Abstract
The aqueous solubilities of allopurinol in some choline chloride (ChCl) based deep eutectic solvents (DESs) with urea, glycerol, citric acid and glycol as neoteric green solvents were measured by the shake-flask method at T = (293.15–333.15) K under atmospheric pressure p = 101.1 kPa. The solubility in these solvents increased more than approximately 27.8-fold for ChCl/urea system, 45.6-fold for ChCl/glycerol system, 31-fold for ChCl/glycol system and 56-fold for ChCl/citric acid system with increasing the weight fraction of DESs. The solubility data was accurately correlated by the λh equation and modified Apelblat equation. The largest percentage of relative average deviation was 1.49 × 10−2, and the maximum value of root-mean-square deviation was 2.43 × 10−5. Also, in order to describe the thermodynamic behaviour of allopurinol in the aqueous DESs solutions, the transfer properties were derived. The negative Gibbs free energy (ΔtrGo) and enthalpy (ΔtrHo) values indicated that the transfer of allopurinol from neat water to aqueous DESs solutions was spontaneous and energetically favourable. Additionally, the ΔtrGo and ΔtrHo values decreased with the increase in the DESs concentrations, indicating that solubilization was more favourable with the increase in the DESs concentrations. In particular, the ΔtrGo values considerably decreased for ChCl/citric acid compared to ChCl/urea, ChCl/glycerol and ChCl/glycol mixtures. ChCl/citric acid is an effective additive for improving the solubility of allopurinol. Furthermore, the excess enthalpy of solution was obtained according to the λh equation.
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