Effect of temperature and composition on solubility and thermodynamics of salicylic acid in aqueous mixtures of betaine-based deep eutectic solvents

Solubility of salicylic acid in binary mixtures of water and betaine/propylene glycol (Bet/PG, molar ratio of 1:5), betaine/ethylene glycol (Bet/EG, molar ratio of 1:3) or betaine/glycerol (Bet/Gly, molar ratio of 1:3) deep eutectic solvents (DESs) was investigated via the shake flask method at 293.15–313.15 K under atmospheric pressure (≈85 kPa). The results indicate that the solubility values are enhanced with mass fraction of each DES and temperature and DES containing PG is the best cosolvent. The Hansen solubility parameter (HSP) of salicylic acid, neat solvents of each DES and water was calculated and used to predict the solubility of drug in each neat solvent. The experimental solubility values were in good agreement with the ones predicted from HSPs. Three cosolvency models (Jouyban-Acree, Jouyban-Acree-van’t Hoff and the modified version of Jouyban-Acree-van’t Hoff) and two activity coefficient models (NRTL and UNIQUAC) were selected to correlate the solubility data. Based on the obtained percentage mean relative deviations ( MRD %) for back-calculated data, all the investigated models show good correlation and validation. Additionally, investigation of the apparent thermodynamic analysis presented an endothermic and entropy-driven dissolution of salicylic acid in all cosolvent compositions including neat DESs and water.