Potentials Of Average Force For An Interaction Site Model Of Aqueous Alcohols: A Molecular Model For The Hydrophobic Bond

An interaction site model with simple square-well potentials of average force is shown to accurately fit experimental second osmotic virial coefficients, B[AB], for a number of aqueous alcohol systems. The model takes into account the nonspherical structure of the alcohol molecules, includes all important orientational effects, and considers all interactions of the sites. One site (C) is centered on the carbon atom and represents the C, CH, CH2, and CH3 groups while the other (O) is on the oxygen and represents the OH group. None of the models considered fits the data if pairwise additivity for the C-C site interactions is assumed. The nonpairwise additivity algorithm that fits the data is of the type expected if the attractive part of the C-C interaction is due mainly to solvent displacement from the solute cospheres (Gurney potential). The CC interaction is represented by a narrow, contact square well, and all models that include a solvent-separated well, either alone or in conjunction with a contact well, give very poor fits to the data. The C-0 interaction is a hard sphere plus a somewhat broader repulsive square mound. The 0-0 interaction includes a single, broad and shallow, square well that accommodates both solvent-separated and contact interactions. Both the computations and experimental data show quite clearly that the effect of the water solvent is to weaken the attractive C-C interaction relative to the gas-phase (vacuum) interaction. Osmotic second virial coefficients for the individual site-site interactions are calculated and shown to be quite independent of small changes in the widths of the potential wells. With the best fit for the C-C site potential, a number of BAB values for hydrocarbons in water are predicted