Evaluating the Importance of Long-Range Interactions in the Thermodynamic Description of the Choline Chloride/Water System

The ability to model salt-containing deep eutectic solvents (DES) is essential for their rational design and application. Most existing thermodynamic models treat salts as neutral species, neglecting ionic interactions. This work investigates the explicit inclusion of ionic effects in modeling the liquid-phase nonideality of the binary choline chloride (ChCl)/water system. Experimental vapor–liquid equilibrium, solid–liquid equilibrium, water activity, and ChCl activity coefficients obtained from isopiestic measurements were used. Short-range molecular interactions were described with the nonrandom two-liquid (NRTL) model, while long-range ionic interactions were incorporated through the Pitzer–Debye–Hückel (PDH) term. Treating ChCl as a fully dissociated electrolyte and including the PDH contribution significantly improved the representation of nonideality across low and high salt concentrations. These results demonstrate the importance of electrolyte-based modeling frameworks for the development, optimization, and recycling of water-containing DES Types II–IV.