Consistency of Solid–Liquid Equilibrium Data in Choline Chloride-Based Deep Eutectic Solvents: An Activity Coefficient Analysis

Abstract

Thermodynamic models used to design deep eutectic solvents (DES) rely on activity coefficients calculated from experimental solid–liquid equilibrium (SLE) data, and their reliability depends on the consistency of the underlying data. The SLE data are particularly uncertain for systems containing choline chloride (ChCl), a thermally unstable, hygroscopic salt that decomposes before melting. This work assesses the consistency of SLE data for ChCl-based DES reported in the literature using component activity coefficients. The activity coefficients of ChCl are calculated directly from the SLE data using the lower and upper bounds of the ChCl melting properties. Because these bounds cover the physically acceptable ChCl melting properties range, the assessment does not depend on the selection of ChCl melting properties. The activity coefficients of ChCl shifted abruptly across its solid–solid transition temperature in all systems examined, indicating that the data above and below the solid–solid transition are internally inconsistent. The data for ChCl-rich mixtures measured above the solid–solid transition were difficult to reconcile with the expected nonideality of the mixtures, consistent with the limited thermal stability of ChCl. These findings are intended to guide the selection of consistent SLE data for developing thermodynamic or machine learning models.