Measurement and Correlation for Solubility of Diosgenin in Some Mixed Solvents*

(陳飛雄)(趙明蕊)(馮露)(任保增),**

1College of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou 450001, China

2Henan Medical College for Staff and Workers, Zhengzhou 451191, China

Measurement and Correlation for Solubility of Diosgenin in Some Mixed Solvents*

CHEN Feixiong(陳飛雄)1, ZHAO Mingrui(趙明蕊)2, FENG Lu(馮露)1and REN Baozeng(任保增)1,**

1College of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou 450001, China

2Henan Medical College for Staff and Workers, Zhengzhou 451191, China

The solubility data of diosgenin in mixed systems of ethanol + 1-propanol (1︰1), ethanol + 1-butanol (1︰1), ethanol + isobutyl alcohol (1︰1), methanol + isobutyl alcohol (1︰1), methanol + isobutyl alcohol (1︰4), ethanol + 1-pentanol (1︰1) and carbon tetrachloride were measured over the temperature range from 289.15 K to 334.15 K by a laser monitoring observation technique at atmospheric pressure, with all mixtures mixed by volume ratio. The Apelblat equation, the ideal solution model, and the λh equation are used to correlate the solubility data. The results show that the three models agree well with the experimental data, providing essential support for industrial design and further theoretical study.

solid-liquid equilibrium, solubility, diosgenin

1 INTRODUCTION

Diosgenin (3-O-{β-D-glucopyranosyl-(1→3)-[β-D-glucopyranosyl-(1→6)]-β-D-glucopyranosyl-(1→4)-[α-L-rhamnopyranosyl-(1→2)-β-D-glucopyranoside}, CAS RN 512-06-1) is an important starting material in the steroidal hormone industry. Diosgenin possesses many biological activities, such as oral contraceptives, sex hormones and other steroids [1-4]. It is used primarily as a precursor for the synthesis of steroidal drugs. The solubility of binary system of diosgenin has been reported [5, 6], but its solubility data for ternary systems are not available in literature. In order to obtain high-yield purification of diosgenin and exploit new synthesis method, engineering design and industrial production for diosgenin, we measure the solubility of diosgenin in ethanol + 1-propanol (1︰1), ethanol + 1-butanol (1︰1), ethanol + isobutyl alcohol (1︰1), methanol + isobutyl alcohol (1︰1), methanol + isobutyl alcohol (1︰4), ethanol + 1-pentanol (1︰1) and carbon tetrachloride in the temperature range from 289.15 K to 334.15 K by using laser monitoring technology in this study, with the systems mixed by volume ratio. The experimental data are correlated with the λh model, Apelblat equation and the ideal solution model.

2 EXPERIMENTAL

2.1 Materials

Diosgenin crystals (mass fraction purity ≥99.5%) used in the experiments were purchased from Zhengzhou Lion Technology Co., Ltd. Ethanol was purchased from Zhengzhou Dezhong Chemical Regent Factory with mass fraction purity of 99.7%. 1-Propanol and 1-pentanol were purchased from Tianjin GuangFu Fine Chemical Research Institute with mass fraction purity of 99.8% and 95%, respectively. 1-Butanol was purchased from Tianjin GuangFu Technology Development Co., Ltd. with mass fraction purity of 99.5%. Isobutyl alcohol and methanol were purchased from Tianjin Kermel Chemical Reagent Co., Ltd. with mass fraction purity of 99%. All the solvents are of AR grade.

2.2 Procedure

The solubility apparatus and method were the same as those in our previous work [5]. The solubility of a solid in a solvent was measured by a synthetic method [7, 8]. To testify the uncertainty of the measurement, the experimental solubility data for benzoic acid in water were compared with literature values [9], and the results were given in the supplementary material (see Appendix Fig. A1). The average relative error of this system is less than 0.02, so this experimental technique is reliable.

3 RESULTS AND DISCUSSION

The λh model developed by Buchowski et al. [8] is a semi-empirical equation,

where λ and h are the model parameters determined by experimental data and listed in Table 1, χ is the mole fraction of the solubility at temperature T, and Tmis the normal melting temperature (K).

The relationship between mole fraction of the solubility and temperature is generally expressed by the Apelblat equation, which can be deduced from Clausius-Clapeyron equation [10],

where A, B and C are empirical constants, and χ is the mole fraction of the solubility at temperature T.

The simplified model for ideal solution is a universal equation for the solid-liquid equilibrium [11] based on the thermodynamic principles. For ideal solution (γi=1), the model is written as

where A and B are model parameters, and χ is the mole fraction of the solubility at temperature T.

The solubility data of diosgenin in some mixed solvents are listed in Table 2. The experimental pointsand calculated values are shown in Figs. 1-3, where T is the absolute temperature, and χ(exp) and χ(cal) are the experimental and calculated mole fractions of the solubility, respectively. Moreover, χ(λh), χ(alp) and χ(ideal) are the calculation values from the λh model, the Apelblat equation and the ideal solution equation, respectively. ADD is the relative error defined as follows:

Table 1 Parameters of Eqs. (1)-(3) for diosgenin in some mixed solvents

Table 2 Mole fraction solubility χ of diosgenin in some mixed solvents

Table 2 (Continued)

Table 2 (Continued)

Table 2 (Continued)

Figure 1 Mole fraction solubility χ of diosgenin in mixed solvents and calculated values from the Apelblat equationLines: calculated values from the equation; ◇ carbon tetrachloride;ethanol + isobutyl alcohol (1︰1); △ methanol + isobutyl alcohol (1︰4);methanol + isobutyl alcohol (1︰1);ethanol + 1-butanol (1︰1); ○ ethanol + 1-propanol (1︰1);□ ethanol + 1-pentanol (1︰1); mixed by volume ratio

Figure 2 Mole fraction solubility χ of diosgenin in mixed solvents and calculated values from the ideal solution modelLines: calculated values from the model; ◇ carbon tetrachloride;ethanol + isobutyl alcohol (1︰1); △ methanol + isobutyl alcohol (1︰4);methanol + isobutyl alcohol (1︰1);ethanol + 1-butanol (1︰1); ○ ethanol + 1-propanol (1︰1);□ ethanol + 1-pentanol (1︰1); mixed by volume ratio

The average absolute deviation (AAD) is

Figure 3 Mole fraction solubility χ of diosgenin in mixed solvents and calculated values from the λh modelLines: calculated values from the model; ◇ carbon tetrachloride;ethanol + isobutyl alcohol (1︰1); △ methanol + isobutyl alcohol (1︰4);methanol + isobutyl alcohol (1︰1);ethanol + 1-butanol (1︰1); ○ ethanol + 1-propanol (1︰1);□ ethanol + 1-pentanol (1︰1); mixed by volume ratio

The root-mean square deviations (RMSD) is defined as

where n is the number of experimental points.

According to R2values in Table 1, RMSD and AAD in Table 2, and Figs. 1-3, it can be found that: (1) the solubilities of diosgenin in ethanol + 1-propanol (1︰1), ethanol + 1-butanol (1︰1), ethanol + isobutyl alcohol (1︰1), methanol + isobutyl alcohol (1︰1), methanol + isobutyl alcohol (1︰4), ethanol + 1-pentanol (1︰1) and carbon tetrachloride over the temperature range from 289.15 K to 334.15 K have the same trend; (2) the solubility has the following order: ethanol + 1-pentanol (1︰1)>carbon tetrachloride>ethanol + 1-butanol (1︰1)> ethanol + 1-propanol (1︰1)>ethanol + isobutyl alcohol (1︰1), so it decreases as the polarity of the solvent in the mixed system increases; (3) compared with the solubility of diosgenin in alcohol solvents [6], its solubility has the following order: 1-propanol>ethanol + 1-propanol (1︰1)>ethanol, and isobutyl alcohol> ethanol + isobutyl alcohol (1︰1)>ethanol. However, its solubility in the mixture of ethanol + 1-pentanol (1︰1) is better than that in both ethanol and 1-pentanol, especially better than that in the non-polar solvent, carbon tetrachloride, the mechanism of which needs to be further studied; (4) the solubility decreases with increasing concentration of methanol in the solution of isobutyl alcohol, with the following order: methanol + isobutyl alcohol (1︰4)>methanol + isobutyl alcohol (1︰1)>methanol [5]; (5) the Apelblat equation is more accurate than other two models for the mixture of ethanol + 1-propanol (1︰1), methanol + isobutyl alcohol (1︰4), ethanol + 1-pentanol (1︰1) and carbon tetrachloride. Ethanol + isobutyl alcohol (1︰1) and methanol + isobutyl alcohol (1︰1) agree well with λh equation, while ethanol + 1-butanol (1︰1) agrees well with the ideal solution model.

4 CONCLUSIONS

New experimental results for the solubility of diosgenin in mixed systems of ethanol + 1-propanol (1︰1), ethanol + 1-butanol (1︰1), ethanol + isobutyl alcohol (1︰1), methanol + isobutyl alcohol (1︰1), methanol + isobutyl alcohol (1︰4), ethanol + 1-pentanol (1︰1) and carbon tetrachloride are presented. The Apelblat equation is more accurate than the λh equation and the ideal solution model for the mixed systems of ethanol + 1-propanol (1︰1), methanol + isobutyl alcohol (1︰4), ethanol + 1-pentanol (1︰1) and carbon tetrachloride. Ethanol + isobutyl alcohol (1︰1) and methanol + isobutyl alcohol (1︰1) agrees well with the λh equation. Ethanol + 1-butanol (1︰1) agrees well with the ideal solution model. These models could be used to correlate the solubility data of diosgenin in industrial production. The mixed solvents and carbon tetrachloride are beneficial for the crystallization of diosgenin, which will provide essential support for industrial design.

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APPENDIX

Figure A1 Solubility of benzoic acid in water

Received 2012-04-26, accepted 2012-11-02.

* Supported by Science and Technology Breakthrough Major Project in Henan Province (112101210200).

** To whom correspondence should be addressed. E-mail: renbz@zzu.edu.cn