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

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.