Thermodynamic models for determination of the solubility of dl-malic acid in methanol plus (acetonitrile, N,N-dimethylformamide, isopropyl alcohol) binary solvent mixtures
Introduction
dl-Malic acid (C4H6O5, CAS RN: 6915-15-7, shown in figure 1), a white crystalline powder, is widely used as an important intermediate in the process of tricarboxylic acid cycle,[1], [2] which can be used as food additives, pharmaceutical intermediates, cosmetics, rinses, metal cleaners, buffering agents, retarders in the textile industry and fluorescent whitening agent of polyester fiber [3], [4]. In industrial mass production, dl-malic acid is chemically synthesized by hydration of either maleic or fumaric acid at high temperature and pressure [1]. Solution crystallization is commonly used as the final separation and purification step in its production process, where the accurate values of solubility of dl-malic acid as a function of the temperature are essential [5]. The solubility of organic compounds in different solvents plays an important role for understanding the (solid + liquid) equilibria (SLE) or phase equilibria in the development of a crystallization process, or (liquid + liquid) equilibria in extraction and extractive or azeotropic distillation processes [6]. More particularly, the knowledge of accurate solubility is needed for the design of separation processes such as extractive crystallization and the safety of operating different processing units such as distillation columns, absorption units, and extraction plants. The solubility of dl-malic acid can also supply basic values for industrial production. To determine proper solvents and to design an optimized production process, it is necessary to know the solubility of dl-malic acid in different solvents [7], [8]. To our knowledge, we find no report of the solubility of dl-malic acid in methanol plus (acetonitrile, N,N-dimethylformamide, isopropyl alcohol) binary solvent mixtures.
In this work, the solubility of dl-malic acid in methanol plus (acetonitrile, N,N-dimethylformamide, isopropyl alcohol) binary solvent mixtures was measured from T = (278.15 to 328.15) K under atmosphere pressure (101.3 kPa). The modified Apelblat equation and Buchowski–Ksiazaczak model were applied to correlate with the experimental results. This is the first attempt at modeling the solubility of dl-malic acid in methanol plus (acetonitrile, N,N-dimethylformamide, isopropyl alcohol) binary solvent mixtures using these specific thermodynamic models. The thermodynamic properties of the dissolution process, including enthalpy, entropy and Gibbs free energy, were calculated by means of the van’t Hoff analysis and Gibbs equation.
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Materials
dl-Malic acid (mass fraction purity 0.99) was purchased from Aladdin (China). Its purity was measured by high performance liquid chromatography (HPLC type DIONEX P680 DIONEX Technologies), and the melting point of dl-malic acid was measured by digital melting point system (type WRS-1B, Shanghai Precision & Scientific Instrument Co., Ltd.) at T = 403.15 K. This melting point falls within the range of (402.65 to 403.65) K values reported in the literature [9], [10], [11]. Methanol, acetonitrile,
Solubility data and thermodynamic models
The solubility of dl-malic acid (x) in methanol plus (acetonitrile, N,N-dimethylformamide, isopropyl alcohol) binary solvent mixtures with the temperature ranging from (278.15 to 328.15) K are presented in TABLE 2, TABLE 3, TABLE 4, and graphically showed in FIGURE 2, FIGURE 3, FIGURE 4.
Modified Apelblat equation
The changing trends of solubility against temperature in the solvent with same ratio are described by modified Apelblat equation. This model is firstly used by Apelblat [15], [16], which can give a relatively
Conclusions
The solubility of dl-malic acid in binary solvent mixtures consisting of (methanol + acetonitrile); (methanol + N,N-dimethylformamide); and (methanol + isopropyl alcohol) at temperatures ranging from (278.15 to 328.15) K were measured by the gravimetric method. The results presented in this paper may be useful, allowing later scholars to reach a better understanding of this thermodynamic behavior in the systems studied, which is useful in many fields. It is obvious that the solubility increases with
Acknowledgements
This research work was financially supported by NSFC (Natural Science Foundation of China) (No. 31471692), Agricultural science and technology achievements transformation projects (2014GB2C100317) and Major projects supported by the natural science foundation of universities in Jiangsu province (14KJA180001).
We thank the editor and the anonymous reviewers.
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