Part A. Molecular Dynamics and Ionic Associations of LiAsF6 in 4-Butyrolactone Mixtures with 2-Methyl Tetrahydrofuran. Part B. Molecular Dynamics and Ionic Associations of LiBF4 in 1,2-Dimethoxymethane

摘要

Solutions of LiAsF6 at 298.2 K were studied by audiofrequency conductance in the concentrations range .0001-.01 Mol/Cu dm and by radiofrequency ultrasonic absorption in the concentration range 0.05 - 0.5 mol/cu dm. The solvents employed were mixtures of 4-butyrolactone with 2-methyl tetrahydrofuran varying in compositions from mole fractions X subscript BL = 0.10 X subscript BL = 0.75. In dilute solutions the audiofrequency conductivity data yield ion association constants which appear to represent both contact and solvent-separated ion pairs for mixtures up to X subscript BL = 0.36. At higher LiAsF6 concentrations and in solvent mixtures of composition X subscript BL = 0.75, and X subscript BL = 0.35, the ultrasonic spectrum shows a single relaxation process which again is attributed to the formation of both contact and solvent separated ion pairs. Part B: Electrical conductance data for LiBF4 in 1,2-Dimethyoxymethane (DMM)at 25 C reveal the electrolyte to be heavily associated to ion-pairs and triple ions. A theoretical expression for the triple ion association constant similar to the Bjerrum one for ion-pairs and to the Maaser-Bjerrum theory of dimers has been developed and applied to the present conductance data. Ultrasonic relaxation absorption data at much higher concentrations than the conductance data reveal association to dimers. This is also evident from the microwave dielectric data showing no electrolyte dielectric effect on the solvent and apparent lack of presence of dipoles. The ultrasonic data can be rationalized by a two-step dimerization mechanism.