CHAPTER 10: Solvation Gibbs Energy: The Equation of State Approach

The solvation Gibbs energy is a key thermodynamic quantity for understanding physicochemical phenomena and for designing physicochemical processes. It is a valuable quantitative measure of the solute–solvent affinity, of crucial importance in separation/partitioning processes. It is also the bridging quantity between the formation free energies of the solute in the (ideal) gas state and in solution. These features make the solvation Gibbs energy particularly important in numerous fields and applications, including life processes and metabolism under both ambient and remote external conditions. Systematic study of the solvation/hydration Gibbs energy over a broad range of external pressure and temperature conditions calls for an equation of state approach. This chapter focuses on two representative routes of the equation of state approach to solvation. In the first, the predictive UMR–PRU (Universal Mixing Rule – Peng–Robinson UNIFAC) cubic equation of state is explored for the estimation of solvation Gibbs energy over an extensive range of external conditions. Its predictions compare favorably with available experimental data from a recent large database with a mean absolute deviation of ca. 0.4 kcal mol⁻¹ for all binary data (here 1 cal = 4.187 J). In the second, a versatile statistical thermodynamic model is explored that permits, in addition, the study of key components of solvation Gibbs energy such as components from cavitation, charging and solute conformations/solvent restructuring contributions. These latter components shed light on the mechanism of solvation and contribute to our understanding of solvation phenomena. The challenges and perspectives of the equation of state approach to solvation are critically discussed.