Solubility and thermodynamic properties of flonicamid in pure and binary solvents in the temperature range of 283.15–323.15 K

The solubility data of flonicamid (FA) in twelve mono-solvents (methanol (MA), ethanol (EA), n-propanol (NPA), isopropanol (IPA), n-butanol (NBA), isobutanol (IBA), ethyl acetate (EAC), butyl acetate (BAC), acetone (AT), acetonitrile (AN), tetrahydrofuran (THF), 1,4-dioxane (DX)) and a binary solvent of EA + EAC were experimentally measured by using a gravimetric method under atmospheric pressure. The tested temperature range for the studied solvents was from 283.15 K to 323.15 K except of 288.15–323.15 K for DX. Powder X-ray diffraction (PXRD) was applied to evaluate the crystal form of FA, and no solvate or new form formation were observed during the whole experiment. The experimental results show that the rising temperature leads to the increase of the FA solubility in all the studied mono and mixed solvents at the tested ranges. For mono-solvents, the solubility of FA at 293.15 K increases in the order: IBA (0.002641) < IPA (0.004243) < NPA (0.004361) < NBA (0.004750) < BAC (0.006844) < EA (0.006864) < EAC (0.01342) < MA (0.01394) < DX (0.01747) < AN (0.02455) < THF (0.03433) < AT (0.04645). For EA + EAC mixed solvents, the solubility of FA increases and then decreases with the rising mole fraction of EAC. That means addition of EAC to EA can alter the FA solubility. Density functional theory (DFT) calculations were performed to analyze the interactive energy between FA and the solvent molecules. Furthermore, the Apelblat equation, van’t Hoff equation, λh equation are applied to correlate the experimental solubility data of FA in mono and mixed solvent systems, while NRTL model and Wilson model are selected for mono-solvents, CNIBS/R-K model and Jouyban-Acree model for mixed solvents. Correlated results demonstrate that the Apelblat model provides the best regression. In addition, the dissolution thermodynamic properties of FA in chosen solvents are analyzed by the van’t Hoff equation. It suggests that the dissolution processes of FA are non-spontaneous, endothermic and entropy increase in these studied systems.