Monte Carlo simulations of alkanes in water: Hydration numbers and the hydrophobic effect

Monte Carlo statistical mechanics simulations have been used to calculate the number of water molecules in the first hydration layer around a series of alkanes in aqueous solution. This key parameter in theories of aqueous solutions is found to have an excellent linear correlation with entropies of solution. Consequently, strong support is provided for the traditional idea that the ordering of water molecules in the first layer around a hydrocarbon is primarily responsible for the large entropy losses associated with the hydrophobic effect. However, the energetic results do not support the necessity of invoking water structure making to explain the substantial exothermic heats of hydrations for alkanes. Insights are also obtained concerning hydrophobic effects on conformational equilibria and the relative solubilities of branched and straight-chain isomers.