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

Synergistic interaction effects of surfactant mixtures adsorbed at fluid interfaces are discussed on the basis of a generalised Frumkin-Damaskin isotherm in terms of a mutual interaction parameter. The adsorption model is discussed by simulations for surfactants of different surface activity and different self-interaction parameters. For long chain homologous N-n-alkyl-N,N-dimethylammonio-acetic acid bromides (at pH 7 predominantly betaines) the adsorption parameters were determined from experimental sigma-log c curves using the original Frumkin-Damaskin isotherm, taking into account the displacement of solvent molecules by adsorbed surfactant molecules. Using the adsorption parameters of these isotherms, an additional interaction parameter was determined which characterises the interaction between the two surfactants. The range of application of this approach is discussed and the conditions for synergistic interaction am defined. If the alkyl chain length of the mixture components is small there are no synergistic effects. In the case of higher differences between the alkyl chain length, the interaction parameters determined are positive and indicate an additional synergistic attraction between the interacting molecules in the adsorption layer.