RISM study of the structure of sulphur dioxide at a plane wall

The reference interaction site model (RISM) was used to study the distribution of SO₂ molecules at a soft plane wall. The RISM formalism was adapted for the treatment of a mixture of SO₂ molecules as solvent, and spherical Lennard-Jones particles as solute. The latter were treated in the regime of infinite dilution, which has as a consequence neglect of the solute-solute distribution and the decomposition of the system of equations to a form in which one can independently solve the structure of the solvent and afterwards the solute-solvent correlations. Increasing the diameter of the solute and extrapolating the results towards an infinite radius, we obtained the density profiles of sulphur and oxygen atoms at the plane wall. The calculations were performed for two strengths of the wall-SO₂ interactions. The results show that the distribution of SO₂ molecules displays oscillatory behaviour. The oscillations are well expressed at the wall which attracts the SO₂ molecules with a strength which exceeds the thermal energy by less than one order of magnitude. In the case when the wall-SO₂ potential exhibits only a repulsive interaction, the density profiles are nearly monotonous. The angular distribution shows a strong orientational anisotropy in the immediate vicinity of the wall, but it decays very fast when going towards the interior of the bulk phase.