QUANTIFICATION OF SYNERGISTIC INTERACTION BETWEEN DIFFERENT SURFACTANTS USING A GENERALIZED FRUMKIN-DAMASKIN ADSORPTION-ISOTHERM

On the basis of a generalized Frumkin-Damaskin isotherm, synergistic i nteractions between mixed surfactants adsorbed at fluid interfaces are discussed in terms of a mutual interaction parameter. By use of exper imental sigma-log c curves of sodium dodecyl sulfate, oxethylated deca nol (5EO), and two homologous (N-n-alkyl-N,N-dimethylammonio)acetic ac id bromides (at pH = 7 predominantly betains), the adsorption paramete rs of these surfactants were determined, including their self-interact ion parameters. By use of these adsorption parameters, an additional i nteraction parameter was determined, which characterizes the interacti on between two different surfactants. This additional interaction para meter is small for mixtures of sodium dodecyl sulfate and the nonionic surfactant. It becomes considerable for mixtures of sodium dodecyl su lfate and (N-n-alkyl-N,N-dimethylammonio)acetic acid bromides, which i ndicates an additional synergistic attraction between the interacting molecules in the adsorption layer. The additional interaction paramete rs depend remarkably on the mixing ratio and correlate to the depressi on of the critical micelle concentration of these mixtures.