On the Correlation between Pair Hydrophobicity and Mixing Enthalpies in Water-Alcohol Binary Mixtures

In this article, we have explored the extent of pair hydrophobicity in water-alcohol binary mixtures upon varying the chain length of the alcohol at several compositions. We have measured the pair hydrophobicity in water-methanol, water-propanol, and water-butanol mixtures. The pair hydrophobicity is measured by the depth of the first minimum (contact minimum) in the potential of mean force profile between a pair of neopentanes. In the case of water-methanol mixtures, the pair hydrophobicity is highest at x_MeOH = 0.25, whereas in water-propanol mixtures, it is highest at x_PrOH = 0.07, and in water-butanol mixture pair, hydrophobicity is highest at even lower alcohol concentration (x_BuOH = 0.03). This indicates that as we increase the chain length of alcohol, the composition at which pair hydrophobicity is highest shifts toward a lower alcohol composition of the binary mixture. We have shown that the composition dependence of pair hydrophobicity echoes the trend observed in the calculated composition dependence of the enthalpy of mixing of the alcohol-water binary mixtures. The association pattern of the hydrophobic part of the alcohol also shows a change in trend around similar alcohol compositions. The hydrogen bond pattern around the alcohol rather than water exhibits a change in trend around those compositions. These results will improve our understanding of the composition-dependent phenomena of biomolecular processes in aqueous binary mixtures.