Electrical conductivity of highly concentrated electrolytes near the critical consolute point: A study of tetra-n-butylammonium picrate in alcohols of moderate dielectric constant

Abstract

The electrical conductivity of highly concentrated solutions of tetra-n-butylammonium picrate (TBAP) in 1-dodecanol (dielectric constant ε=4.6) and 1,4-butanediol (ε=25.9), and in mixtures of both alcohols, is measured in an extended temperature range ≈10−5<τ<≈10−1, where τ=(T−Tc)/Tc is the reduced temperature with Tc, the critical temperature. The electrical conductivity Λ(T) obeys the Vogel–Fulcher–Tammann (VFT) law for the temperatures far from the critical one. In the temperature range τ<10−2 a systematic deviation of the electrical conductivity from the regular VFT behavior is observed. This deviation is attributed to a critical anomaly. At the critical point the amplitude of the critical anomaly is finite with a value which varies between ≈0.4 and ≈2.7% of Λ(Tc), depending on the solvent. The (1−α) critical exponent describes well the conductivity anomaly, α being the exponent of the specific heat anomaly at constant pressure. The value of the Walden product (Λeqvη), with Λeqv, the equivalent conductivity and η, the shear viscosity, allows the degree of dissociation αdiss of TBAP to be determined at the critical point. αdiss becomes larger for increasing values of ε: for TBAP in 1-dodecanol αdiss≈0.25 and in 1,4-butanediol αdiss≈0.73. When the degree of dissociation of the salt is accounted for the Debye screening length is found almost independent on ε.