Hydrophobic and hydrophilic associations of a methanol pair in aqueous solution

The association dynamics of a methanol pair in aqueous solution were theoretically studied with QM/EFP-MD and quantum mechanical methods. Stable contact pairs and solvent separated configurations (SS) were found from simulations with a free energy barrier of 2 kcal/mol, revealing the strong tendency of methanol association. The stable contact pairs were further identified as the hydrophobic (CP_A) and hydrophilic (CP_B) species, with the CP_A having a larger population. Although the free energy difference between the CP_A and CP_B is negligible with virtually no associated free energy barrier, the slow isomerization dynamics of intermolecular rotations ensures their individual identity. Further mechanistic analysis revealed that only the CP_A has a direct path to the SS, showing that hydrophobic attraction initiates the association process. A subsequent intermolecular hydrophilic attraction isomerizes CP_A and CP_B. Therefore, our results show that both the hydrophobic and hydrophilic attractions between methanol molecules play important roles in the association dynamics. The former operates on the longer intermolecular distance, while the latter is effective in contact pairs.