ADSORPTION OF AGGREGATING MOLECULES

As it was shown in the experiments, the aggregation phenomena of adsorbed compounds play significant role on the adsorption extent. For an adsorbent with small pores, such as silica gel, the aggregation of the molecules in the bulk and on the surface may reduce adsorption due to the blocking of the pores (see Section 3.1.2 and Paper I).

As it was shown in Section 3.1.2, the aggregation of bacteria during adsorption has an influence to the adsorption extent. Because of the decrease of hydrophobicity in the bacteria in the aggregated state, the amount of the adsorbed bacteria is larger for the more hydrophilic adsorbent at a higher degree of aggregation (at a higher ionic strength), than for the hydrophobic one (Fig. 11).

Figure 13. Models of layers of a cationic surfactant at solid/liquid interfaces: A) hydrophobic surface: hemicylindrical aggregates; B) hydrophilic surfaces: (left) flexible cylindrical aggregates, (right) monolayer topped with hemicylinders [1].

A)

B)

In this study, the influence of the aggregation (micellization) phenomenon of surfactants on adsorption is described in the example of the cationic surfactant BKC. Surfactants are the surface-active molecules with hydrophobic and hydrophilic parts. In aqueous solutions, they exist as free monomers below the CMC, and due to their amphipathic molecular structure, they form aggregates in the solution and on interfaces at concentrations equal to the CMC and higher. The driving force of this aggregation or micelle formation is hydrophobic interactions between hydrocarbon chains of surfactant molecules [65]. Micelle formation poses additional difficulties for describing the surfactant adsorption equilibrium at concentration values below and above the CMC.

The adsorption behavior of ionic surfactants on a solid surface has been investigated extensively. Adsorption of cationic surfactants onto solid surfaces typically displays two-step adsorption mechanism as a function of surfactant concentration. At low concentrations the adsorption driven forces depend on the nature of the natures of adsorbent and adsorbate. With the increasing of the liquid phase concentration, adsorbate density on the adsorbent surface also increases until the surfactant achieves its critical micelle concentration, after that lateral hydrophobic interaction between adsorbed molecules becomes predominant. At low equilibrium concentrations, individual molecules adsorb onto silica surface [51, 66] and metal oxides, such as alumina oxide and titanium oxide [67, 68] due to electrostatic interaction. The dsorption of cationic surfactant onto nonionic graphite surface was due to hydrophobic interaction [69]. The adsorption onto ion-exchange materials such as zeolite clinoptilolite [63] and clay [70]

was due to exchange mechanism. In the adsorption onto activated carbon an ion-exchange mechanism and an ion pairing were predominant [71]. At a sufficiently high liquid phase concentration - the CMC and higher - surfactants form aggregates on solid surfaces [55, 63, 66-71].

A step forward in understanding the surfactant adsorption was made by Manne et al. [69], when they first showed the direct imaging of saturated surfactant layers adsorbed onto hydrophobic surface. They found that at low concentrations the surfactant molecules

form monolayer periodic structures, which are twice the molecular length and placed a head-to-head and tail-to-tail. They also postulated that this monolayer structure serves as a template for hemimicelle formation as the concentration is increased. Before that, the surface aggregation was only proposed in order to explain the specific shape of the surfactant adsorption isotherms.

The various micelle structures, including bi-layers, hemicylinders and spheres, have been observed to be dependent on the nature of the surfactant and surfaces (Fig. 13) [1, 62, 69, 72-78].

Several models were used for the describing the two-step adsorption of surfactants onto a solid surface. Zhu and Gu [79] used their model for fitting the adsorption isotherms where two-step adsorption was modeled with two binding constants (Eq. 14). The first step is described with the Langmuir isotherm. The second takes into account the surface aggregation, where each molecule of the monolayer acts as an “active center” for aggregation with the increasing surface density of surfactant. This model reproduces both Langmuir and S-shaped adsorption isotherms common in the adsorption of surfactants.

The limitation of this model is in the difficulty the parameters correlate at low values of aggregates number (n):

Another model, the self-consistent field (SCF) lattice theory, is has been successfully used for the surfactants adsorption on a charged surface. Also, it interprets quite well the adsorption with variable ionic strengths and pH values [68, 80].

This thesis presents a simplified adsorption model, describing the formation of surface aggregates. Again, this model describes two-step surfactant adsorption, where single molecules adsorb directly on the adsorbent surface, forming the first monolayer. This step

is described with the Langmuir isotherm model. The model assumes the monolayer adsorption (each site can accommodate only one molecule), the adsorption energy is the same at all sites, the adsorbent has finite adsorption capacity, and there is no interaction between adsorbed molecules [2]. Each molecule in the first layer can act as an active centre (or site), onto which a surface aggregate can grow (Fig. 13 (A)). Assuming that the interaction energy between the molecules in the liquid phase and the surface aggregate is constant, and that the number of molecules in the surface aggregate is unlimited, this distribution can be described with a linear isotherm. The total amount of surfactant adsorbed,q, is the sum of the molecules in the first layer and in the aggregates, and can thus be written as

For the details, see Paper III.

Simultaneous micellization and adsorption equilibria

In this example, the evaluation steps for an apparent adsorption equilibrium with simultaneous micellization are discussed.

Surfactants form micelles at their CMC and higher concentrations. Micelle formation creates additional complication in the simulation of the surfactant adsorption equilibrium.

The calculation example given can be used in the description of surfactant adsorption at concentrations exceeding the CMC. Firstly, the mass fraction of micelles (x) and the total mass of the surfactant (w) in the liquid phase were calculated from the mass balance and presented in Fig. 14(A). This enables the calculation of the concentration of the surfactant free monomers (c_A^L) at any surfactant concentration in the liquid phase (w0) (Fig. 14(B)).

As can be seen from the figure, at the w0 concentration the plot shows a plateau due to micellization when the monomer concentration does not change. It means that the adsorption isotherm in Fig. 14(C) does not rise after the monomers concentration reaches approximately 0.8 mmol/L. The last picture shows the apparent adsorption isotherm,

where both surfactant micellization and adsorption were taken into account. This isotherm shows that the plateau after the liquid phase surfactant concentration is equal or close to the CMC. This is because only monomers adsorb onto the solid phase, whereas micelles stay in the bulk solution.

Figure 14. (A) The mass fraction of surfactants in the micelles (x)vs. the total mass of surfactants in the liquid phase; (B) the molar concentration of free monomers in the liquid phasevs. the initial mass of the surfactant; (C) the adsorption isotherm of the surfactant without micellization taken into account; (D) the adsorption isotherm where the micellization of the surfactant is taken into account.

0 0.2 0.4 0.6 0.8