Dynamic surface tension of heterogemini surfactants with quaternary ammonium salt and gluconamide or sulfobetaine headgroups

Abstract

Dynamic surface tensions of two types of heterogemini surfactants with nonidentical hydrophilic headgroups consisting of a quaternary ammonium salt (cationic) and a gluconamide (nonionic) or sulfobetaine (zwitterionic) group were measured using the maximum bubble pressure method. For these compounds, effects of alkyl chain length, structure of the hydrophilic groups, and surfactant concentration were investigated using diffusion coefficients and parameter x. The parameter x is related to the difference between the energies of adsorption and desorption of the surfactant. The values of x of heterogemini surfactants increased as the alkyl chain length increased, and they were slightly larger than that for the corresponding monomeric surfactant. This is because of an increase in hydrophobicity caused by two alkyl chains, as well as interactions between two different hydrophilic groups. Adsorption rate of the heterogemini surfactants decreased with increasing alkyl chain length, indicating slow dynamics, and inhibited adsorption to the air/water interface as the chain length increased. However, at higher concentrations, the heterogemini surfactants showed rapid and effective adsorption and increased adsorption rates at higher concentrations. Diffusion coefficients of the heterogemini surfactants decreased with increasing concentrations for all chain lengths, indicating diffusion of the solute molecules to the subsurface and adsorption of the solute from the subsurface to the surface.