Imidacloprid (I) in several aqueous co-solvent mixtures: Solubility, solvent effect, solvation thermodynamics and enthalpy–entropy compensation

Abstract

This contribution dealt with the saturated solubility, solvation behavior, and dissolution and transfer belongings of imidacloprid in some aqueous co-solvent mixtures of n-propanol, acetone, ethanol and methanol by means of experiment and mathematical correlations. Solubility experiments were conducted via a commonly used technique, namely shake-flask method, under 101.2 kPa from 283.15 to 323.15 K. The maxima of the obtained imidacloprid solubility was recorded in neat n-propanol/acetone/ethanol/methanol at temperature T = 323.15 K, which corresponded to 227.7 × 10−5, 2851 × 10−5, 237.9 × 10−5 and 409.9 × 10−5 (mole fraction), respectively; and the minimum, in single water (0.6792 × 10−5 at 283.15 K). The Hansen solubility parameters were used here to illustrate the imidacloprid solubility behavior. Solubility modeling through Jouyban–Acree–van’t Hoff, mixture response surface (MRS), Jouyban–Acree and modified Wilson models achieved the relative average deviations of no higher than 3.01%. As well, the extended Hildebrand solubility approach analyzed the saturated solubility of imidacloprid at 298.15 K attaining the maximum of relative average deviation (RAD) of 1.16%. The key contributions of solution descriptors to the imidacloprid solubility were observed as dipolarity-polarizability and solubility parameter studied through molecular interactions of solvent-solvent and solute-solvent species to the imidacloprid solubility in terms of the linear solvation energy relationships. A quantitatively investigation upon local mole fractions was executed by utilizing the powerful tool of inverse Kirkwood–Buff integrals through considering several properties of solutions. The local mole fractions of n-propanol/acetone/ethanol/methanol were higher than that of the bulk ones in n-propanol/acetone/ethanol/methanol-rich and middle proportions, representing preferential solvation of the imidacloprid by co-solvents (n-propanol/acetone/ethanol/methanol). It is speculated within these regions imidacloprid acts as a Lewis acid with the molecules of co-solvents. Thermodynamic calculation of dissolution properties and enthalpy–entropy compensation study revealed an entropy-driven mechanism and endothermic process of imidacloprid dissolved in blended solvents. Transfer property analysis designated more advantageous solubilization ability over the intermediate and rich-co-solvent contents.