Thiamethoxam in aqueous co-solvent mixtures of 1,4-dioxane, N,N-dimethylacetamide, dimethyl sulfoxide and acetonitrile: Solubility solute-solvent and solvent-solvent interactions, and preferential solvation analysis

Abstract The determination of equilibrium solubilities of thiamethoxam in solutions of 1,4-dioxane (1) + water (2), N,N-dimethylacetamide (DMAC, 1) + water (2), dimethyl sulfoxide (1) + water (2) and acetonitrile (1) + water (2) was carried out through the shake-flask method from 278.15 to 323.15 K under 101.2 kPa. The relative contributions of solvent-solvent and solute-solvent interactions on the drug solubility variation were studied via the analysis of linear solvation energy relationships. The significant contributions to the solubility variation were the cavity term and dipolarity-polarizability term in aqueous solution of 1,4-dioxane; and cavity term in aqueous solutions of DMAC/DMSO/acetonitrile. The inverse Kirkwood–Buff integrals were employed to investigate the preferential solvation in light of several solution thermodynamic properties. The preferential solvation parameters for 1,4-dioxane, DMAC and acetonitrile presented positive values in the three aqueous solutions within intermediate and 1,4-dioxane/DMAC/acetonitrile-rich compositions, demonstrating that the drug thiamethoxam was solvated preferentially by 1,4-dioxane/DMAC/acetonitrile. It is conjectured that in these regions thiamethoxam serves as a Lewis acid with the 1,4-dioxane and DMAC molecules; while for the (acetonitrile + water) mixture, the preferential solvation could be attributed to polarization effects. Moreover, the solubility data in mole fraction was mathematically described by the van’t Hoff–Jouyban-Acree model and Jouyban-Acree model attaining the average relative deviations no more than 6.67 %.