An extensive series of neutron diffraction experiments combined with Empirical Structural Refinement Analysis (EPSR) has shown that mixtures of methanol and water exhibit extended structures in solution despite the components being fully miscible in all proportions. Of particular interest is a concentration region (methanol model fraction between 0.27 and 0.54) where methanol and water appear to form separate, percolating networks. This is the concentration range where many transport properties and thermodynamic excess functions reach extremal values. The observed concentration dependence of several of these material properties of the solution may therefore have a structural origin. Molecular segregation in methanol-water mixtures is studied across a wide concentration range as a function of temperature and pressure. Cluster distributions obtained from both neutron diffraction point to significantly enhanced segregation as the mixtures are cooled or compressed. The observed behaviour is consistent with an approach to an upper critical solution point. Such a point would appear to be “hidden” below the freezing line, thereby precluding observation of the two-fluid region. Finally, it is shown that the negative excess entropy of mixing characteristic of aqueous lower alcohols can be understood quantitatively in terms of molecular-scale segregation of the components. A simple model is presented and used to understand the behaviour of methanol-water solutions under extreme conditions using molecular clustering data from neutron diffraction and obviates the need to invoke other restructuring concepts which, though well-known, are unsupported by recent experiments.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:649666 |
| Date | January 2005 |
| Creators | Dougan, Lorna |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/13698 |
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