Investigation of Fundamental Transport Properties and Thermodynamics in Diglyme−Salt Solutions

Abstract

Ionic mobility, the thermodynamics of ionic association, and the structure of associated species are studied in solutions of diglyme containing either lithium triflate or tetrabutylammonium triflate. Infrared spectroscopic, PFG NMR, thermodynamic, and crystallographic data suggest that the solute species existing in diglyme-lithium triflate are "free" ions, contact ion pairs, and dimers. Equilibrium constants, S(o), deltaH(o), and deltaG(o) are calculated for processes occurring between these species. In particular, the equilibrium constant, corrected for nonideality using a modified Debye-Hückel expression, is calculated for the dissociation of contact ion pairs into "free" cations and anions. A second equilibrium constant for the formation of dimers from contact ion pairs is also calculated; these constants do not significantly vary with salt concentration up to about 1.3 x 10(-3) mol cm(-3). The measured temperature dependence of equilibrium constants was used to calculate deltaH(o) and deltaS(o) for the two processes. The value of deltaS(o) = -102 J mol(-1) K(-1) for the dissociation of contact ion pairs shows that the large entropy decrease due to cation solvation outweighs the entropy increase due to dissociation of a contact ion pair. Ionic mobilities are calculated in lithium triflate-diglyme solutions using conductivity data in conjunction with information about the nature and concentrations of solute species obtained from IR spectroscopy. Mobilities in tetrabutlyammonium triflate-diglyme solutions are calculated directly from conductivity data. It was concluded that the concentration dependence of the molar conductivity is due in large part to the variation of the ion mobilities with concentration.