Structural and Dynamical Origins of Ionic Mobilities in Supercritical Water

Abstract

We report the results of simulations of the ionic mobility of Na+ and Cl- in supercritical water at 673 K, including solvent densities below those previously considered in simulation or experimental data. By considering these results along with earlier published analyses, we find that the spatially inhomogeneous solvation structure around the ions and solvent dynamics are strongly coupled in determining transport rates. The appearance of a plateau in the infinite-dilution conductivity over a wide range of intermediate solvent densities is a result of a subtle balance of excess (dielectric) friction and a nonlinear variation in the viscous friction. The result is strongly influenced by the inhomogeneous solvent density around the ions, but cannot be rationalized on the basis of only structural criteria. A reduced effective ionic radius is introduced that is inversely proportional to the Walden product and can be trivially evaluated from experimental conductivity results. It is shown that when represented in this way, conductivity data smoothly and continuously vary with solvent density over the entire density range and are much more readily interpreted. In particular, this effective ionic size exhibits a maximum at a density of ca. 0.2 g/cm3, providing a natural division between high- and low-density solvents. At higher densities, the structure of the first hydration shell of the ions is only weakly dependent on solvent density, while at lower densities, entropic forces increasingly lead to the loss of this primary solvation shell. These results are consistent with the view that, with decreasing solvent density down to this natural division, an increasing imbalance between ion−water and water−water interactions produces an increasingly rigid ionic solvation shell and thus an increasing friction on the ion.

A mode-coupling theory of diffusion in supercritical fluids

“…31,30,84 Accordingly, we start by looking at the local solute-solvent microstructure. We compute the ion-solvent g(r) at three solvent densities (*ϭ0.17, 0.316, and 0.57͒ at T*ϭ1.37, which correspond to the density range and isotherm (Tϭ303 K, T r ϭ1.05) studied experimentally.…”

“…Of particular importance in this regard are the SCF transport coefficients, such as viscosity and diffusivity. As such, molecular diffusion coefficients in supercritical media have been actively studied via experiment, [9][10][11][12][13][14][15][16][17][18][19][20][21] molecular dynamics ͑MD͒ simulation, [22][23][24][25][26][27][28][29][30][31] and statistical mechanical theory. 26 -28,32-34 Due to the difficulties associated with measurements of diffusion coefficients at supercritical conditions, some of the experimental findings remain controversial, 33,35,36 which underscores the need for advancing theoretical treatments of SCF transport properties.…”

“…[22][23][24][25][26][27][28][29][30][31] For neutral solutes, the intermolecular interactions were typically modeled via pairwise potentials of the Lennard-Jones ͑LJ͒ form. [22][23][24][25][26][27][28] The simulated diffusion coefficients were obtained either from mean square displacements or from the time integrals over velocity time correlation functions ͑TCFs͒.…”

“…[37][38][39] In addition to neutral solutes, diffusion coefficients have been also simulated for ions in supercritical water, which is another example of a strongly attractive supercritical solution. [29][30][31] A pronounced plateau in the density dependence of infinitely dilute ionic mobilities has been observed in the medium-density region. 31 The appearance of this plateau was linked to the enhanced solvent density and viscosity in the vicinity of an ion ͑electrostriction effect͒.…”

“…[29][30][31] A pronounced plateau in the density dependence of infinitely dilute ionic mobilities has been observed in the medium-density region. 31 The appearance of this plateau was linked to the enhanced solvent density and viscosity in the vicinity of an ion ͑electrostriction effect͒.…”