| It has been known for a long time from empirical evidence that there are significant qualitative differences between small and large length scale hydrophobic solvation. An understanding of this phenomenon is of particular relevance to the physics of biological processes, which typically involve the interaction with water of species of a wide range of sizes. Atomistic computer simulations, which can feasibly only treat solutes of up to a few nanometers in size, provide relatively limited information about the problem. “Coarse-grained” theories, which characterize the solvent by a reduced set of properties, offer a computationally tractable alternative, while also providing understanding of the physically relevant degrees of freedom governing solvation.; The Lum-Chandler-Weeks (LCW) theory [J. Phys. Chem. B 103, 4570–4577 (1999)] is the first generally applicable theory of this type to study hydrophobic solvation in quantitative detail. In particular, the theory predicts a crossover in the scaling of solvation free energies at a nanometer length scale due to surface-induced “drying” adjacent to large hydrophobic solutes.; In this work, we provide the first systematic computer simulation studies of small and large length scale solvation in fluids close to liquid-vapor coexistence, directed particularly to validating the accuracy of the LCW theory. Both water and a simple model of a monoatomic nonpolar fluid, the Lennard-Jones fluid, are treated. Reasonable agreement between theory and simulation is found. The LCW theory is also used to study the temperature dependence of hydrophobic solvation. This application illustrates how the difference between small and large length scale solvation can by itself explain the experimentally observed variable behavior of entropies of protein folding. Finally, the theory is generalized to treat solutes that interact with the solvent via weak long-ranged attractions in addition to short-ranged repulsions. With a reasonable strength of alkane-water interactions, an accurate prediction of the alkane-water interfacial tension is obtained, while the scaling of solvation free energies is found to be qualitatively similar to that for solutes without attractions. Attractive interactions are shown to be of almost no consequence to the temperature dependence of the solvation free energies relevant to protein folding. |
