Diffusion Coefficients in Ionic Liquids:  Relationship to the Viscosity

Abstract

The relationship between the diffusion coefficient and the viscosity has been examined in computer simulations for a number of ions diffusing in a molten salt (alkali halide) solvent. The comparison gives a measure of a hydrodynamic radius for the diffusing ions which is then compared with the bare ionic radius and a characteristic radius of the coordination complex formed by halide ions around polyvalent cations. K(+) and Cl(-) ions appear to diffuse as isolated spherical particles, whereas the trivalent cations Sc(3+), Y(3+), and La(3+) diffuse as if with an intact coordination shell. These different behaviors can be related to the time scale for the relaxation of the coordination shell, compared to the structural relaxation time of the solvent.

Varying the charge of small cations in liquid water: Structural, transport, and thermodynamical properties

“…At the present time the switching between two different diffusional regimes is not observed in mean residence time behavior that is related to the time scale of coordination shell relaxation, 26,27 but mrt should have some influence. Then it would be tempting to connect mrt with some measure of "local" stress tensor which in Adelman picture 13 is related to local density.…”

“…19,22 In an attempt to explain why the conductivity of alkali ions in water increases with their ionic radius when going from Li + to Rb + , while it then decreases for the larger ion Cs + (or tetramethyl ammonium), the role of dispersion interactions has been investigated recently. [23][24][25] Studies about ionic diffusion in molten salts by Madden and co-workers 26,27 pointed out that the effective hydrodynamic radius switches from a bare value to that of the solvation shell, correlating this switch to the relative time scales of coordination shell relaxation and stress relaxation time.…”

Polarization effects in ionic solids and melts

“…[103] The diffusion coefficients, conductivity and viscosity in the melt are strongly influenced by the lifetimes of the coordination complexes and by the degree to which these complexes are linked together to form a network. [82,104] The inclusion of the polarization effects in the interaction potential is crucial for describing these complexes and their network-forming tendencies correctly.…”

A new approach for determining the critical cooling rates of nucleation in glass‐forming liquids

“…However, on the other hand, once heterogeneous seeds or other factor in lowering thermodynamic barrier of critical nucleus formation, for example, phase separation, exist in a liquid, the I ‐ T curve will shift to melting temperature, T m , and the value will be larger than that for the case of homogeneous nucleation. To a certain extent, the Stokes–Einstein law for translational diffusivity in crystallization process is still valid if the onset temperature of nucleation and/or growth is far from the glass transition temperature, in other words, the diffusivity is comparable to the viscous flow . This extreme case is shown as the curve 3 in Figure , where the maximum heterogeneous nucleation rate is located at a higher temperature compared to the maximum crystal growth rate.…”

“…To a certain extent, the Stokes-Einstein law for translational diffusivity in crystallization process is still valid if the onset temperature of nucleation and/or growth is far from the glass transition temperature, in other words, the diffusivity is comparable to the viscous flow. [15][16][17] This extreme case is shown as the curve 3 in Figure 1, where the maximum heterogeneous nucleation rate is located at a higher temperature compared to the maximum crystal growth rate. This scenario is found in many metallic glass-forming liquids.…”