STUDIES OF IONIC INTERACTIONS IN POLY(PROPYLENE GLYCOL)4000 COMPLEXEDWITH TRIFLATE SALTS

In aprotic electrolytes and, in particular, in most polyether-salt sys tems the molar conductivity increases dramatically with increase in sa lt concentration from a minimum at low concentrations to an unexpected high maximum. Such increases in conductivity are larger than would be accounted for by Fuoss-Onsager type transport theories in which compl ete dissociation is assumed. Examples can be found in poly(propylene g lycol) (PPG) complexed with triflate salts, The series MCF(3)SO(3)/PPG 4000 (M = Li, Na, NH4, ND4) has been studied and illustrates the effec t of cation on the value of the maximum molar conductivity and the rat e of increase of molar conductivity with increase in concentration. Pa rticular attention has been paid to the previously reported enhancemen t of the ionic conductivity for ND4CF3SO3/PPG4000 electrolytes over th at of NH4CF3SO3/PPG4000. Detailed infrared and Raman studies of the po lyether C-O-C vibrations and of the salt vibrations have been carried out as a function of temperature (FT-IR) and salt concentration (FT-IR and FT-Raman). Spectral data are reported which show that the polariz ability of the anions is effected differently in chemical coordination processes in these ammonium electrolytes than are the dipole moments. We interpret the results in terms of ionic interactions and attribute the enhanced conductivity for the deuterated electrolyte to an increa sed ionic mobility due to a looser association of the ND4+ cations to the CF3SO3- anions leading to shorter lifetimes of the various associa tions of ions. We comment on the ionic interactions that have been pre viously proposed to explain the observed increase in molar conductivit y and propose an additional concept, i.e., that with increase in salt concentration there is an increase in the rate of exchange of cations between ion pairs and higher aggregates and between anion-associated s ites and those ether oxygen sites to which the cation is less tightly coordinated.