Monte Carlo study of the adsorption layers of hard spheres near large hard spheres of varying radii

Abstract

Monte Carlo simulation results are presented for the fluid density profiles of the inhomogeneous system formed by a large colloidal hard sphere of variable radius immersed in a hard sphere solvent. The behavior of this system should give insight into the contribution of hard core exclusion effects to the structure of colloids. Such exclusion effects are believed to be the source of the oscillatory short range forces, seen in recent experiments, which are not predicted by classical theories. These profiles are reported as a function of the reduced density for inverse radii between zero and unity (in units of the solvent radius). Periodic boundary conditions and the minimum image condition were used only for the solvent hard spheres. They are not used for the large hard sphere because we wish to examine the case where the large sphere is present in zero concentration. Some error is introduced by our ad hoc treatment of the large sphere. We attempt to minimize this error by taking the number of solvent hard spheres in the unit cell to be as large as is practicable. The reliability of this method is tested by comparison of our results with hard sphere radial distribution functions obtained from conventional bulk simulations by considering the case where the large hard sphere has the same diameter as the solvent hard spheres. We also test our method by comparison of the contact values of the density profiles with interpolated values between the known exact values for the case where the colloidal hard sphere has a diameter equal to that of the solvent hard spheres and the case where the colloidal hard sphere is infinitely large. Both tests indicate that our method gives satisfactory results.