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.