Water structure and solute association in dilute aqueous methanol

¹ Department of Physics and Astronomy, The University of Edinburgh Mayfield Road, EH9 3JZ, Edinburgh, UK
² ISIS Facility, Rutherford Appleton Laboratory Chilton, Didcot, Oxon, OX11 OQX, UK
³ Department of Physics and Astronomy, University College London Gower Street, WC1E 6BE, London, UK

Received: 18 February 2002
Accepted: 2 May 2002

Abstract

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.