Equilibrium Solubility, Model Correlation, and Solvent Effect of Indole-3-acetic Acid in Twelve Pure Solvents

The solid–liquid solubility of indole-3-acetic acid in pure methanol, ethanol, n-propanol, isopropanol, acetone, n-butanol, acetonitrile, chloroform, ethyl acetate, 1,4-dioxane, N,N-dimethylformamide (DMF), and dimethyl sulfoxide (DMSO) was measured by using a isothermal saturation method. The determination temperature range is from 278.15 to 323.15 K. The order of solubility data from high to low was ethyl acetate > DMSO > DMF > n-butanol > acetone > isopropanol > 1,4-dioxane > n-propanol > ethanol > methanol > acetonitrile > chloroform. Two thermodynamic models (modified Apelblat equation and λh equation) were used to correlate the solubility values of indole-3-acetic acid in different monosolvents, and the two equations can correlate the experimental values very well. The values of root-mean-square deviations (RMSDs) and relative average deviations (RADs) between the experimental and calculated solubility values were not exceeding 0.27 × 10–6 and 0.75 × 10–2, respectively. In order to study the effect of solvation interaction on solubility, the solute–solvent and solvent–solvent interactions were discussed.