SPring-8, the large synchrotron radiation facility

Date & Time: 13:30 - 14:30, January 27, 2005

Speaker: Prof. Alan K. Soper
Affiliation: ISIS Facility, Rutherford Appleton Laboratory

Abstract
　　Conventional wisdom has it that when alcohols such as methanol, ethanol,etc., mix with water, they do so completely, forming what is loosely called an "ideal" mixture. Even more intriguing is the suggestion by Frank and Evans [1] that water which surrounds the hydrophobic headgroups of such molecules in solution is more structured than ordinary water, the so-called "iceberg" model. The restructuring is used to explain several thermodynamic properties of these solutions, such as the negative excess enthalpy of mixing and the negative excess entropy of mixing.

　　Using neutron diffraction techniques which invoke hydrogen isotope substitution methods [2], we investigate a series of alcohol-water solutions, namely methanol, ethanol and n-propanol. The data are interpreted using a computer simulation technique called EPSR (empirical potential structure refinement) which attempts to find 3-dimensional models of the solutions whose structure factors mirror as closely as possible the diffraction measurements [3]. This is achieved via an empirical potential derived from the difference between data and simulation, which perturbs an initial so-called reference potential.

　　Perhaps rather surprisingly we see no evidence for the purported enhanced structure of water in the presence of alcohols - if anything the water structure tends to be disordered by the alcohol [4]. Instead we see a pronounced segregation of the solutions into alcohol-rich and water-rich components at the molecular level. Using a simple model this micro-segregation can be used to estimate the observed excess entropy of mixing with surprisingly good accuracy.

The talk introduces the above techniques and outlines the results obtained.
[1] Frank, H. S.; Evans, M. W., J. Chem. Phys., 1945, 13, 507.
[2] Soper, A. K. and Luzar, A., J Chem Phys 1992, 97, 1320.
[3] Soper, A. K., Chem. Phys. 1996, 202, 295. Soper, A. K., Physical Review B, 2002, 72, 104204
[4] Dixit, S.; Poon, W. C. K.; Crain, J.; Finney, J. L.; Soper A. K. Nature 2002, 416, 829. Dixit, S.; Soper, A. K.; Finney, J. L.; and Crain, J. Europhys. Lett. 2002, 59, 377. Dougan, L.; Bates, S. P.; Hargreaves, R.; Fox, J. P.; Crain, J.; Finney, J. L.; R_at, V.; Soper, A. K. J. Chem. Phys. 2004, 121, 6456. Dougan, L.; Hargreaves, R.; Bates, S. P.; Finney, J. L.; R_at, V.; Soper, A. K.; Crain, J. J. Chem. Phys., 2005, 122, 174514.