NATURE OF THE ADSORPTION OF ZWITTERIONIC SURFACTANTS AT HYDROPHILIC SURFACES

This paper describes the adsorption of zwitterionic dodecyl-N,N-dimeth ylammonio alkanoates with polymethylene inter-charge arms of different lengths on silica. The data presented were obtained by in situ ellips ometry, allowing time-resolved studies of the surface excess, the mean thickness, and the refractive index of thin interfacial films. It is shown that the mode of adsorption of zwitterionic surfactants is simil ar to that observed for ethylene-oxide-based nonionic surfactants, The interaction energy between single zwitterionic surfactants and silica is relatively weak and the adsorption process is best described in te rms of surfactant self-assembly, promoted by the presence of the solid surface. The mode of adsorption is only weakly affected by increasing the number of intercharge methylene units, The surface aggregation be havior observed at the silica surface displays many parallels with the corresponding solution phase behavior. Finally, the adsorption of zwi tterionic surfactants is relatively independent of the pH. However, as the pH is lowered to the pK(a) values of the terminal carboxyl group (i.e., as the surfactants become increasingly positively charged) deso rption is observed. (C) 1996 Academic Press, Inc.