INTERFACIAL PROPERTIES OF ZIRCONIUM DIOXIDE IN AQUEOUS-SOLUTIONS OF SODIUM ALKYLBENZENESULFONATES

The interfacial properties of crystalline zirconium dioxide in aqueous solutions of anionic surfactants at an initial pH value of 3.8 have b een studied. Adsorption isotherms, electrophoretic mobilities of the z irconia particles, and the pH of the supernatants were measured for tw o sodium n-alkyl-p-benzenesulfonates, the heptyl compound (SHES) and t he octyl compound (SOBS), and were compared with those for sodium benz enesulfonate (SBS), a molecule containing no alkyl chain. These tradit ional experiments were supplemented by calorimetric measurements of th e enthalpy of dilution and the enthalpy of adsorption. The system sati sfies the condition of charge regulation; the ''head-on'' adsorption o f surfactant ions causes the pH of the equilibrium bulk solution to in crease along the isotherm. The patchwise-like topography of the zircon ia surface brings about a non-uniform distribution of surface charge a nd activates tail-tail attraction even at relatively low surface cover ages. The primary surface aggregates grow and possibly fuse together i n a direction parallel to the surface but for the most part within the surface patches. In consequence, only submonolayer coverages can be a chieved, and at higher bulk concentrations aggregates grow into compac t and fragmented three-dimensional-like structures.