MAGNITUDE OF HYDRATION ENTROPIES OF NONPOLAR AND POLAR-MOLECULES

A mean field model based on continuum representation of bulk water is applied to the calculation of hydration entropies of nonpolar and polar spheres. Polar spheres were assigned charges of +/-0.5e and +/-1e. It is found that above the radius of similar to 3 Angstrom hydration entropies of nonpolar and polar spheres of similar size are identical within the accuracy of our computations. Hydration entropies are similar for nonpolar and half-charged spheres of all radii. It is found that the mean field model exhibits a gaslike behavior. If entropy calculations are performed at constant pressure, the hydration entropy per Angstrom(2) Of solute's accessible area does not exhibit a size dependence and stays around 30 cal/Angstrom(2). In calculations at constant volume, corresponding compression entropy terms are added to the constant-pressure results, leading to a strong size dependence. A comparison of the results to scaled particle theory suggests that the size dependence of the hydrophobic transfer energies may be closely linked to volume effects which can be absent or small in hydrophobic interactions.