Cooperative hydrophobic/hydrophilic interactions in the hydration of dimethyl ether

摘要

Cooperative interactions in the hydration of dimethyl ether DME relative to its purely hydrophobicanalog, propane, are analyzed by expressing the free energy of hydration in terms of an “inner-shell”contribution from water molecular packing and chemical association, and an “outer-shell”contribution described by the mean binding energy of the solute to the solution and fluctuations inthis binding energy. We find that nonadditive, cooperative interactions associated with strongcorrelations in the binding energy fluctuations of the methyl groups and ether oxygen play adominant role in the hydration of DME relative to propane. The electrostatic nature of theseinteractions is revealed in a multi-Gaussian analysis of hydration substates, which shows that theformation of favorable ether oxygen-water hydrogen bonds is correlated with less favorable methylgroup-water interactions, and vice versa. We conclude that the group additive distinction betweenthe hydrophobic hydration of the DME methyl groups and hydrophilic hydration of the ether oxygenis lost in the context of these cooperative interactions. Our results also suggest that the bindingenergy fluctuations of constituent hydrophobic/hydrophilic groups are more sensitive than localwater density fluctuations for characterizing the hydration of heterogeneous interfaces.