquid. Substituting Eq. (4) in Eq. (3), it results:

$$\mathbf{s}\_{\mathbf{i}}^{\*} = \alpha\_{\mathbf{i}} \mathbf{s}\_{\mathbf{i}\cdot\mathbf{1}}^{\*} \tag{5}$$

where

researchers have been analyzed this worrying fact, and numerous modifications

Among these, the three parameters of GAB equation [12–15], introduce a modification to the BET sorption model. The GAB model is basically similar to BET ones in its assumptions. These authors propose that the state of the adsorbed molecules beyond the first layer is the same but different from that in the liquid state. This equation describes satisfactorily the sorption of water vapor in foods up to water activities of 0.8–0.9 [16–19]. The main advantage of the GAB equation is that its parameters have physical meaning. This equation has been adopted by West

For water activities higher than 0.8–0.9, most of the food materials show values of moisture content larger than that predicted by the GAB model. This flaw indicates that state of the adsorbed molecules beyond the first layer introduced by the GAB model is limited to a certain number of sorption layers. Then turn up as plausible to assume a third stage for the water molecules in the outer zone with true

A three-zone model for the structure of water near water/solid interfaces was proposed by Drost-Hansen [21]; in this model, beyond the monolayer, a zone of ordered molecular structures of water is expected to exist adjacent to a surface, the ordering extending into the bulk liquid. This is a transition region over which one structure decays into another. At sufficiently large distances from the surface, bulk

The BET model and its modifications were developed for an energetically homogeneous flat surface without lateral interaction and are not suitable for highly

This roughness plays a significant role in the determination of the adsorption characteristics [23–25], since the shape of the adsorbent surface influences the accessibility of the adsorbate to the active adsorption sites. In this chapter, their fractal dimension will characterize the roughness of the adsorbing surfaces. In addition, taking into account the model of the three zones, the derivation of an equation is presented for BET type multilayer isotherms on rough surfaces. This equation takes into account the influence of the adsorbate-adsorbent interaction of

It is shown that under certain conditions, this equation is reduced to the known classical forms. The capacity of the different isothermal equations to adjust the

Brunauer, Emmett, and Teller proposed an adsorption surface divided into n segments, having 1, 2, 3, …, i number of layers of adsorbed molecules. According to this model, adsorption and desorption occur at the top of these segments. So, the

∗

where a1 and b1 are adsorption and desorption coefficients, the same meaning as in BET theory, E1 is the heat of adsorption of the first layer, R is the gas constant, T is the temperature, and P is the vapor pressure of adsorbate. Between any successive

<sup>1</sup> exp � E1

RT  <sup>1</sup> is:

(1)

equilibrium between the uncovered surface so and the first layer s<sup>∗</sup>

<sup>P</sup>‡ s0 <sup>¼</sup> b1s

a1 P

have been proposed to the BET model to amend this problem [9–11].

liquid-like properties, as postulated by the original BET model.

European Food Researchers [20].

Food Engineering

water structure exists.

rough surfaces [22].

all the adsorbed layers.

2. Mathematical model

66

equilibrium moisture in the food is analyzed.

layers, the equilibrium can be expressed as:

$$\alpha\_{\rm i} = \frac{\rm P}{P^{\rm \circ}} \exp\left(\frac{\Delta \mathcal{G}\_{\rm L}}{\rm RT}\right) \exp\left(\frac{\Delta \mathcal{G}\_{\rm i}^{\rm e}}{\rm RT}\right) \tag{6}$$

Defining

$$\mathbf{h}\_{\mathrm{i}} = \exp\left(\frac{\Delta \mathbf{G}\_{\mathrm{i}}^{\mathrm{e}}}{\mathrm{RT}}\right) \tag{7}$$

and given that P0 <sup>¼</sup> <sup>P</sup>‡ exp ð Þ �ΔGL=RT , it results:

$$\mathbf{a}\_{\mathbf{i}} = \frac{\mathbf{P}}{\mathbf{P}\_0} \mathbf{h}\_{\mathbf{i}} = \mathbf{x} \mathbf{h}\_{\mathbf{i}} \tag{8}$$

being x ¼ P=P0. The fraction of surface occupied by 1st, 2nd, …, ith layer follows the relation:

$$\mathbf{s}\_{i}^{\*} = \mathbf{s}\_{1}^{\*} \prod\_{j=2}^{i} \alpha\_{j} \tag{9}$$

Combining Eqs. (8) and (9), we have

$$\mathbf{s}\_{\mathbf{i}}^{\*} = \mathbf{s}\_{\mathbf{i}}^{\*} \prod\_{\mathbf{j}=2}^{\mathrm{i}} \left( \mathbf{x} \mathbf{h}\_{\mathbf{j}} \right) \tag{10}$$

As s<sup>∗</sup> <sup>1</sup> ¼ ω1s0, it results:

$$\mathbf{s}\_{\mathbf{i}}^{\*} = \mathbf{h}\_{1} \mathbf{s}\_{0} \mathbf{x}^{\mathrm{i}} \prod\_{j=2}^{\mathrm{i}} \mathbf{h}\_{\mathbf{j}} = \mathbf{C} \, \mathbf{s}\_{0} \mathbf{x}^{\mathrm{i}} \prod\_{j=2}^{\mathrm{i}} \mathbf{h}\_{\mathbf{j}} \tag{11}$$

where C <sup>¼</sup> h1 <sup>¼</sup> exp <sup>Δ</sup>G<sup>e</sup> <sup>1</sup>=RT � � is the constant C of BET theory.

Given that the adsorbate molecules, considered as spheres, when adsorbed osculate the surface, for a fractal surface the relationship between the surfaces at the top, s<sup>∗</sup> <sup>i</sup> , and the bottom, si, for a molecular stack of i-layers is [26]:

$$\mathbf{s}\_{i}^{\*} = \mathbf{s}\_{i} \left(\mathbf{2i} \mathbf{-1}\right)^{\mathbf{2} \cdot \mathbf{D}} \tag{12}$$

2.1 Applications of the model to smooth surfaces

DOI: http://dx.doi.org/10.5772/intechopen.82669

N Nm ¼

N Nm ¼

GAB equation (see Eq. (23) in Table 1).

adsorbate/adsorbent systems [29–31].

food products is presented in Table 2.

for D = 2, hi = 1, and i ≥ 2, from Eq. (18)

also obtained by Brunauer et al. [1]. But from Eq. (19), D = 2, hi = 1, and i ≥ 2

1 <sup>N</sup>

k <sup>N</sup>

<sup>i</sup>–<sup>1</sup> <sup>N</sup>

Equations derived from Eq. (20).

N Nm <sup>¼</sup> <sup>C</sup><sup>x</sup> ð Þ 1–x

Nm <sup>¼</sup> <sup>C</sup><sup>x</sup>

Nm <sup>¼</sup> Ck<sup>x</sup>

Nm <sup>¼</sup> <sup>C</sup>xð Þ <sup>1</sup>þ<sup>x</sup>

N Nm <sup>¼</sup> <sup>C</sup><sup>x</sup>

in Table 1).

(limi!<sup>∞</sup> <sup>Δ</sup>G<sup>e</sup>

products.

i

i–1 i

Table 1.

69

2.1.1 Limited sorption

For a nonfractal surface (D = 2), Eqs. (18) and (19) reduce, respectively, to:

<sup>i</sup>¼2<sup>i</sup> <sup>x</sup><sup>i</sup> �

h i

<sup>i</sup>¼<sup>2</sup>∑∞

It is interesting to comment that for hi = 1 and i ≥ 2 (free energy of the multilayer equal to the free energy of bulk water), Eq. (20) reduces to BET equation (see Eq. (22)

Even more, if for the second and higher layers the free energy of the adsorbate differs from that of pure liquid in a constant amount (h = k), Eq. (18) reduces to

Assuming that the adsorbate properties approach to the pure liquid as i increases

Eqs. (22) and (23) are frequently used in bibliography and tested for different

The ability of Eq. (24) to fit experimental data of water sorption on different

Eq. (25) gives a good agreement using data of amilaceous materials, nuts, and meats, whereas Eq. (24) shows a good fitting with fruits, some vegetables, and milk

If the number of the adsorbed layers cannot exceed some finite number n, then

hi Equation References

<sup>1</sup>–ð Þ <sup>n</sup> <sup>þ</sup> <sup>1</sup> <sup>x</sup><sup>n</sup> <sup>þ</sup> <sup>n</sup>x<sup>n</sup>þ<sup>1</sup> ½ �

<sup>1</sup>–<sup>x</sup> <sup>þ</sup> <sup>C</sup><sup>x</sup> � <sup>C</sup>xnþ<sup>1</sup> ½ � (28)

ð Þ <sup>1</sup>–<sup>x</sup> ½ � <sup>1</sup>–xþC<sup>x</sup> (22) [1]

ð Þ <sup>1</sup>–k<sup>x</sup> ½ � <sup>1</sup>–kxþCk<sup>x</sup> (23) [27]

ð Þ <sup>1</sup>–<sup>x</sup> ð Þ <sup>1</sup>–<sup>x</sup> <sup>2</sup> ð Þ <sup>þ</sup>C<sup>x</sup> (24) [28]

ð Þ <sup>1</sup>–<sup>x</sup> ½ � <sup>1</sup>–C�ln 1ð Þ –<sup>x</sup> (25) [28]

<sup>i</sup> ¼ 0), h2 > h3 > … > 1 or h2 < h3 < … < 1, depending on arrangement of

h i

Q<sup>i</sup> <sup>j</sup>¼<sup>2</sup> hi

<sup>i</sup>¼<sup>2</sup> <sup>x</sup><sup>i</sup> Q<sup>i</sup> <sup>j</sup>¼<sup>2</sup> hj h i (20)

<sup>j</sup>¼<sup>i</sup> <sup>x</sup><sup>j</sup> <sup>Q</sup><sup>j</sup>

<sup>i</sup>¼<sup>2</sup> <sup>x</sup><sup>i</sup> Q<sup>i</sup> <sup>j</sup>¼<sup>2</sup> hj h i (21)

<sup>k</sup>¼<sup>2</sup> hk

<sup>C</sup> <sup>x</sup> <sup>þ</sup> <sup>∑</sup><sup>∞</sup>

Kinetic Approach to Multilayer Sorption: Equations of Isotherm and Applications

<sup>C</sup> <sup>x</sup> <sup>þ</sup> ∑∞

the adsorbate in the multilayer region (see Eqs. (24) and (25) in Table 1).

In Table 3, the fitting test corresponding to Eq. (25) can be seen.

<sup>1</sup> <sup>þ</sup> <sup>C</sup><sup>x</sup> <sup>þ</sup> <sup>C</sup> <sup>∑</sup><sup>∞</sup>

<sup>1</sup> <sup>þ</sup> <sup>C</sup><sup>x</sup> <sup>þ</sup> <sup>C</sup>∑<sup>∞</sup>

where D is the fractal dimension; when Eq. (12) is substituted in Eq. (11), it gives:

$$\mathbf{s}\_{\mathbf{i}} = \mathbf{C} \,\mathrm{s}\_{0} \mathbf{x}^{\dagger} (2\mathbf{i} - \mathbf{1})^{\mathrm{D}-2} \prod\_{\mathbf{j}=2}^{\mathrm{i}} \mathbf{h}\_{\mathbf{j}} \tag{13}$$

According to the BET theory, the monolayer capacity, Nm, is:

$$\mathbf{N\_m} = \frac{1}{\sigma} \sum\_{i=0}^{\infty} \mathbf{s\_i} = \frac{\mathbf{s\_0}}{\sigma} \left[ \mathbf{1} + \mathbf{C}\boldsymbol{\omega} + \sum\_{i=2}^{\infty} \mathbf{C}\boldsymbol{\omega}^i (2\mathbf{i} - \mathbf{1})^{\mathbf{D}-2} \prod\_{j=2}^{i} \mathbf{h\_j} \right] \tag{14}$$

where σ is the cross-sectional area of water molecule. The total amount of adsorbent in a given layer n is:

$$\mathbf{N}\_{\mathbf{n}}^{\mathrm{t}} = \frac{\left(2\mathbf{n} - \mathbf{1}\right)^{2 - \mathrm{D}}}{\sigma} \sum\_{i=\mathrm{n}}^{\infty} \mathbf{s}\_{i} = \frac{\mathrm{C} \; \mathbf{s}\_{0}}{\sigma} \left(2\mathbf{n} - \mathbf{1}\right)^{2 - \mathrm{D}} \sum\_{i=\mathrm{n}}^{\infty} \left(2\mathbf{i} - \mathbf{1}\right)^{\mathrm{D} - 2} \mathbf{x}^{\mathrm{i}} \prod\_{j=2}^{\mathrm{i}} \mathbf{h}\_{j} \tag{15}$$

The total number of molecules, N, that form the adsorbed film is:

$$\mathbf{N} = \frac{\mathbf{1}}{\sigma} \left[ \mathbf{s}\_1 + \sum\_{i=2}^{\infty} \mathbf{s}\_i \cdot \sum\_{\mathbf{k}=1}^{i} (2\mathbf{k} \cdot \mathbf{1})^{2 - \mathbf{D}} \right] = \frac{\mathbf{C} \mathbf{s}\_0}{\sigma} \left[ \mathbf{x} + \sum\_{i=2}^{\infty} \mathbf{x}^i (2\mathbf{i} - \mathbf{1})^{\mathbf{D} - 2} \prod\_{j=2}^{i} \mathbf{h}\_j \sum\_{\mathbf{k}=1}^{i} (2\mathbf{k} - \mathbf{1})^{2 - \mathbf{D}} \right] \tag{16}$$

but, from Eq. (15), it is also:

$$\mathbf{N} = \sum\_{i=1}^{\infty} \mathbf{N}\_i^t = \frac{\mathbf{C} \cdot \mathbf{s}\_0}{\sigma} \left[ \mathbf{x} + \sum\_{i=2}^{\infty} (2\mathbf{i} \cdot \mathbf{l})^{2 - \mathbf{D}} \sum\_{j=i}^{\infty} \mathbf{x}^j (2\mathbf{j} - \mathbf{1})^{\mathbf{D} - 2} \prod\_{k=2}^{j} \mathbf{h}\_k \right] \tag{17}$$

Finally, combining Eqs. (14) and (16), the following general equation for sorption isotherms is found:

$$\frac{\mathbf{N}}{\mathbf{N\_m}} = \frac{\mathbf{C} \left[ \mathbf{x} + \sum\_{i=2}^{\infty} \mathbf{x}^i (2\mathbf{i-1})^{\mathrm{D-2}} \cdot \prod\_{\mathbf{j=2}}^i \mathbf{h\_i} \ \sum\_{\mathbf{k=1}}^i (2\mathbf{k-1})^{2\mathrm{-D}} \right]}{\left[ \mathbf{1} + \mathbf{C}\mathbf{x} + \sum\_{i=2}^{\infty} \mathbf{C} \ \mathbf{x}^i (2\mathbf{i-1})^{\mathrm{D-2}} \prod\_{\mathbf{j=2}}^i \mathbf{h\_j} \right]} \tag{18}$$

But combining Eqs. (14) and (17), other equivalent form of the equation for sorption isotherms is reached:

$$\frac{\mathbf{N}}{\mathbf{N\_m}} = \frac{\mathbf{C} \left[ \mathbf{x} + \sum\_{i=2}^{\infty} (2\mathbf{i-1})^{2-\mathbf{D}} \sum\_{j=1}^{\infty} \mathbf{x}^j (2\mathbf{j}-\mathbf{1})^{\mathbf{D}-2} \prod\_{k=2}^{j} \mathbf{h}\_k \right]}{\left[ \mathbf{1} + \mathbf{C}\mathbf{x} + \sum\_{i=2}^{\infty} \mathbf{C} \,\mathbf{x}^i (2\mathbf{i}-\mathbf{1})^{\mathbf{D}-2} \prod\_{j=2}^{i} \mathbf{h}\_j \right]} \tag{19}$$

Eq. (18) is therefore the isotherm equation for multilayer adsorption on fractal surfaces that takes into account the variation of the free energy of adsorption with successive layers.

Kinetic Approach to Multilayer Sorption: Equations of Isotherm and Applications DOI: http://dx.doi.org/10.5772/intechopen.82669

### 2.1 Applications of the model to smooth surfaces

Given that the adsorbate molecules, considered as spheres, when adsorbed osculate the surface, for a fractal surface the relationship between the surfaces at the

where D is the fractal dimension; when Eq. (12) is substituted in Eq. (11), it

1 þ Cx þ ∑

where σ is the cross-sectional area of water molecule. The total amount of

<sup>σ</sup> ð Þ 2n–<sup>1</sup> <sup>2</sup>–<sup>D</sup> <sup>∑</sup>

2 4

x þ ∑ ∞ i¼2 xi

ð Þ 2i–<sup>1</sup> <sup>2</sup>–<sup>D</sup> <sup>∑</sup>

Finally, combining Eqs. (14) and (16), the following general equation for sorp-

<sup>i</sup>¼<sup>2</sup><sup>C</sup> <sup>x</sup><sup>i</sup>

But combining Eqs. (14) and (17), other equivalent form of the equation for

<sup>i</sup>¼<sup>2</sup><sup>C</sup> <sup>x</sup><sup>i</sup>

Eq. (18) is therefore the isotherm equation for multilayer adsorption on fractal surfaces that takes into account the variation of the free energy of adsorption with

<sup>i</sup>¼<sup>2</sup>ð Þ 2i–<sup>1</sup> <sup>2</sup>–D∑∞

ð Þ 2i–<sup>1</sup> <sup>D</sup>–<sup>2</sup> �

∞ j¼i xj

Q<sup>i</sup>

<sup>k</sup>¼<sup>1</sup>ð Þ 2k–<sup>1</sup> <sup>2</sup>–<sup>D</sup> h i

j¼i xj

h i

<sup>j</sup>¼<sup>2</sup> hi <sup>∑</sup><sup>i</sup>

ð Þ 2i–<sup>1</sup> <sup>D</sup>–<sup>2</sup> <sup>Q</sup><sup>i</sup>

ð Þ 2i � <sup>1</sup> <sup>D</sup>�<sup>2</sup> <sup>Q</sup><sup>i</sup>

h i (19)

h i (18)

ð Þ 2j � <sup>1</sup> <sup>D</sup>�<sup>2</sup> <sup>Q</sup><sup>j</sup>

" #

ð Þ 2i � <sup>1</sup> <sup>D</sup>�<sup>2</sup> <sup>Y</sup>

∞ i¼2 C x<sup>i</sup>

> ∞ i¼n

i

j¼2

<sup>i</sup> <sup>¼</sup> sið Þ 2i–<sup>1</sup> <sup>2</sup>–<sup>D</sup> (12)

ð Þ 2i � <sup>1</sup> <sup>D</sup>�<sup>2</sup> <sup>Y</sup>

ð Þ <sup>2</sup><sup>i</sup> � <sup>1</sup> <sup>D</sup>�<sup>2</sup>

ð Þ 2i � <sup>1</sup> <sup>D</sup>–<sup>2</sup> <sup>Y</sup>

ð Þ 2j � <sup>1</sup> <sup>D</sup>�<sup>2</sup> <sup>Y</sup>

<sup>j</sup>¼<sup>2</sup> hj

i

j¼2 hj

xi Y i

j¼2

i

hj ∑ i k¼1

j¼2

j

k¼2 hk

<sup>k</sup>¼<sup>2</sup> hk

<sup>j</sup>¼<sup>2</sup> hj

hj (13)

3

5 (14)

hj (15)

ð Þ 2k–<sup>1</sup> <sup>2</sup>–<sup>D</sup>

3 5

(16)

(17)

<sup>i</sup> , and the bottom, si, for a molecular stack of i-layers is [26]:

s ∗

si <sup>¼</sup> C s0x<sup>i</sup>

According to the BET theory, the monolayer capacity, Nm, is:

2 4

si <sup>¼</sup> C s0

The total number of molecules, N, that form the adsorbed film is:

<sup>¼</sup> Cs0 σ

x þ ∑ ∞ i¼2

<sup>i</sup>¼<sup>2</sup> <sup>x</sup><sup>i</sup>

<sup>1</sup> <sup>þ</sup> <sup>C</sup><sup>x</sup> <sup>þ</sup> ∑∞

<sup>1</sup> <sup>þ</sup> <sup>C</sup><sup>x</sup> <sup>þ</sup> ∑∞

Nm <sup>¼</sup> <sup>1</sup>

adsorbent in a given layer n is:

<sup>n</sup> <sup>¼</sup> ð Þ 2n–<sup>1</sup> <sup>2</sup>–<sup>D</sup>

si ∑ i k¼1

but, from Eq. (15), it is also:

N ¼ ∑ ∞ i¼1 Nt <sup>i</sup> <sup>¼</sup> C s0 σ

> N Nm ¼

sorption isotherms is reached:

successive layers.

68

N Nm ¼

tion isotherms is found:

� �

Nt

s1 þ ∑ ∞ i¼2

<sup>N</sup> <sup>¼</sup> <sup>1</sup> σ <sup>σ</sup> <sup>∑</sup> ∞ i¼0

<sup>σ</sup> <sup>∑</sup> ∞ i¼n

ð Þ 2k–<sup>1</sup> <sup>2</sup>–<sup>D</sup>

<sup>C</sup> <sup>x</sup> <sup>þ</sup> <sup>∑</sup><sup>∞</sup>

<sup>C</sup> <sup>x</sup> <sup>þ</sup> ∑∞

si <sup>¼</sup> s0 σ

top, s<sup>∗</sup>

Food Engineering

gives:

For a nonfractal surface (D = 2), Eqs. (18) and (19) reduce, respectively, to:

$$\frac{\mathbf{N}}{\mathbf{N\_m}} = \frac{\mathbf{C} \left[ \mathbf{x} + \sum\_{i=2}^{\infty} \mathbf{i} \,\mathbf{x}^i \cdot \prod\_{j=2}^i \mathbf{h}\_i \right]}{\left[ \mathbf{1} + \mathbf{C} \mathbf{x} + \mathbf{C} \sum\_{i=2}^{\infty} \mathbf{x}^i \prod\_{j=2}^i \mathbf{h}\_j \right]} \tag{20}$$

$$\frac{\mathbf{N}}{\mathbf{N\_m}} = \frac{\mathbf{C} \left[ \mathbf{x} + \sum\_{i=2}^{\infty} \sum\_{j=1}^{\infty} \mathbf{x}^{j} \prod\_{\mathbf{k}=2}^{j} \mathbf{h}\_{\mathbf{k}} \right]}{\left[ \mathbf{1} + \mathbf{C} \mathbf{x} + \mathbf{C} \sum\_{i=2}^{\infty} \mathbf{x}^{i} \prod\_{\mathbf{j}=2}^{i} \mathbf{h}\_{\mathbf{j}} \right]} \tag{21}$$

It is interesting to comment that for hi = 1 and i ≥ 2 (free energy of the multilayer equal to the free energy of bulk water), Eq. (20) reduces to BET equation (see Eq. (22) in Table 1).

Even more, if for the second and higher layers the free energy of the adsorbate differs from that of pure liquid in a constant amount (h = k), Eq. (18) reduces to GAB equation (see Eq. (23) in Table 1).

Assuming that the adsorbate properties approach to the pure liquid as i increases (limi!<sup>∞</sup> <sup>Δ</sup>G<sup>e</sup> <sup>i</sup> ¼ 0), h2 > h3 > … > 1 or h2 < h3 < … < 1, depending on arrangement of the adsorbate in the multilayer region (see Eqs. (24) and (25) in Table 1).

Eqs. (22) and (23) are frequently used in bibliography and tested for different adsorbate/adsorbent systems [29–31].

The ability of Eq. (24) to fit experimental data of water sorption on different food products is presented in Table 2.

In Table 3, the fitting test corresponding to Eq. (25) can be seen.

Eq. (25) gives a good agreement using data of amilaceous materials, nuts, and meats, whereas Eq. (24) shows a good fitting with fruits, some vegetables, and milk products.

#### 2.1.1 Limited sorption

If the number of the adsorbed layers cannot exceed some finite number n, then for D = 2, hi = 1, and i ≥ 2, from Eq. (18)

$$\frac{\mathbf{N}}{\mathbf{N\_m}} = \frac{\mathbf{C}\mathbf{x}}{(\mathbf{1}-\mathbf{x})} \frac{[\mathbf{1}(n+1)\mathbf{x}^n + \mathbf{n}\mathbf{x}^{n+1}]}{[\mathbf{1}\mathbf{-x} + \mathbf{C}\mathbf{x} - \mathbf{C}\mathbf{x}^{n+1}]} \tag{28}$$

also obtained by Brunauer et al. [1]. But from Eq. (19), D = 2, hi = 1, and i ≥ 2


Table 1. Equations derived from Eq. (20).


(a), adsorption; (d), desorption, (s), sorption.

\* E% ¼ 100 � ∑<sup>n</sup> Np � N<sup>e</sup> � � � �=Np p : predicted e : experimental

#### Table 2.

Food sorption isotherms fitted with Eq. (24).

$$\frac{\mathbf{N}}{\mathbf{N\_m}} = \frac{\mathbf{Cx}}{(\mathbf{1} - \mathbf{x})} \frac{(\mathbf{1} - \mathbf{x}^n)}{(\mathbf{1} - \mathbf{x} + \mathbf{Cx})} \tag{29}$$

known as Pickett [10] or Rounsley [50] isotherm equation.

#### 2.2 The fractal isotherm

If the surface of the adsorbent behaves like a fractal with 2 < D < 3 and assuming that the free energy distribution in the adsorbed film is the same like in BET theory (hi = 1), the following fractal isotherm equations can be obtained from Eqs. (18) and (19), respectively:

$$\frac{\mathbf{N}}{\mathbf{N}\_{\rm m}} = \frac{\mathbf{C} \left[ \mathbf{x} + \sum\_{i=2}^{\infty} \mathbf{x}^{\rm i} (2\mathbf{i} - \mathbf{1})^{\rm D - 2} \sum\_{j=1}^{i} (2\mathbf{j} - \mathbf{1})^{2 - \rm D} \right]}{\left[ \mathbf{1} + \mathbf{C}\mathbf{x} + \mathbf{C} \sum\_{i=2}^{\infty} \mathbf{x}^{\rm i} (2\mathbf{i} - \mathbf{1})^{\rm D - 2} \right]} \tag{30}$$

Table 4 illustrates the usefulness of Eq. (30) on starchy materials.

Mullet, white muscle (a) 25°C 6.1 10.1 3.7

=Np p : predicted e : experimental

2.3 A four parameters equation

(a), adsorption; (d), desorption, (s), sorption.

Food sorption isotherms fitted with Eq. (25).

E% ¼ 100 � ∑<sup>n</sup> Np � N<sup>e</sup> 

In all the modeled materials, it is found that the value of the parameter D is approximately 2.8. This indicates that the tested products show a high roughness.

Material Temperature C Nm%, d.b. E%\* References

Barley (d) 25°C 16.6 8.4 7.5 [36] Corn shelled (d) 25°C 21.6 8.5 7.5 [37] Flour (a) 30°C 57.7 6.8 9.1 [38] Hard red winter (d) 25°C 23.1 9.3 5.4 [39] Native manioc starch (d) 25°C 22.4 9.5 6.8 [37]

Oats (d) 25°C 16.2 8 8.2 [37] Rough rice (d) 60°C 8.2 5.7 3.2 [40] Rye (d) 25°C 26.4 8.3 7 [37] Sorghum (a) 38°C 19.0 8.4 6.1 [41] Starch (a) 30°C 9.6 9.4 1.6 [38] Tapioc starch (d) 25°C 19.9 10.8 8.2 [37] Wheat, durum (d) 25°C 22.1 9.1 5 [39]

Moroccan sweet almonds (a) 25°C 6.5 3.7 2.2 [42] Para nut (a) 25°C 9.4 2.7 4.7 [32] Peas, dried (a) 25°C 75.1 8.3 3.3 [42] Pecan nut (a) 25°C 6.8 2.9 4.7 [32] Rapeseed, guile (d) 25°C 10.7 4.6 3.2 [43]

Soybean seed (s) 15°C 5.6 8 5.4 [44]

Beef, minced (a) 30°C 4.4 9.2 5.4 [45] Beef, raw/minced (s) 10 °C 2.3 14.3 8.4 [46] Cod, freeze dried/unsalted (a) 25°C 5.6 12.2 5.8 [47] Fish flour (a) 25°C 4.2 6.8 5.3 [48] Mullet roe, unsalted (a) 25°C 5.3 4.8 1.4 [49]

Rapeseed, tower (a) 25°C 8.4 5 2.3

Native potato starch (a) 25°C 11.9 11.5 3.1

Kinetic Approach to Multilayer Sorption: Equations of Isotherm and Applications

Starchy foods

DOI: http://dx.doi.org/10.5772/intechopen.82669

Nuts and oilseeds

Meats

\*

71

Table 3.

Assuming that the total surface area available for sorption is formed by two types of surfaces or regions: (a) a region representing a fraction, α, of the total adsorbing

$$\frac{\mathbf{N}}{\mathbf{N\_m}} = \frac{\mathbf{C} \left[ \mathbf{x} + \sum\_{i=2}^{\infty} (2\mathbf{i-1})^{2-\mathbf{D}} \sum\_{j=1}^{\infty} \mathbf{x}^j (2\mathbf{j} - \mathbf{1})^{\mathbf{D}-2} \right]}{\left[ \mathbf{1} + \mathbf{C}\mathbf{x} + \mathbf{C} \sum\_{i=2}^{\infty} \mathbf{x}^i (2\mathbf{i} - \mathbf{1})^{\mathbf{D}-2} \right]} \tag{31}$$

To illustrate the effect of roughness on the shape of the isotherms, in Figure 1, the influence of D values, for C = 20, can be seen.


#### Kinetic Approach to Multilayer Sorption: Equations of Isotherm and Applications DOI: http://dx.doi.org/10.5772/intechopen.82669

#### Table 3.

N Nm

2.2 The fractal isotherm

Milk products

Food Engineering

Fruits

Vegetables

\*

70

Table 2.

Eqs. (18) and (19), respectively:

(a), adsorption; (d), desorption, (s), sorption.

Food sorption isotherms fitted with Eq. (24).

E% ¼ 100 � ∑<sup>n</sup> Np � N<sup>e</sup> � � �

> N Nm ¼

N Nm ¼

the influence of D values, for C = 20, can be seen.

known as Pickett [10] or Rounsley [50] isotherm equation.

<sup>C</sup> <sup>x</sup> <sup>þ</sup> ∑∞

<sup>C</sup> <sup>x</sup> <sup>þ</sup> ∑∞

<sup>¼</sup> Cx ð Þ 1 � x

Material Temperature C Nm%, d.b. E%\* References

Edam cheese (a) 25°C 27.6 2.1 4.7 [32]

Apple (d) 20°C 60.3 4.6 7.1 [33]

Banana (a) 25°C 1.5 4.8 18.1 [32] Figs (a) 30°C 1.7 4.9 5.8 [33] Pear (a) 25°C 6.1 5.2 6.2 [32]

Plums (a) 30°C 1.9 4.9 10.8 [33]

Carrots (s) 37°C 2.9 4.5 5.8 [34] Onion, tender (a) 25°C 64.7 4.1 5 [32]

Spinach (s) 37°C 64.3 3.2 1.2 [34] Sugar beet (d) 25°C 14.3 4.3 6.4 [35]

Emmental cheese (a) 25°C 77.7 1.9 10.3 Yoghurt (a) 25°C 25.2 2.8 2.2

Apricots (a) 30°C 1.7 3.9 10.4

Pineapple (a) 25°C 4.7 4.7 25

Sultana raisins (a) 30°C 48.4 4.5 8.3

Radish (a) 25°C 9.7 4.6 8.4

�=Np p : predicted e : experimental

If the surface of the adsorbent behaves like a fractal with 2 < D < 3 and assuming that the free energy distribution in the adsorbed film is the same like in BET theory (hi = 1), the following fractal isotherm equations can be obtained from

<sup>i</sup>¼<sup>2</sup> <sup>x</sup><sup>i</sup>

<sup>1</sup> <sup>þ</sup> <sup>C</sup><sup>x</sup> <sup>þ</sup> <sup>C</sup>∑∞

<sup>1</sup> <sup>þ</sup> <sup>C</sup><sup>x</sup> <sup>þ</sup> <sup>C</sup>∑∞

<sup>i</sup>¼<sup>2</sup>ð Þ 2i–<sup>1</sup> <sup>2</sup>�<sup>D</sup>∑∞

To illustrate the effect of roughness on the shape of the isotherms, in Figure 1,

ð Þ 2i–<sup>1</sup> <sup>D</sup>�<sup>2</sup> <sup>∑</sup><sup>i</sup> <sup>j</sup>¼<sup>1</sup>ð Þ 2j–<sup>1</sup> <sup>2</sup>�<sup>D</sup> h i

<sup>i</sup>¼<sup>2</sup> <sup>x</sup><sup>i</sup>

<sup>i</sup>¼<sup>2</sup> <sup>x</sup><sup>i</sup>

<sup>j</sup>¼<sup>i</sup> <sup>x</sup><sup>j</sup> ð Þ 2j � <sup>1</sup> <sup>D</sup>�<sup>2</sup> h i

ð Þ 2i–<sup>1</sup> <sup>D</sup>�<sup>2</sup> h i (30)

ð Þ 2i � <sup>1</sup> <sup>D</sup>�<sup>2</sup> h i (31)

<sup>1</sup> � <sup>x</sup><sup>n</sup> ð Þ

ð Þ <sup>1</sup> � <sup>x</sup> <sup>þ</sup> Cx (29)

Food sorption isotherms fitted with Eq. (25).

Table 4 illustrates the usefulness of Eq. (30) on starchy materials.

In all the modeled materials, it is found that the value of the parameter D is approximately 2.8. This indicates that the tested products show a high roughness.

### 2.3 A four parameters equation

Assuming that the total surface area available for sorption is formed by two types of surfaces or regions: (a) a region representing a fraction, α, of the total adsorbing

#### Figure 1.

Shape of the isotherm for different values of D (C = 20).


#### Table 4.

Food sorption isotherms fitted with Eq. (30).

surface that only adsorbs a limited number of adsorbate layers, that is, internal surface such as pores; (b) the remainder fraction, (1 � α), of the total adsorbing surface, where unlimited sorption may occur, that is, external surface and macropores where large values of n are required to fill them. From Eq. (29), it can be written [54]:

$$\frac{\text{N}}{\text{N}\_{\text{m}}}\_{\text{m}} = \frac{\text{Cx}(\text{1} - a\mathbf{x}^{n})}{(\text{1} - \text{x})(\text{1} - \text{x} + \text{Cx})} \tag{32}$$

In Table 5, the fitting of moisture sorption on different products using Eq. (32)

Eq. (32) gives a good agreement with experimental data. The inclusion of the

Material T°C C Nm%, d.b. α n E%\* References

Casein (a) 25 12.04 5.74 0.9864 3.72 0.97 [51]

Anis (a) 25 15.03 4.32 0.9900 9.42 1.15 [32]

Casein (d) 25 9.72 7.96 0.8923 2.34 0.77 Coffee (a) 20 2.21 3.23 0.8784 14.94 3.62 Dextrin (a) 10 12.02 13.01 0.8752 0.99 0.98 Potato starch (a) 20 8.57 8.03 0.9230 3.47 1.21

Avocado (a) 25 10.55 3.52 0.9900 10.13 3.43 Banana (a) 25 0.41 18.65 0.6432 3.87 9.06 Cardamom (a) 25 25.30 6.02 0.6734 2.64 0.67 Celery (a) 25 5.39 7.06 0.8406 9.09 3.43 Chamomile (a) 25 16.78 6.07 0.5010 4.67 0.94 Emmenthal (a) 25 9.68 3.42 0.9604 13.25 1.66 Cinnamon (a) 25 20.18 6.26 0.9999 3.63 0.19 Clove (a) 25 29.72 4.25 0.6638 3.08 1.45 Coriander (a) 25 10.95 5.87 0.7814 2.21 0.84 Eggplant (a) 25 6.56 7.76 0.4396 7.96 4.00

30 2.42 20.28 0.8478 1.41 0.19 [55]

40 2.36 18.52 0.8556 1.37 1.49 50 2.61 15.59 0.8588 1.50 1.91 60 1.79 17.80 0.9037 1.26 1.78 70 1.64 15.68 0.8945 1.30 2.71

Influence of temperature on desorption isotherms of champignon mushroom (Agaricus bisporus). Solid line,

alpha parameter allows modifying the amplitude of the isotherm plateau.

Kinetic Approach to Multilayer Sorption: Equations of Isotherm and Applications

DOI: http://dx.doi.org/10.5772/intechopen.82669

can be seen.

73

Eq. (32); dotted line, Pickett Eq. (29).

Champignon mushroom (a) (Agaricus bisporus)

Figure 2.

In Figure 2, data of champignon mushroom and its fit using Eqs. (32) and (29) for different temperatures are presented.

Kinetic Approach to Multilayer Sorption: Equations of Isotherm and Applications DOI: http://dx.doi.org/10.5772/intechopen.82669

#### Figure 2.

Influence of temperature on desorption isotherms of champignon mushroom (Agaricus bisporus). Solid line, Eq. (32); dotted line, Pickett Eq. (29).

In Table 5, the fitting of moisture sorption on different products using Eq. (32) can be seen.

Eq. (32) gives a good agreement with experimental data. The inclusion of the alpha parameter allows modifying the amplitude of the isotherm plateau.


surface that only adsorbs a limited number of adsorbate layers, that is, internal surface such as pores; (b) the remainder fraction, (1 � α), of the total adsorbing surface, where unlimited sorption may occur, that is, external surface and macropores where large values of n are required to fill them. From Eq. (29), it can be

Material Temperature C Nm%, d.b. D E%\* Source

Wheat (a) 30°C 9.1 8.1 2.8 4.3 [38] Corn (d) 25°C 12.5 9.8 2.9 1.0 [37] Potato (a) 25°C 4.0 9.5 2.8 3.3 [51] Tapioca (d) 25°C 14.3 10.2 2.9 2.4 [37] Native manioc (a) 25°C 15.8 9.3 2.8 1.7 [37]

Rough rice (d) 25°C 10.7 9.0 2.9 2.6 [52] Rough rice (d) 40°C 8.3 8.9 2.9 1.3 [53]

Wheat durum (d) 25°C 12.6 8.7 2.8 0.9 [39] Corn, continental (d) 20°C 12.5 9.5 2.9 1.0 [53]

Sorghum (d) 20°C 16.4 9.6 2.9 0.9 Sorghum (d) 50°C 6.9 8.8 2.9 1.1

Corn, continental (d) 50°C 4.0 7.9 2.8 1.9

=Np p : predicted e : experimental

<sup>¼</sup> Cx <sup>1</sup> � <sup>α</sup>xn ð Þ

In Figure 2, data of champignon mushroom and its fit using Eqs. (32) and (29)

ð Þ <sup>1</sup> � <sup>x</sup> ð Þ <sup>1</sup> � <sup>x</sup> <sup>þ</sup> Cx (32)

N Nm

for different temperatures are presented.

(a), adsorption; (d), desorption; (s) sorption.

Food sorption isotherms fitted with Eq. (30).

E% ¼ 100 � ∑<sup>n</sup> Np � N<sup>e</sup> 

written [54]:

\*

72

Table 4.

Figure 1.

Food Engineering

Starches

Cereals

Shape of the isotherm for different values of D (C = 20).


region between the monolayer and the outer zone where the adsorbate has the properties of the bulk liquid through a free energy excess that differentiates the

Kinetic Approach to Multilayer Sorption: Equations of Isotherm and Applications

This general equation, depending on the simplifications assumed, gives the classical BET and GAB equations. But, taking into account an asymptotic reduction of the free energy excess, for a flat surface, two different equations were obtained. One of them appears useful to model starchy materials, extending the isotherm plateau, and the other, successfully model fruit isotherms, reducing the isotherm

Considering only the roughness, and assigning bulk liquid properties at all layers

This fractal dimension can vary from 2 to 3. The rising of its value result in an outspread the isotherm plateau. Particularly, for highly rough surfaces, the multilayer growing is limited by geometrical restrictions. In this case, the magnitude of

It results from the present analysis that modifications of the BET model based only on the three-zone model or geometric considerations conduct to similar results. So, Eqs. (25), (30), and (32) predict lower sorption capacity with the increment of water activity, giving better agreements with experimental isotherms than the

This fact forewarns that the fractal dimension in the model could be affected

The authors gratefully acknowledge the Morón University for providing finan-

Francisco S. Pantuso, María L. Gómez Castro, Claudia C. Larregain, Ethel Coscarello

© 2019 The Author(s). Licensee IntechOpen. This chapter is 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,

Agroalimentary Research Laboratory, FAyCA, Morón University,

\*Address all correspondence to: rjaguerre@gmail.com

beyond the first, an equation that includes the fractal dimension is obtained.

the interactions practically has no effect on the shape of the isotherm.

from unsuitable accounting for the adsorbate-adsorbent interactions.

adsorbed phase from the bulk liquid.

DOI: http://dx.doi.org/10.5772/intechopen.82669

plateau.

classical BET equation.

Acknowledgements

cial support.

Author details

75

and Roberto J. Aguerre\*

Morón, Buenos Aires, Argentina

provided the original work is properly cited.

#### Table 5.

Food sorption isotherms fitted with Eq. (32).

The modeling of the sigmoid isotherms is facilitated, typical form found in the adsorption of water in food products.

## 3. Conclusions

In the framework of the BET model, a general isotherm equation was obtained that includes the roughness of the adsorbent surface and characterizes the transition

#### Kinetic Approach to Multilayer Sorption: Equations of Isotherm and Applications DOI: http://dx.doi.org/10.5772/intechopen.82669

region between the monolayer and the outer zone where the adsorbate has the properties of the bulk liquid through a free energy excess that differentiates the adsorbed phase from the bulk liquid.

This general equation, depending on the simplifications assumed, gives the classical BET and GAB equations. But, taking into account an asymptotic reduction of the free energy excess, for a flat surface, two different equations were obtained. One of them appears useful to model starchy materials, extending the isotherm plateau, and the other, successfully model fruit isotherms, reducing the isotherm plateau.

Considering only the roughness, and assigning bulk liquid properties at all layers beyond the first, an equation that includes the fractal dimension is obtained.

This fractal dimension can vary from 2 to 3. The rising of its value result in an outspread the isotherm plateau. Particularly, for highly rough surfaces, the multilayer growing is limited by geometrical restrictions. In this case, the magnitude of the interactions practically has no effect on the shape of the isotherm.

It results from the present analysis that modifications of the BET model based only on the three-zone model or geometric considerations conduct to similar results.

So, Eqs. (25), (30), and (32) predict lower sorption capacity with the increment of water activity, giving better agreements with experimental isotherms than the classical BET equation.

This fact forewarns that the fractal dimension in the model could be affected from unsuitable accounting for the adsorbate-adsorbent interactions.
