Gibbs cosurfactants

The type of behavior shown by the ethanol-water system reaches an extreme in the case of higher-molecular-weight solutes of the polar-nonpolar type, such as, soaps and detergents [91]. As illustrated in Fig. Ul-9e, the decrease in surface tension now takes place at very low concentrations sometimes showing a point of abrupt change in slope in a y/C plot [92]. The surface tension becomes essentially constant beyond a certain concentration identified with micelle formation (see Section XIII-5). The lines in Fig. III-9e are fits to Eq. III-57. The authors combined this analysis with the Gibbs equation (Section III-SB) to obtain the surface excess of surfactant and an alcohol cosurfactant. 

The role of the cosurfactant in reducing the interfacial tension can be understood from application of the Gibbs adsorption equation in the form (14). 

The reason for the lowering of y when using two surfactant molecules can be understood from consideration of the Gibbs adsorption equation for multicomponent systems [9]. For two components, sa (surfactant) and co (cosurfactant). Equation (15.4) becomes. 

Surface active additives (cosurfactants, demulsifiers, etc.), such as fatty alcohols in the case of ionic surfactants, may affect the emulsifier partitioning between the phases and its adsorption, thereby changing the Gibbs elasticity and the interfacial tension. The surface-active additive may also change the surface charge (mainly by increasing the... 

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