Statistical thermodynamics of solute molecule in model mixed fluid

The solvent effect in a mixed solvent on the thermodynamical properties of a Solute molecule is studied with a simplified model system. The solvent molecules in the model system are distinguished only by the interaction with the solute molecule, which is assumed as a square-well interaction. The exact relationship between the thermodynamical quantities in the single-component solvent and those in the mixed solvent are derived through the bond-number (BN) distribution of the solute molecule; the bond number is the number of interacting molecules in the potential well of the solute molecule. The BN distributions are evaluated for the hard-sphere fluid using the Monte Carlo method over a wide density region. The mean field approximation does not work well at low mole fraction of the interacting component in the mixed solvent clue to the fluctuation of the local composition. The effect of the fluctuation of the local composition was reproduced by the modified independent-site model under the mean field approximation, in which the fluctuation of the composition is assumed to take place independently at hypothetical sites around a solute molecule.