Solubility and thermodynamic properties of an energetic intermediate in four binary solvents (water + n-butanol, n-pentanol, isobutanol and isoamyl alcohol)

Abstract The solubility of 4-nitropyrazole (4-NP) in four different binary solvents (i.e., water + n-butanol, water + n-pentanol, water + isobutanol, and water + isoamyl alcohol) was firstly determined by a gravimetric method within a temperature range of 288.15 K to 328.15 K at atmospheric pressure (P = 0.1 MPa). The experimental results indicated that the solubility of 4-NP in selected binary solvents increases with the increasing temperature and decreases with the rising water mass-fraction ranging. Besides, the (solid + liquid) equilibrium values in the studied solvents were correlated based on the modified Apelblat equation, Jouyban-Acree model, CNIBS/R-K model, and Wilson model. Moreover, all of the 324 experimental values were used to correlate the parameters of the Wilson model, and the correlated results were superior to the other three models by comparing the average 104RMSD and R2. The average root-mean-square deviation (104RMSD) values were 1.87, 31.83, 6.04, and 1.18, respectively. In addition, Hansen solubility parameters were employed to provide a reasonable illustration for the solubility order of 4-NP in the 4 binary mixed solvents, and the order of 4-NP solubility in each binary solvent mixture was the result of the combined effects of hydrogen bonding, dispersion, and polarity. Furthermore, the Wilson model was applied to calculate the thermodynamic properties of 4-NP including mixing enthalpy, mixing entropy, and mixing free Gibbs energy in the studied binary solvent mixtures at a given temperature. It is easy to understand that dissolution is an exothermic process from the results. Solid-liquid surface tension (γ) and surface entropy factor (f) of 4-NP were estimated by using the experimental solubility data, and the results showed that γ and f increase with the increasing mass-fraction of water content in the binary mixed solvent and decrease with the increasing temperature. The experimental solubility values, model parameters, and thermodynamic properties of 4-NP in different mixed solvents will provide the necessary support for its preparation, crystallization process, and further theoretical studies.