MODEL-CALCULATIONS OF CHANGES OF THERMODYNAMIC VARIABLES FOR THE TRANSFER OF NONPOLAR SOLUTES FROM WATER TO WATER-D2

A recently introduced modified hydration shell hydrogen bond model for rationalizing the thermodynamic consequences of hydrophobic hydration is adapted for use with heavy water. The required adjustment of parameters employs the assumption that breaking hydrogen bonds in water-d2 involves a greater enthalpy change and a larger entropy increase than bond breaking in ordinary water. It also makes some use of information derived from studies of gas solubilities in the two solvents, although a review of the data leads to serious questions about the reliability of results obtained in this way. The model permits calculations of hydrogen bonding contributions to the changes, DELTA-G(t)-degrees, DELTA-H(t)-degrees, DELTA-S(t)-degrees, and DELTA-C(p,t,)-degrees for transfer of nonpolar solutes from water to water-d2 and implies that such data should show regular trends. Although some of the numerical results depend strongly on the values chosen for the parameters, the pattern defined by these trends is nearly independent of parameters. Predicted values of DELTA-C(p,t)-degrees are large and positive for all nonpolar solutes, while DELTA-S(t)-degrees is expected to be negative near O-degrees-C, becoming progressively less negative on warming and eventually positive. Both of these quantities should be proportional to the molecular surface area of the solute. Analogous predictions regarding DELTA-G(t)-degrees and DELTA-H(t)-degrees can also be made, but only if it is permissible to neglect possible contributions to these quantities from van der Waals interactions.