Water structure and solute association in dilute aqueous methanol

The molecular structure of a dilute aqueous methanol solution ( 1 : 19 molar ratio) is studied using neutron diffraction with hydrogen/deuterium isotope substitution. The Empirical Potential Structure Refinement (EPSR) method is used to create a structural model of the solution which is consistent with the measured diffraction data. Direct evidence of methanol association is found with more than 80% of the methanol molecules in clusters of 3 to 8 molecules. Contrary to several previous studies, and conventional views on the hydrophobic interaction, no enhancement of the water structure surrounding the non-polar groups is observed. Consistent with results from other related systems, a compression of the second-neighbour water-water contact distance is observed that may be a structural feature of the hydrophobic driving force.