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 pola r spheres. Polar spheres were assigned charges of +/-0.5e and +/-1e. I t is found that above the radius of similar to 3 Angstrom hydration en tropies of nonpolar and polar spheres of similar size are identical wi thin 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 calculati ons are performed at constant pressure, the hydration entropy per Angs trom(2) Of solute's accessible area does not exhibit a size dependence and stays around 30 cal/Angstrom(2). In calculations at constant volu me, 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 dependen ce of the hydrophobic transfer energies may be closely linked to volum e effects which can be absent or small in hydrophobic interactions.