Browsing by Author "Frech, R."

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    Investigation of Fundamental Transport Properties and Thermodynamics in Diglyme?Salt Solutions
    (Journal of Physical Chemistry B, 2006) Petrowsky, M.; Frech, R.; Suareza, S.N.
    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�, ?H�, and ?G� are calculated for processes occurring between these species. In particular, the equilibrium constant, corrected for nonideality using a modified Debye?H�ckel 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 ? 10-3 mol cm-3. The measured temperature dependence of equilibrium constants was used to calculate ?H� and ?S� for the two processes. The value of ?S� = ?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.
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    NMR Studies of Ionic and Molecular Motion in Diglyme:LiCF3SO3 Complexes
    (2004) Jayakody, J.R.P.; Suarez, S.; Greenbaum, S.G.; Petrowsky, M.; Frech, R.
    Low molecular mass diglyme forms complexes with lithium salts similar to the high molecular mass and much-studied poly(ethylene oxide) (PEO), and thus serves as a useful model for investigating ion solvation,association, and transport in polymer electrolytes. NMR measurements of diglyme:LiCF3SO3 complexes were performed on all three mobile components (solvent, cation, and anion) at 1H, 7Li and 19F Larmor frequencies of 301.0, 116.9 and 283.2 MHz respectively. The diglyme/Li ratio varied from 5:1 to 40:1, and variable temperature measurements were carried out for a 20:1 sample. Spectra, spin-lattice relaxation times (T1) and self-diffusion coefficients (D) were obtained; D measured by standard pulsed field gradient methods. Spin-lattice relaxation times and self-diffusion coefficients for the anion (19F), cation (7Li) and solvent ( 1H) as a function of diglyme concentration at 20oC are displayed in Figures 1 and 2, respectively. As shown in Figure 1 the T1?s are observed to increase with decreasing salt concentration, consistent with the known increase in viscosity of electrolytes at high salt concentrations. The self-diffusion coefficient is a more direct indicator of mass transport, and thus also decreases with increasing salt concentration. At high salt concentrations (n = 5 and 6) all three species gave similar D?s, indicating strongly correlated motion. For diglyme/salt ratios greater than n=6, the proton (solvent) D's were higher than those of both the cation and anion. A comparison of the anions and cations shows the cations having greater D's than the anions above n = 20. These results will be discussed in the context of cation-anion association and cation-diglyme complexation information gained from vibrational spectroscopic data on the same system. Finally, variable pressure self-diffusion measurements were performed for the first time on any polymer electrolyte using a static magnetic field gradient. For the 10:1 diglyme/LiTf complex, diffusion activation volumes for the anion and host were the essentially the same, again indication strongly correlated motion.