A NEW THEORETICAL APPROACH TO ADSORPTION OF IONIC SURFACTANTS AT WATER OXIDE INTERFACES - EFFECTS OF OXIDE SURFACE HETEROGENEITY

On the basis of the scaled particle theory, a new theoretical approach is developed for adsorption of ionic surfactants at water/oxide interfaces. The adsorbed phase is treated as a mixture of oblate aggregates of various dimensions, interacting via excluded volume interactions. A three-parameter equation for the adsorption isotherm is developed, and then successfully applied to fit experimental isotherms above the cac (critical aggregation concentration). By adjusting just one more parameter in the corresponding equation for the heat of adsorption, very good agreement with calorimetric experimental data is achieved. Having determined the adsorption parameters, one can calculate the distribution of the surface aggregates among their corresponding aggregation numbers. The calculated average aggregation number treated as a function of the degree of surface coverage exhibits behavior found in fluorescence decay spectroscopy experiments. The adsorption proceeds both via the increasing number of surface aggregates and via the increasing size of the aggregates. It is concluded that the surface heterogeneity of oxides may be a leading factor in the process of surface aggregation.