HYDROPHOBIC INTERACTIONS - CONFORMATIONAL EQUILIBRIA AND THE ASSOCIATION OF NONPOLAR MOLECULES IN WATER

Recently developed proximity approximations have been used to calculat e inhomogeneous water density profiles around non-polar molecular solu tes. Relative Helmholtz energies of hydrophobic hydration are calculat ed from these density profiles using two inherently different approach es: Helmholtz energy perturbation and a multiparticle correlation func tion expansion. Entropic contributions to the hydration Helmholtz ener gy are also calculated using the multiparticle correlation function ex pansion for the entropy truncated at the level of pair correlations. W e show that the proximity approximations describe water structure arou nd a tetramethylammonium ion in good agreement with neutron diffractio n experiments, and provide an accurate description of water structure around simple alkanes and benzene as reflected in their entropies of h ydration. Further, we reproduce two important features of hydrophobic interactions: a highly favoured contact minimum and a solvent separate d minimum in the PMFs for methane-methane and neopentane-neopentane as sociation in water. Our calculations also show that the more compact c onformations of n-butane and n-pentane are favoured in water, as expec ted based on traditional ideas regarding hydrophobic interactions.