Dynamic Structure and Ion Solvation in Associated Liquids

Ber Bunsenges Phys Chem

Radscheit²

1976

The properties of metal‐ammonia solutions in the concentration range of the metal‐nonmetal transition between 0.1 and 10 mol percent metal (MPM) are discussed. The high‐frequency electrical properties show a number of characteristic features in this intermediate concentration range, such as additional dielectric relaxations at about 0.1 MPM and 0.7 MPM for LiNH3 and NaNH3, respectively, and a resonance‐like behavior at about 1.0 MPM and 1.5 MPM for LiNH3 and NaNH3, respectively. For both LiNH3 and NaNH3 the real part of the high‐frequency dielectric function changes to large negative values around 2.5 MPM, indicating a transition to metallic behavior at high frequencies. The high‐frequency properties, which are also discussed in terms of electron conductivity, are explained quantitatively as a function of concentration, temperature, and frequency using a model of polaron hopping. The metal‐nonmetal transition is related to structural changes in the solutions which lead to phase separations in LiNH3 and NaNH3 at lower temperatures. The structural properties of the solutions are investigated using sound velocity and ultrasonic attenuation data. Concentration and temperature dependence of the ultrasonic attenuation in LiNH3 and NaNH3 can be described quantitatively in terms of structural fluctuation.

Metal‐Nonmetal Transition in Metal‐Ammonia Solutions

Ber Bunsenges Phys Chem

Radscheit²

1976

Brillouin Light Scattering and Structural Relaxations in Ionic Solutions

Ber Bunsenges Phys Chem

Polke²

1981

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...

ChemInform Abstract: DYNAMIC STRUCTURE AND ION SOLVATION IN ASSOCIATED LIQUIDS

Chemischer Informationsdienst

Wolz²

1974

The dynamic n‐state model is based on time‐dependent transport coefficients and applied to the ultrasonic absorption in solutions of simple ions in water, liquid ammonia, and methanol. The absorption measurements were made over a wide range of temperatures at frequencies between 5 and 100 MHz. The characteristic differences of the thermodynamic parameters, the relaxation times, the molar volumes, and the solvation numbers are discussed for the liquids and ions investigated.