On the heat-capacity change of pairwise hydrophobic interactions

Computer simulations [S. Shimizu and H. S. Chan, J. Am. Chem. Soc. 123, 2083 (2001); D. Paschek, J. Chem. Phys. 120, 10605 (2004)] have demonstrated that the heat-capacity change associated with the interaction of two nonpolar spherical particles, at room temperature, shows a complex behavior with a significant maximum at the distance corresponding to the desolvation barrier configuration and a small minimum at the distance corresponding to the contact configuration. Taking advantage of the detailed analysis performed by Paschek, the two-state model of Muller is applied to estimate the energetic strength and the intactness of the H bonds in the hydration shell of a xenon atom and in the concave part of the joint Xe-Xe hydration shell. In both hydration shell regions the H bonds are energetically stronger but more broken than those in bulk water. In addition, those in the concave part of the joint Xe-Xe hydration shell are, in absolute, stronger and more broken. These thermodynamic features coupled to simple geometric arguments allow the calculation of heat-capacity values that are in agreement with those provided by computer simulations for the pairwise Xe-Xe interaction.