Solubilities of p-coumaric and caffeic acid in ionic liquids and organic solvents
Graphical abstract
Highlights
► New solubility data of p-coumaric and caffeic acid in ionic liquids and organic solvents. ► Normal melting point temperature and enthalpy of fusion of p-coumaric and caffeic acid. ► Thermogravimetric analysis for p-coumaric and caffeic acid. ► Correlation with UNIQUAC and NRTL.
Introduction
Cinnamic acid derivatives (CADs), such as p-coumaric acid and caffeic acid, are natural antioxidants belonging to the family of phenolic acids and are widely distributed throughout the plant kingdom. Fruits, vegetables, spices, aromatic herbs, cereals, coffee beans and beverages, olives, propolis, sunflower, barks and wine are natural sources of these compounds [1], [2], [3], [4]. Due to their almost universal distribution, they are an integral part of the human diet. Epidemiological studies suggest a link between the consumption of whole grain products containing CADs and prevention of chronic, degenerative diseases associated with oxidative damage (namely coronary heart diseases, cardiovascular diseases, diabetes, arthritis, cataract formation, aging and cancer) [5], [6]. Many of the health protective effects of phenolic compounds have been ascribed to their antioxidant, antimutagenic, anti-inflammatory, antimicrobial, antiviral, antialergic, immunoprotective, ultraviolet (UV) filtering properties and other biological and pharmacological activities [7], [8]. The above benefits in combination with a growing concern about the safety of synthetic antioxidants [9] make CADs high value raw materials for the synthesis of different molecules with industrial interest, such as drugs, cosmetics, antiseptics and flavors [10].
Common problems encountered in chemical synthesis, recovery and separation processes, such as the use of toxic, volatile and flammable organic solvents as well as difficulties in product purification can be handled with the use of ionic liquids (ILs); a new class of solvents that can lead to more environmentally friendly applications and processes as compared to those where conventional organic solvents are used. This characteristic arises mainly from their negligible vapor pressures at room temperature and therefore low volatility and flammability, which minimizes health and safety risk in industry and the chance of loss to atmosphere making their recycling and reusability feasible. Of course, the development of cost and energy efficient technologies for solute recovery from ionic liquids is a challenge towards their industrial application.
Ionic liquids are molten salts, consisting of large asymmetric organic cations and organic or inorganic anions. The low symmetry, high vibrational freedom and charge delocalization of the ions composing an IL reduce the stability of the crystalline phases, and thus their melting temperatures [11]. Unlike molecular liquids, their ionic nature results in a unique combination of distinctive properties, such as high thermal stabilities, large liquidus range and high solvating capacity for organic, inorganic and organometallic compounds [12], [13]. The last is one of the most attractive features of ILs especially for recovery and separation processes, as well as final product purification. It results from ILs’ ability to be tailormade for a specific purpose by careful selection among a huge diversity of cations and anions, or substitutes to the cation. Therefore, ILs are often referred to as “designer solvents” [14].
For the design and optimization of processes where CADs are involved, suitable solvents – either classical organic ones or ionic liquids – have to be selected. The successful completion of such a task requires the knowledge of reliable phase equilibrium data. With reference to solubilities of antioxidants in organic solvents, there are some data in the literature, such as solubility of flavonoids in acetonitrile, acetone and tert-amyl alcohol [15], or solubility of luteolin in methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, acetone, hexane, and DMSO [16], but for phenolic acids only their solubility in water has been measured [8], [17]. Concerning ILs, although much experimental work has been conducted so far on measurements of thermophysical properties of the pure ILs and phase equilibrium in systems containing ILs, solubility data of antioxidants in ILs are very scarce [18], [19], [20]. Some other works have also reported solubility data in ILs of antibiotics [21], dibasic carboxylic acids [22], drugs [23], [24], [25] and inorganic salts [26].
In this work, the solubilities of two CADs, namely p-coumaric acid and caffeic acid, were measured in six imidazolium based ILs composed of the PF6−, BF4−, TF2N− and TFO− anions and they were compared to those measured in two organic solvents, t-pentanol and ethyl acetate. In addition, the normal melting point temperature and the enthalpy of fusion of the two acids were determined by differential scanning calorimentry and their possible decomposition upon melting was tested by thermogravimetric analysis. Moreover, by utilizing the solubility data and the van’t Hoff equations, the apparent thermodynamic functions relative to the solution of the two CADs have been determined. Finally, the capability of the UNIQUAC and the NRTL activity coefficient models to correlate the solubility data was tested and two UNIFAC versions were used to predict the solubilities of the two acids in the organic solvents.
Section snippets
Materials
p-Coumaric acid (p-CA; purity > 98%; CAS No. 501-98-4; C9H8O3) was purchased from Sigma Aldrich Co. Caffeic acid (CA, purity > 98% CAS No. 331-39-5; C9H8O4) was purchased from Acros Organics. 2-methyl-2-butanol (t-pentanol; purity > 99%; CAS No. 75-85-4; C5H12O) was purchased from Sigma Aldrich Co. Ethyl acetate (purity > 99.8%; CAS No. 141-78-6; C4H8O2) was purchased from Merck KGaA. 1-butyl-3-methyl-imidazolium hexafluorophosphate (bmimPF6; purity > 98%; CAS No. 174501-64-5; C8H15F6N2P),
Normal melting point temperatures and heats of fusion
The DSC curves for p-coumaric and caffeic acid, presented in figure 2, show an endothermic peak resulting most probably from melting procedure. The onset temperature determined by DSC method should correspond to normal melting temperature of each acid. However, as shown in the DSC curve of caffeic acid, the endothermic peak is followed by an exothermic one, indicating that another phenomenon except from melting occurs either just after melting is complete or even simultaneously.
The TgA curves
Thermodynamic functions of solution
According to van’t Hoff analysis, the apparent standard enthalpy change in solution is obtained from the slope of a ln x2 versus 1/T plot, where x2 is the solute solubility in mole fraction. In recent thermodynamic treatments, some modifications have been introduced to the van’t Hoff treatment to transform the intercept facilitating their use in thermodynamic calculations. According to the Krug et al. [41], [42] approach, the following modified van’t Hoff expression is used:
Thermodynamic modeling
For the calculation of solid solubility in mole fraction, x2, in a solvent, the following standard thermodynamic equation is applied:where γ2, ΔfusH, and Tm stand for the activity coefficient, the enthalpy of fusion, and the melting temperature of the solid solute, whereas ΔfusCp is the difference between the heat capacity of the solid and that of the subcooled liquid at the melting temperature.
By neglecting the terms that include ΔfusCp, equation
Conclusions
The solubilities of p-coumaric acid and caffeic acid have been measured in six alkyl-substituted imidazolium-based ILs composed of the BF4−, PF6−, TF2N− and TFO− anions and in two organic solvents; t-pentanol and ethyl acetate. The results showed that the BF4− and TFO− – based ILs are better solvents than the organic ones, which in turn are better than the corresponding PF6− and TF2N− – based ILs. Furthermore, it is noticed that the increase of the alkyl chain length on the cation leads to a
Acknowledgements
The financial support of the National Technical University of Athens, Greece (program “PEVE 2009” for basic research) is gratefully acknowledged.
References (49)
- et al.
Free Radical Biol. Med.
(1996) - et al.
J. Chem. Thermodyn.
(2006) - et al.
Fluid Phase Equilib.
(2004) - et al.
J. Chem. Thermodyn.
(2012) - et al.
Fluid Phase Equilib.
(2007) - et al.
Fluid Phase Equilib.
(2012) - et al.
J. Ind. Eng. Chem.
(2010) - et al.
J. Supercrit. Fluids
(2007) - et al.
J. Supercrit. Fluids
(2003) - et al.
Anal. Chim. Acta
(1989)
Chem. Phys. Lett.
J. Sci. Food Agric.
J. Agric. Food Chem.
J. Agric. Food Chem.
J. Agric. Food Chem.
J. Am. Coll. Nutr.
Am. J. Clin. Nutr.
J. Sci. Food Agric.
J. Chem. Eng. Data.
Energy Fuels
Chem. – A Eur. J.
J. Chem. Eng. Data
J. Chem. Eng. Data
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