Molecular Origin of Hydration Heat Capacity Changes of Hydrophobic Solutes:  Perturbation of Water Structure around Alkanes

The heat capacities of hydration (ΔCp) of the first four of the linear alkane series, methane, ethane, propane, and butane, were calculated by a combination of Monte Carlo simulations and the random network model (RNM) of water. The contribution from the water−water interaction and the solute−water interaction accounted for about 45% and 20%, respectively, of the experimental values. The water−water contribution to ΔCp arises from distortions in the water structure in the hydration shell. The hydrogen bonds between the water molecules in the first hydration shell were shorter and less bent (had a smaller root-mean-square angle, θ) compared to those in pure water. In the case of propane and butane, the greatest decrease in θ was for waters hydrating the methyl groups, with a somewhat smaller decrease for waters hydrating the methylene groups. Thus the contribution to ΔCp per hydrogen bond from water around the methyl groups is a little larger than from water around the methylene groups. This also implies t...