Solubility of 5-Fluorocytosine in Different Pure and Binary Mixed Solvents: Measurement, Model Correlation, Solvent Effect, and Preferential Solvation

In this work, the phase equilibrium solubility of 5-fluorocytosine (5-FC) in dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), 1-butanol, acetonitrile, 1-propanol, 1,4-dioxane, acetone, ethyl acetate, 2-propanol, 2-butanone, and mixture systems (DMSO + 2-propanol and DMF + 2-propanol) at a series of temperatures (283.15–323.15 K) is determined, and model correlation, solvent effect, and preferential solvation analysis are researched. The solubility of 5-FC is related to temperature, which decreases with decreasing temperature. For monosolvent systems, the mole solubility order is DMSO > DMF > 1-butanol > 1-propanol > 2-propanol > acetone > 2-butanone > 1,4-dioxane > acetonitrile > ethyl acetate. The solvent effect has been studied in monosolvents, hydrogen-bonding acceptor (HBA) interactions and nonspecific dipolarity/polarizability interactions favor dissolution, and solvent self-cohesiveness is not beneficial to dissolution. For two mixture systems, the mole solubility of 5-FC in DMSO + 2-propanol solvent systems is higher than that in DMF + 2-propanol solvent systems. A dimensionless parameter (δ) is defined to observe the deviation from ideality of mixed solvents whose values are all greater than zero. The equilibrium constant of the solvent exchange process (Kps) can be used to evaluate the preferential solvation of 5-FC. DMSO or DMF dissolves 5-FC in the local region around 5-FC preferentially. The equilibrium solubility data is correlated by four correlation models (modified Apelblat, λh, CNIBS/R-K, and Jouyban–Acree models). The greatest relative average deviation (RAD) and root-mean-square deviation (RMSD) values are 2.01% and 11.87 × 10–5 in pure solvent systems and 2.20% and 5.99 × 10–5 in mixture systems, respectively. These four models show excellent correlation with the experimental solubility data.