Calorimetric study of adsorption of non-ionic surfactants on silica gels: Estimating the role of lateral interactions between surface aggregates

A new approach to surfactant adsorption, based on applying the scaled particle theory, is used here for modeling non-ionic surfactant adsorption. In the case of adsorption of ionic surfactants investigated in our previous publications, only the excluded area interactions were taken into account. This is because coulombic interactions between identically charged polar heads hinder close contacts between surface aggregates, thus decreasing the role of possible short-range interactions. This cannot be assumed a priori in the case of non-ionic surfactants. Thus, the present generalized approach also takes into account the possible short-range interactions between the adsorbed surfactant aggregates and accounts for the effects of the surface energetic heterogeneity of a polar substrate. The developed equations are used for a simultaneous quantitative analysis of both adsorption isotherms and the accompanying differential heats of adsorption of three non-ionic surfactants adsorbed on a porous glass surface. The analysis shows that introducing additional terms describing the short-range interactions does not improve the fit of adsorption isotherms and heats of adsorption. This would suggest that also in the case of non-ionic surfactants, the overwhelming contribution to the free energy of adsorption comes from the excluded area interactions, and that the short-range interactions between the surface aggregates play only a negligible role.