Adsorption of cationic surfactants on hydrophilic silica: effects of surface energetic heterogeneity

The thermodynamics of adsorption of cationic surfactants on a hydrophilic silica surface is discussed. The adsorption isotherms of these surfactants on negatively charged silica surfaces exhibit a two-step character, as a rule. The heights of these steps and the concentration at which the second step begins depend strongly on the surface charge determined by the pH and indifferent electrolyte concentration in the equilibrium bulk solution. In our previous publication we have developed a theoretical description based on the model of the adsorbed phase being a mixture of monolayered and bilayered aggregates of different sizes. Only the "excluded area" interactions were taken into account. The potential energy per molecule was assumed to decrease linearly with the aggregate size. The theoretical expressions obtained for adsorption isotherms and heats of adsorption were next fitted to experimental data of cationic surfactants adsorption on precipitated silica. Although our model was able to reproduce well the two steps observed on the adsorption isotherms of these surfactants, it failed to predict the behavior of isosteric heats of adsorption at low surface coverages. In this paper our model is extended by taking into account the effects of surface heterogeneity on single monomer adsorption. This leads to a much better description of entropic effects accompanying surfactant adsorption in the low coverage region. It is therefore concluded that individual adsorption of surfactant ions, which is determined primarily by the specific solid-surfactant interactions, is strongly affected by the surface heterogeneity effects. (C) 1998 Elsevier Science B.V. All rights reserved.