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Molecular segregation observed in a concentrated alcohol-water solution

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Original languageEnglish
Pages (from-to)829-832
Number of pages5
JournalNature
Volume416
Issue number6883
DOIs
Publication statusPublished - 25 Apr 2002

Abstract

When a simple alcohol such as methanol or ethanol is mixed with water(1,2), the entropy of the system increases far less than expected for an ideal solution of randomly mixed molecules(3). This well-known effect has been attributed to hydrophobic headgroups creating ice-like or clathrate-like structures in the surrounding water(4), although experimental support for this hypothesis is scarce(5-7). In fact, an increasing amount of experimental and theoretical work suggests that the hydrophobic headgroups of alcohol molecules in aqueous solution cluster together(2,8-10). However, a consistent description of the details of this self-association is lacking(11-13). Here we use neutron diffraction with isotope substitution to probe the molecular-scale structure of a concentrated alcohol-water mixture (7:3 molar ratio). Our data indicate that most of the water molecules exist as small hydrogen-bonded strings and clusters in a 'fluid' of close-packed methyl groups, with water clusters bridging neighbouring methanol hydroxyl groups through hydrogen bonding. This behaviour suggests that the anomalous thermodynamics of water-alcohol systems arises from incomplete mixing at the molecular level and from retention of remnants of the three-dimensional hydrogen-bonded network structure of bulk water.

    Research areas

  • FREQUENCY RAMAN-SPECTROSCOPY, AQUEOUS-SOLUTIONS, METHANOL MIXTURES, TERTIARY BUTANOL, NEUTRON-DIFFRACTION, SELF-ASSOCIATION, SIMULATION, HYDRATION, SOLVENT, ICE

ID: 1490581