Liquid-vapor interface of a polydisperse fluid

We report a grand canonical Monte Carlo simulation study of the liquid-vapor interface of a model fluid exhibiting polydispersity in terms of the particle size sigma. The bulk density distribution, rho(0)(sigma), of the system is controlled by the imposed chemical potential distribution mu(sigma), the form of which is specified such that rho(0)(sigma) assumes a Schulz form with associated degree of polydispersity approximate to 14%. By introducing a smooth attractive wall, a planar liquid-vapor interface is formed for bulk state points within the region of liquid-vapor coexistence. Owing to fractionation, the pure liquid phase is enriched in large particles, with respect to the coexisting vapor. We investigate how the spatial variation of the density near the liquid-vapor interface affects the evolution of the local distribution of particle sizes between the limiting pure phase forms. We find [as previously predicted by density-functional theory, Bellier-Castella , Phys. Rev. E 65, 021503 (2002)] a segregation of smaller particles to the interface. The magnitude of this effect as a function of sigma is quantified via measurements of the relative adsorption. Additionally, we consider the utility of various estimators for the interfacial width and highlight the difficulty of isolating the intrinsic contribution of polydispersity to this width.