Solid–liquid phase equilibrium and dissolution properties of ethyl vanillin in pure solvents
Introduction
As one of the important separation and purification methods, solution crystallization are widely used to make many kinds of products, such as pharmaceuticals, fine chemicals, spices and so on. The design and operation of crystallization processes will directly affect the quality of the final product, such as purity, yield, and crystal characteristics. The physiochemical properties, including solubility and dissolution thermodynamics, are the foundation of crystallization process. Compared with other process parameters, solubility of solute in different solvents will determine the selection of crystallization method and the optimization of crystallization process [1], [2], [3]. Therefore, it is crucial to know the accurate equilibrium solubility data of target product in different solvents.
Ethyl vanillin (EVA, CAS Registry NO. 121-32-4), 3-ethoxy-4-hydroxybenzaldehyde (C9H10O3, as shown in Fig. 1), is one kind of white or pale yellow acicular crystal. It has been widely used as the flavor additives in the foods, perfumes and commodity industries, and also as the medical intermediates for the pharmaceutical industry [4]. As one of the most valuable synthetic perfumes, EVA has gradually become a substitute for vanillin [5]. It sends out a strong chocolate flavor and the aroma is approximately 3.5 times as rich as the same amount of vanillin, which can reduce the cost and enhance economic efficiency of industrial production [6], [7], [8]. In the production of EVA, crystallization process is one of the important unit operation which will directly determine the quality of the final product. The thermodynamic data of EVA, such as solubility and dissolution enthalpy, is important for the designing and optimizing of the crystallization process of EVA. However, little information on the thermodynamic data of EVA can be found from literature.
In this work, the solubility data of EVA in different pure solvents (including methanol, ethanol, n-propanol, i-propanol, acetonitrile, acetone, methyl acetate and ethyl acetate) were obtained from 273.15 K to 318.15 K by the synthetic method [9], [10]. The Apelblat equation, the λh equation and two local composition models (Wilson and NRTL model) were chosen to correlate and analyze the experimental solubility data of EVA. In addition, to understand the dissolution behavior of EVA in the selected solvents, dissolution thermodynamic properties, such as the dissolution Gibbs energy, dissolution enthalpy and dissolution entropy, were calculated and discussed, basing on the experimental solubility data and the Wilson model.
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Materials
Ethyl vanillin (EVA) with purity (mass fraction) of 0.990, supplied by Aladdin Industrial Corporation, Shanghai, China, was used without further purification. All of the organic solvents used in this study, including methanol, ethanol, n-propanol, i-propanol, acetonitrile, acetone, methyl acetate and ethyl acetate, were purchased from Tianjin Jiangtian Chemical Co. of China with mass fraction purity higher than 0.995. The details, including source and purity of all the materials used in this
The modified Apelblat equation
The modified Apelblat equation is a widely used semi-empirical equation which was deduced from the Clausius-Clapeyron equation. It can be used to correlate the solid-liquid equilibrium and the equation is shown as following [15], [16].where x1 is the mole fraction solubility of solute, T refers to the absolute temperature. A, B and C are empirical parameters. A and B refer to variation of activity coefficient in real solution, and C represents the effect of temperature on the
The characterization of samples
The PXRD pattern of all the EVA samples were found to be the same, which is given in Fig. 2. It confirmed that all the samples of EVA used in this study are the same crystalline form with high crystallinity, and the EVA have characteristic peaks at 2θ of 12.10°, 20.94°, 22.54°, 26.24° and 37.22°.
The DSC analysis results of EVA used in this study are shown in Fig. 3. It can be seen from the DSC data that the melting temperature (352.98 K) is in agreement with the literature value Tm = 350.15–352.15
Conclusions
In this study, the solubility of EVA in eight pure solvents were measured at the temperatures ranging from 273.15 K to 318.15 K by using a static analytical method. The solubility data of EVA in all selected solvents increase with the increasing of temperature. Furthermore, the experimental solubility data were correlated and analyzed by four thermodynamic models. The correlated values are in good agreement with the experimental values throughout the entire range of temperature tested. Finally,
Conflict of interest
The authors declare no competing financial interest.
Acknowledgments
The authors are very grateful to the financial supported from National Natural Science Foundation of China (No. 51478308) and Major National Scientific Instrument Development Project (No. 21527812).
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