NMR AND NEUTRON-SCATTERING INVESTIGATION OF UNDERCOOLED AQUEOUS-SOLUTIONS OF APOLAR SOLUTES

Multinuclear spin-lattice relaxation rates and self-diffusion coeffici ent measurements are reported over wide ranges of temperature, pressur e, and concentration in undercooled aqueous solutions of tetramethylam monium bromide. These dissolved organic cations with apolar surface gr oups provide model systems to investigate the effect of Coulombic, hyd rophobic, and H-bond interactions upon the solvent and solute dynamics within the random, transient H-bond network of the water. Also report ed for the first time are neutron diffraction difference experiments i n these undercooled solutions to investigate how changes in dynamic di sorder are related to changes in the average static structure of the h ydration water as temperature is decreased. A substantial sharpening o f the orientational order between water molecules is observed in the u ndercooled state.