Aqueous Ion Transport Properties and Water Reorientation Dynamics from Ambient to Supercritical Conditions

Abstract

Ion transport properties including the friction coefficient, Walden product (product of conductivity and viscosity), and the limiting equivalent conductance are predicted in water at elevated temperatures using a semicontinuum model. Molecular dynamics computer simulation is used to determine water rotational reorientation times in the first coordination shell compared with the bulk, and the results are incorporated into a hydrodynamic expression for the ionic friction coefficient. Along the coexistence curve of water, the effective Stokes−Einstein radius implied by the model is relatively constant. However, for Cl-, K+, and Rb+, this radius increases at typical supercritical water conditions, where the motion of the first shell water molecules is coupled more closely to that of the ion. For Na+, the coupling is already quite strong at higher solvent densities. The increment to the friction coefficient in excess of the bare ion Stokes−Einstein result contributes a larger fraction of the total in supercritical water at typical densities (up to 0.29 g/cm3) than it does in higher density subcritical water, as a result of electrostriction. The limiting equivalent conductance increases approximately linearly with decreasing solvent density in the supercritical regime, in qualitative accord with the experimental extrapolations of Quist and Marshall (J. Phys. Chem. 1968, 72, 684−703) and in contrast to the plateau with decreasing density inferred from much more recent experiments by Zimmerman et al. (J. Phys. Chem. 1995, 99, 11612−11625).

Reorientional dynamics of water molecules in anionic hydration shells

“…Examination of multiply charged ions (3,25) is also of interest, because for these cases the ion-water attraction is very strong, and the frame reorientation likely will dominate the jump reorientation component of the reorientation time, leading to a rotational ''solventberg'' picture in analogy to that of ion translational mobility discussions (26,27). Indeed, there is a clear reorientation-translation coupling because the reorienting water goes from the first to the second hydration shell (28) ʈ in the jump component of the reorientation, and the jump and frame component perspective may prove useful in describing aspects of ionic mobility (3,4,29,30). † † Finally, the success of the mechanism and model in the present paradigm system for water in inhomogeneous asymmetric environments is encouraging for the challenging study of detailed water H-bond dynamics in the hydration shells of (usually charged) biomolecules, where both experiments and simulations point to a dynamical regime noticeably different from the bulk, with implications in protein recognition and drug binding (31)(32)(33).…”

“…† † We have shown here that NMR and fsIRS probe the different components of the reorientation for water in the first hydration shell of a single ion (Cl Ϫ ). For ionic mobility, a transition between the dominance of different components would be reflected in the differing behavior of the mobilities of different ions (3,4,29,30), a feature that may also be found for hydration shell water reorientation for different ions. Table 2 and correspond to an OH⅐⅐⅐Cl Ϫ bond weaker and longer than the OH⅐⅐⅐O bond, as observed experimentally (8,14).…”

Biomolecular hydration: From water dynamics to hydrodynamics

“…In fact, from Ϫ10 to 60°C, the SE radius a varies by only 1.7%, whereas the viscosity changes by a factor 6. The same approximation has been used in studies of small-ion mobilities (31,32).…”

“…2, as for rotation, with the only difference that ␣ R is replaced by ␣ T ϭ ␣ R 1͞3 (25,26,31,32,42). The steps leading up to Eq.…”

Temperature dependence of protein-hydration hydrodynamics by molecular dynamics simulations

“…[1]) can arise either due to a change in the viscosity (η) or the radius (a) or both. Experimental measurement of the bulk water rotational correlation time (τ s ) follows the viscosity over a wide range of temperatures (43) and in fact the observed change in the radius is only 1.7% in the temperature range of -10º to 60°C (44). Therefore, the assumption that modulation of the translational dynamics of hydration water molecules is predominantly due local changes in the viscosity induced by frictional coupling between the protein and surface waters is reasonable.…”