NMR and Neutron Scattering Investigation of Undercooled Aqueous Solutions of Apolar Solutes

Abstract

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. IntroductionThe 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 thermodynamic data, only very few investigations of the molecular dynamics in these systems exist3v4 and direct structural studies were reported only re~ently.~+6Aqueous 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

Aqueous solutions of tetraalkylammonium halides: ion hydration, dynamics and ion–ion interactions in light of steric effects

“…Regarding TMA + rotation, experimental data suggest t c of the order of 5 ps at low concentrations, with only a small increase up to 5 m concentrations (less than a factor of 1.5). 10,13,55,59,60 This is indeed as we expect from the compact nature of TMA + . Quantitative agreement with our simulated data is however not found, our t rot of 5 ps would correspond to t c of 0.8 ps.…”

“…For completeness, experimental methyl rotational time (t rot B) is 1-5 ps depending on the system studied and the technique employed. 13,50,61 A difference in terminal methyl group rotation has already been reported in a low temperature NMR study, leading to a correlation time larger by a factor of six in the case of TBA + than in the case of TMA + . 62 The measurements of global as well as terminal methyl group rotation for TAA ions is a priori also available from quasi-elastic neutron scattering.…”

“…Unfortunately, the evidence for either one of them is not clearly established. Neutron diffraction, [5][6][7] microscopic simulation 8,9 as well as dynamic studies by NMR and neutron scattering, [10][11][12][13][14][15] conclude against increased water structuring and see no clathrate formation, with the possible exception of TBA + in the low temperature regime only. 12 At the same time, neutron diffraction and microscopic simulation agree on the unusual tangential orientation of water molecules around individual TAA ions, in stark contrast to alkali cations.…”

Towards an understanding of the molecular basis of hydrophobicity

“…Further work is being done to investigate this phenomenon. The diffusion coefficients of water in the hydration shell of methane are always smaller than those for water in the bulk region, confirming previous findings of slowed-down dynamics in the vicinity of a nonpolar solute [20][21][22][23][24].…”

“…This results in a negative contribution to AS~r. The ordering has also been associated with slowed-down dynamics [20][21][22][23][24].…”