A general expression of the canonical partition function for mixture fluids in terms of solvation free energy is presented. Following the same approach as set forth in generalized van der Waals theory, we show that the physical assumptions made in existing thermodynamic models from the perspective of molecular solvation can be prevailed. For example, the temperature dependence on the coordination number, i.e. the number of solvent molecules surrounding the solute, has an impact not only on the temperature dependence of the solvation free energy but on the non-linear solvent reaction field response as well. More importantly, the new formulation provides a unified scheme for deriving the commonly used equations of state (EOS) and newly developed COSMO-type liquid activity coefficient models. We show that from this new formulation it is possible to develop a new class of thermodynamic models that behave like existing EOS and liquid models in the low and high-density limits, respectively.