Evidence of Vogel–Tamman–Fulcher behavior in ionic binary fluids

Abstract

The electrical conductivity σ of the six concentrated binary ionic mixtures of isobutyric acid–water with XM [KCl] at the critical concentrations was measured over an extended temperature range above the critical consolute point. Far from the critical temperature T c, the electrical conductivity is accurately described by the Vogel–Fulcher–Tammann (VFT) law. However, in a temperature range ΔT = T − T c ≤ 2 K, the electrical conductivity exhibits a monotonous deviation from the VFT behavior. This anomaly is finite at T c. The asymptotic behavior of the electrical conductivity anomaly is described by a power law (t)(1− α ), with t = (T − T c)/T c, the reduced temperature, and α the critical exponent of the specific heat anomaly at constant pressure. This critical anomaly is similar to the one observed in other different concentrated critical electrolytes. The degree of dissociation αdiss of the salt for the critical mixture is estimated from the value of the Walden product computed at T c. When the salt is added, the degree of dissociation αdiss shows dependence on the (K+, Cl−) concentrations: 0.15 < αdiss < 0.68.