Remarks on the hydration entropy of polar and nonpolar species

Around room temperature, experimental hydration entropy values are large negative, regardless of the polar or nonpolar nature of the solute. They cannot be rationalized by claiming that an enhancement of water tetrahedrality occurs upon solute insertion, because neutron diffraction measurements and computer simulations failed to detect such a structural enhancement in the hydration shell of several solutes. The negative hydration entropy values and their trend versus the solute molecular volume can be rationalized by recognizing that solute insertion causes a reduction in the number of configurations accessible to water molecules for simple geometric reasons. This entropy reduction is due to the solvent-excluded volume effect upon cavity creation, and can be estimated by means of analytic relationships provided by classic scaled particle theory. The agreement between the calculated estimates of the entropy change upon cavity creation and the experimental hydration entropy values is good. This result is discussed in depth.