Direct measurement by neutron diffraction of the solvation of polar and apolar molecules. The hydration of the tetramethylammonium ion

Using the isotope-substitution neutron diffraction first difference method, we have studied the cation hydration in three concentrations of tetramethyl-ammonium chloride in D2O. At low (0.50 mol kg–1) concentration, the results are consistent with a defective and disordered cage model, as would perhaps be expected for higher tetra-alkylammonium ions. As concentration is increased to 1.00 mol kg–1, where measurements in an H2O/D2O mixture give information on water orientations, there appears to be a qualitative change: the behaviour of the TMA appears to be more like that of a strong cation. Chloride substitution confirms that the anion hydration is normal, and we explain this intermediate concentration behaviour in terms of the effects of the anion rather than the TMA. At high concentrations a further change occurs and the hydration structure seems to return to a defective and disordered cage, presumably with the anion incorporated into it. These results appear to indicate subtle changes in hydration behaviour which are thought to be of chemical and biological importance.