Surfactant adsorption contribution

If an ionic surfactant is present, the potentials should vary as shown in Fig. XIV-5c, or similarly to the case with nonsurfactant electrolytes. In addition, however, surfactant adsorption decreases the interfacial tension and thus contributes to the stability of the emulsion. As discussed in connection with charged monolayers (see Section XV-6), the mutual repulsion of the charged polar groups tends to make such films expanded and hence of relatively low rr value. Added electrolyte reduces such repulsion by increasing the counterion concentration the film becomes more condensed and its film pressure increases. It thus is possible to explain qualitatively the role of added electrolyte in reducing the interfacial tension and thereby stabilizing emulsions. 

Polarity of Vinyl Acrylic Latex and Surfactant Adsorption Contact angle measurements, dispersion and polar contribution to latex film surface tension and polarity of polymer calculated according to the method of Kaelble (10) of the three latex films are whown in Table V. It is seen that the polarity of the latex film decreases with increase in butyl acrylate content of the vinyl acrylic co-polymer. The polarity of the 70/30 (VA/BA) latex is very similar to that of the polybutyl acrylate homopolymer estimated to be about 0.21 (1). ... 

Binding forces in the process of surfactant adsorption on the surface of ionic crystals were studied in detail by Richter and Schneider74) who analyzed the adsorption conditions for low 0 values and values of 0 in the range of a monomolecular layer. For low values the following energetic contributions may play a role ... 

Clearly, neutron reflectivity has contributed much to our understanding of the nature of surfactant adsorption at the solid-solution interface. It has already been successfully applied to an extensive range of systems, as illustrated in this chapter. 

Surfactant adsorption is addressed in Chapter 10 by Lapata, Harwell, and Scamehorn, which describes the different adsorption regions. Following their contribution is a chapter by Louvisse and Gonzalez, in which surfactant adsorption and its effect on contact angles are measured. 

In general, the total adsorption F of an ionic species include contributions from both the adsorption layer (surfactant adsorption layer + adsorbed counterions in the Stem layer), F , and the diffuse layer,... 

Narrowly defined, the main contributions to film pressure or interfacial tension decrease come from the osmotic term and the repulsion of the electrical double layers of ionic surfactants including the effects of counterions. Interactions in mixed adsorption layers are of broad interest for the description of the state of surfactant adsorption layers. For the clarification of the adsorption mechanism at liquid interfaces the replacement of solvent molecules, mainly water, has been intensively studied by Lucassen-Reynders(1981). 

Kretzschmar Voigt (1989) have recently examined the contribution of interacting forces in surfactant adsorption layers to the film pressure. A detailed knowledge of the geometry of the electrical double layer with respect to the plane of the interface is an essential item in the theoretical description of charged monolayers, thin liquid films and membranes. Fig. 2.12. shows an illustration of structural and energetic aspects of the surfactant monolayer formation. 

As illustrated in Fig. 4. a number of surfactant properties experience discontinuity in slope at a specific concentration called the critical micelle concentration (CMC). Below the CMC, an insignificant concentration of micelles is present, and essentially all the surfactant is present as monomer. Above the CMC. incremental surfactant forms micelles, and the monomer concentration remains nearly constant. Surfactant molecules contribute differently to a given property when present as monomer versus in a micelle, accounting for the discontinuity in slope in Fig. 4. The monomer concentration is often nearly proportional to the thermodynamic activity of the surfactant, and it is this activity that determines such properties as adsorption at interfaces, surface tension reduction, and precipitation. Monomer concentrations become nearly constant above the CMC (for singlecomponent surfactants), resulting in many physical properties plateauing above the CMC. For example, as shown in Fig. 4, surface tension at the air-water interface levels off above the CMC—this is the dominant method of measuring the CMC. E en practical properties, such as... 

Chapters 26—29 all discuss hydrodynamic aspects of emulsified systems. The contribution by Danov, Kralchevsky, and Ivanov presents a very fundamental and thorough survey of different phenomena in emulsions related to dynamic and hydrodynamic motions, such as the dynamics of surfactant adsorption mono-layers, which include the Gibbs surface elasticity, and characteristic time of adsorption, mechanisms of droplet-droplet coalescence, hydrodynamic interactions and drop coalescence, interpretation of the Bancroft rule with regard to droplet symmetry, and, finally, kinetics of... 

Depending on the particle-surfactant system, one or more of the above contributions can be responsible for adsorption. The dominating one would depend on the nature and concentration of the surfactant, the surface chemistry of the particle, and solution properties such as pH and ionic strength. Electrostatic and lateral interaction forces are usually the major factors determining the adsorption of surfactants on oxides and other non-metallic minerals. Chemical interactions become more dominant for surfactant adsorption on salt-type minerals, such as carbonates and sulfides. 

For optimum surfactant adsorption at solid/liquid interfaces, mixed micelles have more efficient packing, which in turn contributes to better detergency by lowering the CMC and interfacial tension. 

Surfactant adsorption theories are based on different physical and geometrical models of the adsorbed layer, resulting in a variety of surface equations of state or equivalently in several different adsorption isotherms. The usual approach in the theoretical description of the adsorption of ionic surfactants is the generalization of an adsorption isotherm (or equation of state) of nonionic surfactants by incorporating the electrostatic contribution in the adsorption free energy [4, 5, 6, 7, 8]. The validity of the ionic models derived is usually tested by applying the models for the description of the surface... 

A review providing insight into the contribution of calorimetry to the revelation of surfactant adsorption phenonomena will be published shortly [49]. Table 1 provides a list of references to the particular systems studied so far. 

The standard free energy of surfactant adsorption, AG°, can be determined by nonlinear fits of surface tension isotherms with the help of a theoretical model of adsorption. The models of Frumkin, van der Waals, and Helfant-Frisch-Lebowitz have been applied, and the results have been compared [16]. Irrespective of the differences between these models, they give close values for the standard free energy because all of them reduce to the Henry isotherm for diluted adsorption layers. The results from the theoretical approach have been compared with those of the most popular empirical approach [17]. The latter gives values of the standard free energy, which are considerably different from the respective true values, with ca. 10 kJ/mol for nonionic surfactants, and 20 kJ/mol for ionic surfactants. These differences are due to contributions from interactions between the molecules in dense adsorption layers. The true values of the standard free energy can be determined with the help of an appropriate theoretical model. The van der Waals model was... 

Next, we consider the flexural properties of surfactant adsorption monolayers, which are important for the formation of small droplets, micelles, and vesicles in the fluid dispersions. The contributions of various interactions (van der Waals, electrostatic, steric) into the interfacial bending moment and the curvature elastic moduli are described. The effect of interfacial bending on the interactions between deformable emulsion droplets is discussed. 

With polymer dispersions a range of polarities can be achieved by suitable choice of monomer and the relation between surface polarity and surfactant adsorption can be studied [13]. Defining polarity, Xp, as f/y where yP is the polar contribution to polymer surface tension and y is the surface tension of the polymer, Vijayendran [13] has found that the logarithm of the area per molecule of NaLS at the polymer surface increases with the increase in polarity of the polymer surface. Some of his data are shown in Fig. 9.7. These results account for, at least in part, the low aggregate stability of polar emulsions commonly encountered in practice, as the increased area per molecule on the more polar polymers indicates that there will be decreased stability against flocculation. In none of these systems were there ionizable groups at the surface all are therefore hydrophobic systems where adsorption is likely to be monomolecular. 

Surfactants and their biotransformation products enter surface waters primarily through discharges from wastewater treatment plants (WWTPs). Depending on their physicochemical properties, surface-active substances may partition between the dissolved phase and the solid phase through adsorption onto suspended particles and sediments [1,2]. Several environmental studies have been dedicated to the assessment of the contribution of surfactant residues in effluents to the total load of surfactants in receiving waters. This contribution reviews the relevant literature describing the presence of linear alkylbenzene sulfonates (LASs) and in particular of their degradation products in surface waters and sediments (Table 6.3.1). 

This transition may j-.e. reducing the specific surface energy, f. The reduction of f to sufficiently small values was accounted for by Ruckenstein (15) in terms of the so called dilution effect". Accumulation of surfactant and cosurfactant at the interface not only causes significant reduction in the interfacial tension, but also results in reduction of the chemical potential of surfactant and cosurfactant in bulk solution. The latter reduction may exceed the positive free energy caused by the total interfacial tension and hence the overall Ag of the system may become negative. Further analysis by Ruckenstein and Krishnan (16) have showed that micelle formation encountered with water soluble surfactants reduces the dilution effect as a result of the association of the the surfactants molecules. However, if a cosurfactant is added, it can reduce the interfacial tension by further adsorption and introduces a dilution effect. The treatment of Ruckenstein and Krishnan (16) also highlighted the role of interfacial tension in the formation of microemulsions. When the contribution of surfactant and cosurfactant adsorption is taken into account, the entropy of the drops becomes negligible and the interfacial tension does not need to attain ultralow values before stable microemulsions form. 

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