Anomalous Correlations Between Ionic Conductivity Isotherms and Phase Diagrams in Hydrofluoric and Acetic Acid Aqueous Systems

There is an empirically established correlation between ionic conductivity isotherms and solid–liquid phase diagrams in most electrolyte solutions, wherein the composition of maximum conductivity (x _max ) typically coincides with the eutectic composition (x _eutectic ). In this work, we investigate pronounced deviations from this correlation observed in hydrofluoric acid (HF) and acetic acid (AA) aqueous solutions, two systems that behave anomalously compared to other common acids. Ionic conductivity data across multiple temperatures and concentrations were compiled from literature and supplemented with new experimental measurements. Hydrofluoric acid exhibits nearly linear isotherms that bypass all eutectic features without showing any conductivity maximum, whereas acetic acid displays a parabolic isotherm with an x _max occurring far earlier than its eutectic point, representing an extreme case of x _max ≪ x _eutectic . We attribute these deviations to peculiar chemical equilibria and a less-understood liquid microstructure that likely give rise to distinct microdomain organization and modified ion-transport dynamics. To further probe these anomalies, activation energy and pre-exponential factor values were extracted as functions of molar fraction and examined for both acids. These findings highlight the need for refined theoretical frameworks capable of capturing transport behaviour in systems where traditional conductivity–phase diagram correlations break down.