Interfaces of polydisperse fluids : surface tension and adsorption properties

We consider a system of spherical colloidal particles with a size polydispersity and use a simple van der Waals description in order to study the combined effect of both the polydispersity and the spatial non-uniformity induced by a planar interface between a low-density fluid phase (enriched in small particles) and a high-density fluid phase (enriched in large particles). We find a strong adsorption of small particles at the interface, the latter being broadened with respect to the monodisperse case. We also find that the surface tension of the polydisperse system results from a competition between the tendancy of the polydispersity to lower the surface tension and its tendancy to raise the critical-point temperature (i.e. its tendancy to favor phase separation) with the former tendancy winning at low temperatures and the latter at the higher temperatures.