Brillouin Light Scattering and Structural Relaxations in Ionic Solutions

Abstract

K. G. Breitschwerdt and E. Polke: Brillouin Light Scattering and Structural Relaxations in Ionic Solutions 1059 Therefore, the dynamical properties of all water molecules in the 2.2 molal LiI solution are influenced by the ions. This shows that the microdynamical model can only be applied at low concentrations, where the limit of its validity will depend on the size of the ions. Obviously this limit lies for lithium halide solutions significantly below 2 molal. Therefore, the discrepancies in self-diffusion coefficients and rotational correlation times for the hydration water of Lit and I -between the MD simulation and the model are no surprise.For the water molecules in the first hydration shell of I -it would be expected in the limit of zero concentration a larger self-diffusion coefficient and a smaller rotational correlation time compared with pure water. Accordingly the application of the microdynamical model leads to D do = 1.4 and r -/ r o = 0.4 for 298 K, while the simulation results in D -/ D o = 0.78 and 7 ~ /T" = 1.2 ( Table 2) for 305 K. This reversed effect is a consequence of a second hydration shell of Li + . There are not enough water molecules for a separated second shell of Li+ and a first shell of 1 ~. As part of the water molecules belong to both it is easy to understand that the simulation leads t o very similar results for the water molecules in the hydration shell of I ~ and bulk water (Table 2 and 3), where actually bulk water should be written with quotation marks, as the water molecules belong to the second hydration sphere of Li+. It is important to note that the ST2 model employed in the simulation is not responsible for the reversed effect. From an MD simulation where a single ion is surrounded by 215 water molecules it has been shown by Geiger [l] that for the hydration sphere of a negatively charged ion the self-diffusion coefficient is increased and the rotational correlation time decreased compared with pure water as expected.The microdynamical model leads to a self-diffusion coefficient for the water molecules in the first hydration shell of Li+ which isunrealisticallysmaller than the one for the ion itself. The MD simulation gives a value higher than the one for Li' but not too strongly different (Tables 1 and 2). This similarity results from the long residence time of the water molecules in the first hydration shell of Li+ . The simulation shows that 52% of the water molecules remain in the first hydration sphere of Li + over the whole simulation time of 10 ps and another 31% stayed there for more than 9 ps. This is in agreement with estimations of the residence time given in the literature [4, 161. For the rotational correlation time there is good agreement between the values resulting from the model and the MD simulation being r + / r o = 2.3 and 2.1, respec-tively. It should be mentioned that in an analysis of lithium halide solutions where the self-diffusion coefficient of Li + and its hydration water are assumed to be equal while it is not distinguished between bulk w...