Theory of Liquids and Liquid Mixtures, their Structures and Phase Equilibria

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Benjamin Widom of Cornell University is supported by an award from the Chemical Theory, Models and Computational Methods program to study solutions, in water, of substances that are hardly soluble. The expression "oil and water don't mix" is almost but not entirely true, and the slight solubility of oils and other substances in water is often of crucial importance. It affects the efficiency of separations by distillation and extraction in the chemical process industries; it is a primary factor in drug design in the pharmaceutical industry; it is a concern in water pollution and other environmental problems; and it underlies the most fundamental biological phenomena. It is widely accepted, for example, that it is the relative incompatibility of the oil-like constituents of a protein and the surrounding water that is responsible for the protein's folding into its proper biologically active form. Low solubility in water is an example of what are known collectively as "hydrophobic" (literally water-hating) effects. These effects are the objects of the present theoretical and computational research, which is directed toward the understanding of their molecular origins and consequences: What makes a substance hydrophobic? In what other ways does hydrophobicity manifest itself in addition to low solubility? How does the structure and other properties of the surrounding water itself respond to the presence of dissolved hydrophobic molecules and what are the physical and chemical consequences of such molecular-scale rearrangements?Much of this research is directed to understanding the solvent-induced attraction between hydrophobic solutes, particularly that measure of the attraction contained in the second osmotic virial coefficient. It is desirable to reconcile the values of this coefficient as found from equations of state and thermodynamic measurements with the integral of the solute-solute pair correlation function, as required by one of the Kirkwood-Buff relations. In recent work it has been shown how the virial coefficient may be extracted from an equation of state with parameters set to give realistic properties of the pure solvent and a realistic account of those effects of the solute-solvent interaction such as on the solubility of the solute and on its partial molecular volume in the solvent. The resulting osmotic coefficient agrees well with that extracted from experimental solubility measurements. Independently, the several pair correlation functions in the two-component mixture are obtained by computer simulation. These, on being integrated, are consistent with the solvent compressibility, the solute partial molecular volume, and the gas-phase virial coefficients, yet inconsistent with the previously obtained second osmotic virial coefficient. This is a serious discrepancy, which is important to resolve, for a proper understanding of structural effects in hydrophobic hydration, and hydrophobic effects more generally, depends on it. There still remains the problem of constructing analytical forms for the several pair correlation functions to test earlier analytical indications that the amplitude of the solute-solute correlation function, which is difficult to measure because of the low solubility, greatly exceeds that of the other two so that the solutes remain effectively correlated to longer distances than do the solvent molecules despite the two having the same formal exponential decay length. A remaining project is that of determining, by computer simulation and analytically, the solute-solute pair correlation function on the solute-poor side of a closed-loop coexistence curve, to trace the evolution from entropy-based to enthalpy-based solvophobicity as the temperature varies between that of the two critical solution points.