Surfactant interactions

The conformational dynamics of chain segments near the head groups is more restricted than that of those far from the micellar core [8]. Moreover, to avoid the presence of energetically unfavorable void space in the micellar aggregate and as a consequence of the intermolecular interactions, surfactant molecules tend to assume some preferential conformations and a staggered position with respect to the micellar core [9]. A schematic representation of a reversed micelle is shown in Figure 1. 

Surfactants will be the last type of organic electrolyte to he studied in this section. These molecules are amphiphilic, due to their dual hydropho-bic/hydrophilic character. This produces their accumulation at soHd-water interfaces, where both the hydrophobic and the hydrophihc parts participate in favorable intermolecular interactions. Surfactants are widely used in many industrial and commercial products and processes and have an environmental impact on wastewaters. In recent years, several studies have been pubUshed on the adsorption of surfactants on carbon materials [11, 45-48], and the main results obtained in these works are presented below. 

Figure 13.11 Effect of surfactant on aqueous phase associative thickener interactions. Surfactant molecules form mixed aggregates wifo thickener hydrophobes, first strengthening the aggregates, then displacing thickener hydrophobes to form a network. At veiy high surfactant concentrations, thickeno- l diophobes are completely masked by surfactant molecules, preventing network foimation...

In addition to the electrostatic consequences of specific charge-charge interactions, surfactant adsorption by ion exchange or ion pairing results in the orientation of the molecules with their hydrophobic groups toward the aqueous phase therefore, the surface becomes hydrophobic and less easily wetted by that phase. Once the solid surface has become hydrophobic, it is possible for adsorption to continue by dispersion force interactions (Fig. 9.15). 

It is difficult to summarize all the phenomena discussed in this volume. However, major topics include ultralow interfacial tension, phase behavior, microstructure of surfactant systems, optimal salinity concept, middle-phase microemuIsions, interfacial rheology, flow of emulsions in porous media, wettability of rocks, rock-fluid interactions, surfactant loss mechanisms, precipitation and redissolution of surfactants, coalescence of drops in emulsions and in porous media, surfactant mass transfer across interfaces, equilibrium dynamic properties of surfactant/oil/brine systems, mechanisms of oil displacement in porous media, ion-... 

The thermodynamics of small systems developed by Hill [183] has been applied to non-ionized, non-interacting surfactant systems by Hall and Pethica [184]. In this approach the aggregation number is treated as a thermodynamic variable, thereby enabling variations in the thermodynamic functions of micelle formation with the mean aggregation number n to be examined. The thermodynamic functions of micellization assuming solution ideality are... 

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