Statistical Thermodynamics of Molecular Association in Model Mixed Fluid on the Basis of the Bond-Number Distribution

Abstract

Solvent effects on molecular association in mixed fluids are studied with a simple model system, in which a pair of spherical molecules (solute) X are associated in a mixed fluid composed of spherical molecules A and B. The solvent molecules A and B behave as identical hard spheres except for the interaction with the solute molecule X; B interacts through the square-well potential with X, while A acts as the hard sphere. The solvent effect on the equilibrium constant is expressed by the statistics of the number of interactions (bond number BN) of the molecular associate X 2 with B molecules. The statistics of the BN distribution is expressed by a bivariate distribution function with respect to the singly-bonded and the doubly-bonded solvent molecule B. Rigorous relations are derived for the BN distribution in the mixture and in the single-component fluid composed of B. The BN distributions are evaluated for the hard-sphere fluid using Monte Carlo methods. The molecular association is promoted at low mole ...