HIGH-PRESSURE NMR RELAXATION STUDY OF THE SOLUTE AND SOLVENT DYNAMICSOF UNDERCOOLED AQUEOUS TETRAETHYLAMMONIUM BROMIDE SOLUTIONS

Multinuclear spin-lattice relaxation rate measurements are reported ov er wide ranges of temperature, pressure, and concentration in undercoo led aqueous solutions of tetraethylammonium bromide (Et(4)NBr). These dissolved organic cations with apolar surface groups provide model sys tems to investigate their influence upon the dynamics of the water mol ecules in the hydration shell as well as the influence of the hydratio n shell upon the dynamics of the dissolved hydrophobic ions. The isoba ric relaxation time curves of all nuclei investigated show pronounced minima, indicating a strong slowing down of the dynamics of the solven t molecules in the hydration shell as well as of the dissolved hydroph obic ions. A VTF-like temperature dependence and corresponding scaling behavior allow glass transition temperatures to be deduced which sign ify the onset of motional arrest in these metastable solutions. Segmen tal motions of the aliphatic chains of the organic cations are discuss ed in terms of correlation times, and corresponding order parameters a re given characterizing their spatial restrictions. At low temperature s all proton relaxation rates become equal to those of the methyl grou ps due to strong cross-relaxation effects, indicating that methyl grou ps act as relaxation sinks in these systems.