Solid–liquid equilibrium of dicyandiamide in different solvents
Introduction
As a low-cost, high-efficiency separation and purification process, crystallization is widely used in fine chemical, pharmaceutical, biological as well as environmental industries. Accurate solubility data are required for process design to obtain products with desired qualities.
Solubility of materials is one of the most fundamental physiochemical properties, which depends several factors such as chemical composition, temperature, the pH, the presence of additional species in the solution and the use of different solvents [1]. Especially when dealing with organic species (or inorganics in nonaqueous solvents) a wide variety of solvents and solvent mixtures can be employed. Although there is a vast amount of literature reporting the solubility of many binary and ternary systems in aqueous solution [2], [3], [4], [5], many more combinations of solvent and solute remain to be investigated. This is because the measurement of solubility using the current experimental method is time consuming and usually requires large amount of pure solute which is often unavailable or can be very expensive.
The cyano substituted quinidine, dicyandiamide (molecular weight 84.08), is widely used as a slow fertilizer. Moreover, it can be used as a latent curing agent in heat-cured epoxy resins for laminates or prepreg fabrication, coatings and adhesives [6], [7]. Fig. 1 shows the chemical structure of dicyandiamide. In industrial manufacturing, dicyandiamide goes through several purification and separation processes to purify it; in such processes, solution crystallization and further recrystallization are the key steps. The determination of its solubility in different solvents is then essential for rapid design and optimization of isolation, purification and formulation processes in industry. A literature survey on available solubility data of dicyandiamide in different solvents reveals no systematic study [8], [9]. The present papers relating to dicyandiamide are mainly concerned with its production and application. Given this situation, to ascertain the suitable solvent, a systematic determination of the solubility of dicyandiamide in potential solvents is necessary.
In the present work, experimental solubility data for dicyandiamide in five different solvents, water, methanol, ethanol, glycol and acetone, were determined at temperatures ranging from 273.15 to 338.95 K at atmospheric pressure using a gravimetric method. Additionally, the melting temperature, enthalpy change of fusion and differential molar heat capacity at the melting point of dicyandiamide were measured using differential scanning calorimetry (DSC). Further, these data were used to calculate the ideal solubility of dicyandiamide and estimate its activity coefficient in different solvents.
Section snippets
Theory
The temperature dependence of mole fraction equilibrium solubility of crystalline can be described by the following thermodynamic relationship [10]:where x1, and γ1 represent the mole fraction solubility of the solute 1, ideal mole fraction solubility of the solute and activity coefficient of the solute in solution, respectively.
The ideal solubility of a crystalline solute in liquid solvent in Eq. (1) is given bywhere ΔHf refers
Materials
Dicyandiamide (supplied by Ningxia Beilite Chemical Industry Co., Ltd., China) was recrystallized one time from double-distilled water by cooling crystallization. Its purity was analyzed by a HPLC equipped with a UV detector (HP 1100 America) and the mass fraction purity is >0.95. The standard uncertainty for mass fraction is ±0.005. The methanol, ethanol, glycol and acetone used for experiments were of analytical reagent grade, all supplied by Shanghai Chemical Reagent Co., China and used
Dicyandiamide solubility
The solubility measurement technique in this study was validated by comparing the temperature-dependent mole fraction of dicyandiamide in water and ethanol, with results reported by Hetherington et al. and Ren et al. [8], [9] (shown in Fig. 2). It can be noticed that the measured solubility data are in good agreement with those obtained in references [8], [9].
Table 2 and Fig. 2 show the measured dicyandiamide solubility in different solvents. The results show that, for all cases, dicyandiamide
Conclusions
The solubility of dicyandiamide in water, methanol, ethanol, glycol and acetone were determined at temperatures ranging from 273.15 to 338.95 K at atmospheric pressure using gravimetric method. The melting temperature, Tm, molar enthalpy of fusion at the melting point, ΔHf, and the differential heat capacity ΔCp of dicyandiamide were also measured using DSC. Subsequently, ideal solubility of dicyandiamide was calculated based on Eqs. (2), (4), respectively. On this basis, influence of the
Acknowledgments
We thank the financial support by the Doctoral Fund of Ministry of Education of China (2011M500511) and the 111 Project of Ministry of Education of China (B08021) for their financial assistances in this project.
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