The thermodynamics of solutions
This paper explores the relationship between thermodynamics and two other types of investigation. Structure determinations by neutron diffraction and computer simulation give a clear picture of first-shell solvation for a number of cations in water. Replacement of a water molecule in the complex M(H20): by an organic ligand S in a mixed aqueous solvegt S-H20 may result in distortion which raises the free energy of transfer(AtG ) of the cation M+ from water above that expected from an unhindered base-line. The sequence of these deviations is that predictable from the known geometries, i.2. Li+>Na'>Cs+>Ag+, H' . relative viscosities of solutions are examined by Transition-State Tneory. Electrolytes like CsCl lower the free energy, and markedly, the enthalpy and entropy of activation for viscous flow of water, but do not necessarily break down solvent structure as in the classical view. Enchanced co-ordination of solvent to the ions could occur in a transition-state solvent more weakly structured and bonded than the ground-state solvent. Enthalpies of transfer of the "hydrophobic" solute E-butanol in methanoluater mixtures suggest that TBA makes strong solute-solvent bonds, but breaks solvent-solvent bonds. The large positive activation parameters for viscous flow in highly aqueous mixtures suggest that water encages this type of solute in the ground state, thereby enhancing the solute-solvent interaction. The INTRODUCTION Our basic thermodynamic process is the transfer of a solute between standard states in two different solvents; it will be accompanied by changes in the free energy,&G enthalpy, Aty , and entropy, AtS , of the system, which reflect differences in the solvation of the solute in the two solvents (ref. I). Most of our solutes will be electrolytes. Most of the transfers will involve binary aqueous mixtures. We shall first show how recent structural studies (refs 2 & 3) help us to understand steric influences on the free energies of transfer, AtG , of some simple electrolytes from water to mixed aqueous solvents. We shall then discuss briefly a simple theoretical model (ref. 4 ) for the enthalpy of transfer, AtH*, in binary solvent systems. Finally, we shall consider viscous flow (ref. 5). This process played a crucial part in the development of the established models (refs. 6 & 7 ) for ions in solution. It can be treated quasi-thermodynamically, by Transition-State theory (ref. 8). Our transfer quantities now involve something we call the transitionstate solvent. This access to an unusual type of solvent helps us to a clearer understanding of the solvation process. 8 8 6