The Percus-Yevick approximation for repulsive hard spheres with surface adhesion

We have adapted Baxter's sticky hard sphere (SHS) model so that it more adequately describes systems where the interparticle potential well is preceded by an energy barrier. In the original model the particles interact via a pair potential with a narrow attractive region next to a repulsive core. Colloidal particles, however, often exhibit long range repulsion whereas they do attract each other at shorter ranges. This motivated our effort to extend the model. It is shown that the Percus-Yevick equation can also be solved analytically for this particular potential, provided that the regions of attraction and repulsion are sufficiently small. It is found that the structure functions of both models are identical. The new feature of the repulsive sticky hard sphere (RSHS) model is that the fraction of aggregated particles may increase with temperature, a phenomenon which is experimentally observed in such systems. Structure functions from small angle X-ray studies on water/AOT/iso-octane microemulsions can be fit to those predicted by the RSHS model. The thus obtained binding enthalpy is comparable with earlier determinations from dielectric studies.