Statistical Thermodynamics of Molecular Association in Model Mixed Fluid on the Basis of the Bond-Number Distribution(Cross-disciplinary physics and related areas of science and technology)

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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 fractions of B through the fluctuation in the local composition. The fluctuation of the BN of the doubly-bonded molecules plays an important role in non-monotonic dependence of the association constant on the composition of fluid mixture.
社団法人日本物理学会の論文
2006-12-15