Solution systems model hydrophobic effect

Here we present and discuss an example calculation to make some of the concepts discussed above more definite. We treat a model for methane (CH4) solute at infinite dilution in liquid under conventional conditions. This model would be of interest to conceptual issues of hydrophobic effects, and general hydration effects in molecular biosciences [1,9], but the specific calculation here serves only as an illustration of these methods. An important element of this method is that nothing depends restric-tively on the representation of the mechanical potential energy function. In contrast, the problem of methane dissolved in liquid water would typically be treated from the perspective of the van der Waals model of liquids, adopting a reference system characterized by the pairwise-additive repulsive forces between the methane and water molecules, and then correcting for methane-water molecule attractive interactions. In the present circumstance this should be satisfactory in fact. Nevertheless, the question frequently arises whether the attractive interactions substantially affect the statistical problems [60-62], and the present methods avoid such a limitation. 

What is the likely future use of MC and MD techniques for studying interfacial systems Several promising approaches are possible. Continued investigation of double layer properties, "hydration forces", "hydrophobic effects", and "structured water" are clearly awaiting the development of improved models for water-water, solute-water, surface-water, and surface-solute potentials. 

It should be taken into account, however, that the approximation used is extremely rough. Firstly, the medium of a nonpolar organic solvent appears to be very inadequate model of the hydrophobic nonaqueous compartments (phases) in biological systems. Secondly, organic solvents employed for the commonly used partitioning systems are far from being inert to the solutes being partitioned. The effect of the solute-solvent interactions on the partition coefficient of the solute cannot be usually quantified. 

Let us note here some of the basic principles underlying models, theories, and calculations of hydrophobic effects. The quantity of first interest is the free energy associated with the interaction of the solute with the aqueous environment. This is the chemical potential or partial molar Gibbs free energy of the solute. This quantity provides a driving force for rearranging molecules in thermodynamic systems and many quantities of interest are fundamentally connected to this free energy. Consider first an atomic solute of type A. For example, perhaps A = Ar. The chemical potential at extreme dilution may be expressed as... 

In view of the different behavior of n-Bu4NBr in mixtures of DMF and NMF and of DMF and water, we recently (6) derived an equation for the excess enthalpy of solution in the DMF-water mixture (AHE(M)) by use of a simple hydrophobic hydration model. Summarizing this derivation, we conceived the enthalpies of solution in the DMF-H20 system (AH°(M)) as being the result of two effects (a) When the hydrophobic hydration of tetraalkylammonium ions is absent, the corresponding enthalpy of solution in pure water AH K O) and in the mixture AHJ(M) should be correlated by ... 

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