Determination and Model Correlation of Rhein in 12 Pure Solvents from 283.15 K to 323.15 K

In this work, the mole fraction solubility of rhein in 12 pure solvents including methanol, ethanol, n-propanol, isopropyl alcohol, 1-butanol, isobutyl alcohol, 1,4-dioxane, 2-butanone, cyclohexanone, acetone, acetonitrile, and N,N-dimethylformamide (DMF) at the temperature range from 283.15 K to 323.15 K was studied. The largest mole fraction solubility is observed in 1,4-dioxane (1.450 × 10–3) at 323.15 K, followed by in DMF (6.964 × 10–4), cyclohexanone (4.026 × 10–4), acetone (2.432 × 10–4), 2-butanone (2.009 × 10–4), 1-butanol (5.646 × 10–5), isopropyl alcohol (5.082 × 10–5), n-propanol (4.552 × 10–5), isobutyl alcohol (3.375 × 10–5), ethanol (3.798 × 10–5), methanol (3.083 × 10–5), and acetonitrile (1.655 × 10–5). The solubility first increases and then decreases with the increase of carbon chain length in alcohols. Through the analysis of 1,4-dioxane and DMF properties, the solute molecules should belong to weakly polar compounds, which is also reflected in the alcohols. However, for ketone solvents, the solubility decreases with the decrease of dipolarity/polarizability. This may be caused by the joint action of the solvent cavity term effect and hydrogen bonds. The largest relative average deviation (RAD) from the data is no more than 5%, which suggests that the calculated solubility data using the modified Apelblat model and λh model agreed very well with the experimental ones for each solvent. Compared with the values reported by Cheng and co-workers, there is an increase of 87.61-fold of solubility between 1,4-dioxane and acetonitrile at 323.15 K. Hence, the solubility data achieved in the present work could be very useful for the purification and crystallization of rhein.