Properties of amphoteric surfactants studied by ζ-potential measurements with latex particles

To study the electric properties of adsorption monolayers of an amphoteric surfactant, in our case lauryl dimethyl amine oxide (LDAO), we measured the zeta-potential of latex particles covered with this surfactant. Its adsorption on the latex particles is estimated by dynamic surface tension measurements coupled with light scattering experiments. The influence of pH, ionic strength, and surfactant concentration on the electric properties of the adsorption monolayer has been examined. We propose a theoretical model which describes the dependence of zeta-potential on both pH and the ionic strength. The model accounts for the adsorption of H+, Na+, and Cl- ions on the amphoteric surfactant headgroups. Very good agreement between theory and experiment is achieved. The zeta-potential measurements, coupled with the theoretical model, can be considered as a method for quantitative characterization of the ionization properties of amphoteric surfactants in adsorption layers. The latter properties are, in general, different from the surfactant ionization properties in the bulk of solution. We hope this approach can find application for characterizing various formulations used in the house hold and personal-care detergency, foaming, emulsification, etc.