RESEARCH NOTE pubs.acs.org/IECR
The Effect of Temperature on the Solubility of 11-Cyanoundecanoic Acid in Cyclohexane, n-Hexane, and Water Dongwei Wei,* Hui Li, Cheng Liu, and Baohe Wang Tianjin Research and Development Center for Petrochemical Technology, Tianjin University, Tianjin 300072, China ABSTRACT: The solubility of 11-cyanoundecanoic acid (11-CUA) in cyclohexane, n-hexane, and water was measured in the temperature range from 293.15 K to the melting temperature of the solute using a static analytical method. Enthalpy of fusion, ΔfusH, and melting temperature, Tm, were measured to be 38.33 kJ/mol and 329.85 K, respectively. The solubility data have been correlated as functions of the temperature using nonideal solution models, namely, the Buchowski (λh) equation and three local composition models (Wilson, NRTL, and UNIQUAC). The Buchowski equation gives good agreement with all experimental data. The results indicate that the molecular structure and interactions affect the solubilities significantly.
1. INTRODUCTION 1,10 -Peroxydicyclohexylamine may be converted into 11-cyanoundecanoic acid (11-CUA, NC(CH2)10COOH, MW 211.304, CAS Registry No. 5810-18-4) by the process of U.S. Pat. No. 3808240.1 The 11-cyanoundecanoic acid may be reacted further to give starting materials for the production of polyamides (e.g., Nylon 12). It is important that the monomers for the production of polyamides should be as pure as possible in order to obtain the optimum properties in the resulting polymer. It is therefore most important that the monomers and the materials from which the monomer are prepared should be as pure as possible. The process for recovering 11-CUA from the crude production by pyrolyzing 1,10 -peroxydicyclohexylamine may include distillation. However, since 11-CUA has low volatility and poor thermal stability, a substantial amount of 11-CUA is inevitably decomposed during the distillation period. This results in a low recovery yield of 11-CUA. Further, since the crude material contains impurities having a boiling point close to that of 11-CUA, it is difficult to obtain high purity 11-CUA. U.S. Pat. No. 4,165,328 discloses a process for separating 11-CUA, cyclohexanone, and ε-caprolactam from the oil-water mixture that contains the pyrolysis product of 1,10 -peroxydicyclohexylamine, by which the respective ingredients can be efficiently separated and the intended 11-CUA can be obtained with high purity.2 In accordance with the invention, the recovered 11-CUA is usually composed of 75% to 95% by weight of 11-CUA, 4% to 11% by weight of its isomeric compounds, and 0.1% to 0.2% by weight of tarry materials. To remove the isomers of 11-CUA and tarry materials, cyclohexane or n-hexane was found to be advantageous as the extraction solvent of 11-CUA because of its high extraction selectivity; that is, it dissolves little or no tarry materials. The extracted solution is cooled to a temperature, thereby to crystallize out only 11-CUA. Thus, 11-CUA of high purity and extremely reduced Hazen unit is obtained with high recovery. So far, no papers reported the solubility data of 11-CUA in cyclohexane or n-hexane. The aim of this paper is to explore the solubility of 11-CUA in cyclohexane, n-hexane, and water at several temperatures. This work also tests the capability of selected r 2010 American Chemical Society
solubility correlation models to describe the experimental data. Four nonideal solution models, namely the Buchowski (λh) equation and three local composition models (Wilson, NRTL, and UNIQUAC), were chosen to correlate the solubility to the solution temperature.
2. EXPERIMENTAL SECTION 2.1. Chemicals. 11-CUA, supplied by Sigma-Aldrich with purity of 98%, was further recrystallized from n-hexane and kept in a desiccator with dry silica gel. The purity of the sample after recrystallization was determined to be 0.995 in the mole fraction by a differential scanning calorimeter. Analytically pure grade cyclohexane and n-hexane, obtained from Tianjin Kewei Chemical Reagents, were dried with molecular sieves before use. The purity of the solvents was confirmed by gas chromatography to be >99.5%, and water content was determined by Karl Fisher titration to be n-hexane . water. 3.3. Data Regression Analysis. The dependence of 11-CUA solubility in pure solvent on the temperature can be described by many thermodynamics approximation methods. The Buchowski (λh) Equation. Buchowski et al. described the behavior of solid solubility in liquid as the Buchowski equation.6,7 Although only two parameters (λ and h) are involved, this equation is thermodynamically correct and gives an excellent description of experimental data without considering the activity coefficients of the components: λð1 - x1 Þ 1 1 ¼ λh ð1Þ ln 1 þ x1 T Tm
Figure 1. Experimental solubility of benzoic acid in water compared with literature data: O, this work; b, literature data.5
are estimated to be (0.2 K for the temperature and (2% for the enthalpy of fusion. Solubility Determination. The solubility was measured by a static equilibrium method that was described in our previous work, so only a brief presentation is made here.3,4 The experiments were carried out in a magnetically stirred, jacketed glass vessel (50 cm3). A constant temperature was maintained by circulating water through the outer jacket from a thermostatically controlled water bath. The actual value of temperature in the vessel was measured by a microthermometer (uncertainty of (0.1 K). Solutions with excess of solids were magnetically agitated for 24 h; a longer time had no effect on the solubilities. After the attainment of equilibrium, the sample of the upper portion was withdrawn, appropriately diluted, and analyzed by acid-base titration. Phenolphthalein was used as color indicator. Titration with 0.1 N NaOH solution continued until the first color change. The experimental setup and its accuracy were validated by comparing the experimental solubility data of benzoic acid in water with those in literature.5 As shown in Figure 1, the solubilities obtained in this work are in good agreement with literature values. The deviation of the measured solubilities from literature values was n-hexane > water. In light of the fact that we fit equations to data, the Buchowski (λh) equation and to generate equation parameters for dissolution of 11-CUA in the solvents studied herein. The average error percentage of the predicted solubility values obtained from calculations using the Buchowski equation was below 5%. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ NOMENCLATURE ΔcP = difference between the heat capacities of the solute in the solid and liquid states (J/mol-K) cLP = molar heat capacities of the liquid solute cSP = molar heat capacity solid solute Δg12 = energy parameter for NRTL equation (=g12-g22) (J/mol) Δg21 = energy parameter for NRTL equation (=g21-g11) (J/mol) ΔfusH = enthalpy of fusion at temperature Tm (J/mol) N = number of experimental data q = area parameter r = volume parameter R = gas constant (8.314 J/mol-K) T = temperature (K) Tm = normal melting temperature (K) Δu12 = energy parameter for UNIQUAC equation (= u12 - u22) (J/mol) Δu21 = energy parameter for UNIQUAC equation (= u21 - u11) (J/mol) v = molar volume (cm3/mol) x = mole fraction in the solution z = coordination number (= 10) Greek Letters
r12 = nonrandomness parameter j = volume fraction ϑ = area fraction γ = activity coefficient Δλ12 = energy parameter for Wilson equation (λ12 - λ11) (J/mol) Δλ21 = energy parameter for Wilson equation (λ21 - λ22) (J/mol)
RESEARCH NOTE
Subscripts
1 = solute (11-CUA) 2 = solvent Superscripts
cal = calculated value C = combinatorial id = ideal solution R = residual
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