Studies of Ionic Interactions in Poly(propylene glycol)4000 Complexed with Triflate Salts

Abstract

In aprotic electrolytes and, in particular, in most polyether−salt systems the molar conductivity increases dramatically with increase in salt 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 complete dissociation is assumed. Examples can be found in poly(propylene glycol) (PPG) complexed with triflate salts. The series MCF3SO3/PPG4000 (M = Li, Na, NH4, ND4) has been studied and illustrates the effect of cation on the value of the maximum molar conductivity and the rate of increase of molar conductivity with increase in concentration. Particular attention has been paid to the previously reported enhancement of the ionic conductivity for ND4CF3SO3/PPG4000 electrolytes over that of NH4CF3SO3/PPG4000. Detailed infrared and Raman studies of the polyether 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 polarizability 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 increased ionic mobility due to a looser association of the ND4+ cations to the CF3SO3- anions leading to shorter lifetimes of the various associations of ions. We comment on the ionic interactions that have been previously proposed to explain the observed increase in molar conductivity 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 sites and those ether oxygen sites to which the cation is less tightly coordinated.