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. In contrast, the reported HF conductivity isotherms increase with HF mole fraction throughout the measured composition range, including through the first eutectic region and beyond xHF = 0.5, without exhibiting a resolved maximum or the high-concentration downturn observed in any other aqueous system, 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. These deviations are discussed in relation to composition-dependent ionization, dielectric response, molecular association and hydrogen-bond organization. Because no direct structural measurements were performed in the present work, the proposed microdomain and transport descriptions are treated as literature-supported working hypotheses rather than uniquely established mechanisms. To further probe these anomalies, activation energy and pre-exponential factor values were extracted as functions of mole 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.