Free Energy of Transfer and Its Physical Components

Experimental free energies of solvation span a wide range of values, from positive tens to negative hundreds of kilocalories per mole (for those values where the solution/gas equilibrium constants fall outside the range of about 10 to 10, experimental techniques other 

Equilibrium electrostatic interactions between a solute and a solvent are always nonpositive - tliey are zero if the solute is characterized by no electrical moments (e.g., a noble gas atom) and negative otherwise, i.e., attractive. It is easiest to visualize the electrostatic interactions as developing in a stepwise fashion. Consider a solute A characterized by electrical moments for simplicity, consider only die dipole moment. When A passes from the gas phase into a solvent, the solvent molecules, if diey have permanent moments of their own, reorient so that, averaged over thermal fluctuations, their own dipole moments oppose that of the solute. In an isotropic liquid with solvent molecules undergoing random thermal motion, the average electric field at any point will be zero however, the net orientation induced by the solute changes this, and the lield induced by introduction of the solute is sometimes called the reaction field . 

Of course, the presence of an electric field means dial a term accounting for the interactions of charged particles with this lield should be included in the solute Hamiltonian. When it is included, the effect is to increase the solute polarity in a fashion proportional to the solute polarizability and the strength of the external lield. Thus, die dipole moment of A increases. The solvent, seeing this increase, itself polarizes and moreover increases its own orientation to oppose A s dipole, and so on. 

Another physical effect associated with solvation is cavitation. It is again helpful to visualize the solvation process as a stepwise procedure. Here, we imagine the first step as being creation of a cavity of vacuum within the solvent into which the solute will be inserted as a second step. The energy cost of the cavity creation is the cavitation energy. Note that energy is always required to create the cavity - if it were favorable to create bubbles of vacuum in the liquid, the solvent would not remain in the liquid phase. 

Having enumerated the various processes involved in the transfer of a solute molecule from the gas phase to solution, it must be emphasized that it is not possible to separately measure their contributions to the fundamental observable, AG. One can, of course, attempt to design systems where one expects only a single contribution to dominate, in the hopes of learning more about the nature of that contribution from experimental measurements, but inferences drawn therefrom become less certain as they are applied to systems less like those originally 

---