J. zyxwvutsrq Phys. Chem. zyxwvu 1994,98, zyxwvut 8161-8168 8161 NMR and Neutron Scattering Investigation of Undercooled Aqueous Solutions of Apolar Solutes S. Bradl and E. W. Lang' Institut fur Biophysik und Physikalische Biochemie, Universitht Regensburg, P.O. Box 101042,93040 Regensburg, FRG J. Z. Turner Crystallography Department, Birkbeck College, London WCl E 7HX, U.K. A. K. Soper ISIS Science Division, Rutherford Appleton Laboratory, Oxon OX1 1 OQX, U.K, Received: December 28,1993; In Final Form: April 25, 1994' Multinuclear spin-lattice relaxation rates and self-diffusion coefficient measurements are reported over wide ranges of temperature, pressure, and concentration in undercooled aqueous solutions of tetramethylammonium bromide. These dissolved organic cations with apolar surface groups provide model systems to investigate the effect of Coulombic, hydrophobic, and H-bond interactions upon the solvent and solute dynamics within the random, transient H-bond networkof the water. Also reported for the first time areneutron diffraction difference experiments in these undercooled solutions to investigate how changes in dynamic disorder are related to changes in the average static structure of the hydration water as temperature is decreased. A substantial sharpening of the orientational order between water molecules is observed in the undercooled state. Introduction The structure and the dynamic properties of a great number of aqueous solutions of atomic ions have been studied thoroughly both with neutron diffraction1 and nuclear magnetic relaxation2 techniques. Aqueous solutions with dissolved organic ions with apolar surface groups like symmetric tetraalkylammonium (R4N+) ions are less well investigated. Despite many thermo- dynamic data, only very few investigations of the molecular dynamics in these systems exist3v4 and direct structural studies were reported only re~ently.~+6 Aqueous solutions of R4N+ ions allow the competing influence of the Coulomb effect of the charge density and the hydrophobic effect of the apolar surface on the dynamic structure of the H-bond network of water to be studied. Though the concept of hydrophobic hydration is widely used in chemistry and biology to describe the reduction in entropy upon solvation of apolar solutes in water, its molecular basis is not well understood yet. Thermodynamic, spectro~copic,3~~ and computer simulation738 results suggest that the solvation of apolar groups (hydrophobic hydration) increases the amount of order in the random, transient H-bond network of the coordinated water molecules and reduces their mobility. The hydration structures involved may resemble clathrate-like cages which would be favored by the network organization of liquid water.g Further insight into the dynamic structure may be obtained in undercooled aqueous R4N+ solutions. Because of reduced thermal excitations hydrophobic hydration structures would become more stable and long-lived and hence would cause a strong slowing down of all molecular motions of all constituents. A recent report'" of the deuterium (2H)spin-lattice relaxation rates (SLR) R1 and self-diffusion coefficients D, witnessed at ambient temperatures a slowing down of orientational and translational motions of hydration water molecules due to the interaction with the apolar surface and facilitated motions at low temperatures in the undercooled phase of aqueous tetramethylammonium bromide (Me4NBr) solutions. These data will be supplemented in this study by proton (1H) and carbon-1 3 (13C) relaxation rate measurements providing insight into the overall and internal dynamics of the dissolved ions. 0 Abstract published zyxwvutsrqp in Aduance ACS Abstracts, July zyxwvutsrqp 1, zyxwvutsrq 1994. 0022-365419412098-8 161 %04.50/0 These dynamical characteristics need to be confronted with direct structural information concerning the hydration shell around these apolar solutes. The latter is provided by partial pair correlation functions g&) as deduced from neutron diffraction experiments with isotope substitution (NDIS). The hydrogen-hydrogen correlation function gHH(r) for the water in relatively concentrated aqueous solutions gives information on changes in the water structure caused by the presence of the solute relative to the structure in pure water. Previous measure- ments of the HH-pair correlation function for water in tetram- ethylammonium chloride, Me4NC1, at room temperature showed no appreciable change compared to pure water over a range of solute concentrations5~6despite the fact that NMR measurements demonstrate an increased slowing down of the solvent mobility with increasing solute concentration at ambient temperatures. At low temperatures changes in the hydration water dynamics occur with facilitated molecular motions in the solutions compared to pure water in the deeply undercooled state. Motivated by these complex changes in the dynamics of the hydration water in undercooled solutions as revealed by our recent NMR experiments,IO we report in this study for the first time neutron diffraction experiments with H / D substitution in the water in undercooled aqueous TMABr solutions. The neutron diffraction results presented in this work are a first step of an investigation into whether such changes in dynamic disorder are related to changes in the average static structure of the water in solutions of tetraalkylammonium ions as temperature is decreased. Experiment NMR Experiments. Tetramethylammonium bromide (Med- NBr) was purchased from Aldrich Chemicals (Steinheim, FRG) and heavy water (DzO, 99.8 at. '36) was obtained from Merck (Darmstadt, FRG). The salt was dried on a vacuum line for 24 h, and the solutions were prepared by weighing the proper amount of salt to 3 mL of heavy water. Afterwards the solutions were degassed on a vacuum line by at least five freeze-pumpthaw cycles and then filled into the high-pressure glass capillary as described elsewhere.11JZ To reach high degrees of undercooling, emulsions had to be used, however. As supporting alkane phase, deuterated methylcyclohexane (MCH-dl4, Riedel-de Haen, 0 1994 American Chemical Society