Hydration water dynamics in undercooled aqueous solutions of hydrophobic ions

Abstract

Deuterium spin-lattice relaxation times (zH T I ) are reported for the first time as a function of temperature (180-300 K), pressure (p I 225 MPa), and composition (c I 5 m) in undercooled aqueous solutions of tetraalkylammonium ions (R4N+). A comparison with related investigations of undercooled alkali-metal halide solutions allows the competing influence of coulombic, hydrophobic, and H-bond interactions upon the dynamic structure of the random, transient H-bond network to be studied. Solvent dynamics are seen to be closely related to the glass-forming tendency of undercooled tetramethylammonium (Me4N+) and tetrapropylammonium (Pr4N+) bromide solutions and to the clathrate-forming tendency of undercooled tetrabutylammonium (Bu4N+) bromide solutions.

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].…”

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

“…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.…”

“…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. 8,9,16 Defining y as the angle between the vector joining the cation (central N atom in the case of TAA) to the oxygen atom of a water molecule and the dipole moment vector of the same water molecule, tangential orientation refers to cases when these two vectors are at or close to 90 degrees.…”

Microscopic probing of the size dependence in hydrophobic solvation

“…A widely used model system for studying hydrophobic effects is tetraalkyl-ammonium bromides (R 4 NBr) which consist of apolar chains for which the length can be varied and display effects of both hydrophobic and electrostatic interactions. [26][27][28] A neutron diffraction study of R 4 NBr aqueous solutions could not find significant differences in H-bonded structure in comparison to pure water. 29 Infrared Raman spectroscopy lacks distinctive features to identify different H-bond structures; the contribution from hydrophobic effects is usually much weaker than the pure water signal 30,31 which may introduce ambiguity to spectral interpretation.…”