Quantitative surface analysis of paclobutrazol molecule and comprehensive insight into its solubility in aqueous co-solvent solutions

Abstract

As powerful tools, the minimum negative surface electrostatic potential together with minimum surface average local ionization energy, were used herein to illustrate the electrostatic characteristics of acidity and basicity at a microscopic level, correspondingly. The groups of -N = and –OH of the paclobutrazol molecule are primary sites of electrophilic and nucleophilic attack. The mole-fraction solubility of paclobutrazol in three co-solvent blends of isopropanol + water, acetone + water and acetonitrile + water covering the temperature range from 278.15 to 318.15 K was experimentally acquired by the use of isothermal saturation method under 101.2 kPa. Under the identical experimental conditions, e.g. same temperature and mass composition of isopropanol (acetone or acetonitrile), the solubility of paclobutrazol in mole fraction was much larger in acetone + water blends than in isopropanol/acetonitrile + water blends. The minimal solubility was recorded in neat water solvent at 278.15 K; and the maximal one, in acetone/acetonitrile/isopropanol at T = 318.15 K. No appearance of crystaltransition as well as solvate formation was observed in experiments as indicated by X-ray power diffraction patterns. Solubility data were mathematically correlated by three models including Jouyban-Acree, Apelblat and modified van’t Hoff-Jouyban–Acree. Investigation on preferential solvation of crystalline paclobutrazol was performed by the powerful approach of inverse Kirkwood–Buff integrals at 298.15 K. The preferential solvation parameters of isopropanol, acetone or acetonitrile presented positive values in blends with middle and isopropanol/acetone/acetonitrile-rich composition regions, which indicated the preferential solvation of paclobutrazol by the co-solvents. In addition, the extended Hildebrand solubility approach was employed to quantitatively describe the solubility behavior at 298.15 K in isopropanol/acetone/acetonitrile + water blends studied in this paper and in ethanol/n-propanol/1,4-dioxane + water blends previously reported by us, attaining the average relative deviations < 7.61 %.