An extensive series of neutron diffraction experiments and molecular dynamics simulations 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 mole fraction between 0.27 and 0.54) where both 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.

Hydrogen/deuterium isotopic substitution neutron diffraction techniques have been used to measure the solute−solute intermolecular structural correlations in 0.06, 0.11, and 0.16 mole fraction tertiary butanol−water solutions. Empirical potential structure refinement (EPSR) procedures have been used to extract detailed information relating to the intermolecular structure in these systems. A trend from hydrophobic to hydrophilic character of the solute−solute correlations as a function of solute concentration is observed. Of particular note is the domination of nonpolar to nonpolar solute contacts at 0.06 mole fraction concentration compared with a more complex mixture of nonpolar and polar solute−solute intermolecular contact configurations at the higher alcohol concentration.

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Neutron diffraction involving seven isotope substitution experiments is used to extract the methyl hydrogen to the water hydrogen partial structure factor in a 2.0 m aqueous solution of tetramethylammonium chloride, in order to investigate the orientation of water molecules around the TMA ion. The correlation function of the TMA centers with water molecule centers is obtained and shows that the average distance is about 4.7 Å, in agreement with the result found previously using a nitrogen isotope substitution. The technique of spherical harmonic reconstruction is used to obtain orientational correlation functions between TMA ions and water molecules. The results show that the TMA ion has a predominantly ‘‘apolar’’ character in water but that a range of water molecule orientations around the ion are possible.

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