Multi‐Nuclear Relaxation Time Studies in Undercooled Aqueous Electrolytes

Lang, Elmar; Fink, William H.; Radkowitsch, H.; Girlich, D.

doi:10.1002/bbpc.19900940328

Cited by 12 publications

(6 citation statements)

References 21 publications

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“…Cs+) atomic ions, as is demonstrated in Figure 2a which compares the 7\(2H) of water molecules directly coordinated to Li+, Cs+, and (CH3)4N+ with the relaxation in pure D20. 2 It is seen that if the H-bond network is strongly perturbed because of thermal excitations, these large apolar ions reduce the average mobility of the water molecules. At low temperatures, however, H-bond interactions in the random, transient H-bond network of pure water slow down reorientational fluctuations stronger than ion-water interactions.…”

Section: Methodsmentioning

confidence: 99%

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

Bradl¹,

Lang²,

Turner³

et al. 1994

J. Phys. Chem.

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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

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Section: Methodsmentioning

confidence: 99%

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

Bradl¹,

Lang²,

Turner³

et al. 1994

J. Phys. Chem.

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“…In water the efg at the deuterium nucleus originates almost exclusively from intramolecular charge distributions. The relevant interaction Hamiltonian is given in a laboratory-fixed frame (L) by20 HQ(t) = eQ/2I(Il)fr]T(-ir Tí2mRL2_m(t) (1) /is the spin quantum number (2H, / = 1). The T2m are secondrank irreducible tensor operators, and the R2m are components of the second-rank irreducible field gradient tensor.…”

Section: Theorymentioning

confidence: 99%

Hydration water dynamics in undercooled aqueous solutions of hydrophobic ions

Bradl¹,

Lang²

1993

J. Phys. Chem.

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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.

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“…The relaxation rates of the more dilute solutions ( c = 0.3 m) might be calculated from a two-site approximation. , with N h the hydration number and R 1,hyd and R 1,bulk the experimental relaxation rates of the 1.6 m solution and pure water, taken at the same distance from the relevant singular temperature T s ( p , R ) of the solution with composition R , respectively. This singular temperature, which enters the fractional power law dependence 14,35 of the tumbling correlation time τ or,w of the bulk term, is, however, not known in these solutions.…”

Section: Results and Data Evaluationmentioning

confidence: 99%

“…The structural and dynamic distortions brought about by dissolved atomic ions are rather well investigated. Due to electrostatic interactions mainly, atomic ions orient adjacent water molecules in a characteristic manner depending on the nature and the charge density of the ions.…”

Section: Introductionmentioning

confidence: 99%

“…Cations orient the water dipole away from the ion with the dipole tilted relative to the radial direction, and anions orient water molecules into a configuration which seems to lie in most cases between an H-bonded and a bifurcated arrangement. These ion−water interactions also alter the mean residence time of a water molecule in the coordination shell in comparison to the average residence time of a water molecule in the H-bond network. , …”

Section: Introductionmentioning

confidence: 99%

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High-Pressure NMR Relaxation Study of the Solute and Solvent Dynamics of Undercooled Aqueous Tetraethylammonium Bromide Solutions

et al. 1996

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Multinuclear spin−lattice relaxation rate measurements are reported over wide ranges of temperature, pressure, and concentration in undercooled aqueous solutions of tetraethylammonium bromide (Et4NBr). These dissolved organic cations with apolar surface groups provide model systems to investigate their influence upon the dynamics of the water molecules in the hydration shell as well as the influence of the hydration shell upon the dynamics of the dissolved hydrophobic ions. The isobaric relaxation time curves of all nuclei investigated show pronounced minima, indicating a strong slowing down of the dynamics of the solvent molecules in the hydration shell as well as of the dissolved hydrophobic ions. A VTF-like temperature dependence and corresponding scaling behavior allow glass transition temperatures to be deduced which signify the onset of motional arrest in these metastable solutions. Segmental motions of the aliphatic chains of the organic cations are discussed in terms of correlation times, and corresponding order parameters are given characterizing their spatial restrictions. At low temperatures all proton relaxation rates become equal to those of the methyl groups due to strong cross-relaxation effects, indicating that methyl groups act as relaxation sinks in these systems.

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Multi‐Nuclear Relaxation Time Studies in Undercooled Aqueous Electrolytes

Cited by 12 publications

References 21 publications

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

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

Hydration water dynamics in undercooled aqueous solutions of hydrophobic ions

High-Pressure NMR Relaxation Study of the Solute and Solvent Dynamics of Undercooled Aqueous Tetraethylammonium Bromide Solutions

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