Ferulic acid solubility in supercritical carbon dioxide, ethanol and water mixtures
Introduction
Research on natural products with therapeutic benefits to the consumer has gained prominence in recent years. These products are usually plant extracts rich in bioactive compounds such as phenolic compounds. Phenolic compounds, also known as polyphenols, are secondary metabolites of different vegetable species. Depending on their structure, they can be subdivided in different classes of compounds, including simple phenols, phenolic acids, flavonoids and tannins [1]. Phenolic acids are characterized by having a benzene ring, a carboxylic acid group and one or more hydroxyl and/or methoxyl groups in the molecule. The structural characteristics of these acids confer to them antioxidant properties, enhancing the health benefits and contributing to human body well-being [2].
Among the phenolic compounds is ferulic acid (FA), whose structure is shown in Fig. 1. FA is one of the most abundant phenolic acids in nature and it is present in grains (oats, barley, rye, corn and wheat), fruits (grape, orange, banana, tomatoes), vegetables (beets, spinach, radish) and drinks (coffee, yerba mate tea, orange juice) [3], [4], [5], [6]. This compound exhibits a variety of biomedical, pharmaceutical and industrial applications [7], and its presence is associated with anti-inflammatory, antimicrobial, anti-allergic and anti-cancer activities in addition to the prevention of cardiovascular diseases [7], [8], [9].
Supercritical technology is an interesting alternative widely used to obtain natural extracts. Extractions using supercritical carbon dioxide (scCO2) have some advantages over traditional extraction techniques, since they generate extracts free of toxic residues. Furthermore, carbon dioxide is cheap, inert, non-flammable and presents low critical temperature and pressure values, making attractive for extraction and fractionation of thermosensitive compounds. However, extraction with scCO2 is appropriate to obtain nonpolar or low polarity compounds, while the use of co-solvents is necessary to extract compounds of higher polarity.
Knowledge on solubility of compounds in pure scCO2 or in scCO2 with the presence of co-solvents is important for optimizing the processes employing scCO2 as a solvent or anti-solvent. The behaviour of ferulic acid in scCO2 was studied in prior works [10], [11], [12] and solubility values reported are relatively low, between 10−6 and 10−5 in mole fraction. Therefore, the use of co-solvents is required when extracts enriched in this compound are desired. Ethanol and water are the major co-solvents used in supercritical extraction because they are also “generally recognized as safe” (GRAS) and “green”.
Equilibrium data of systems containing phenolic compounds, CO2 and ethanol are rare. Studies with catechin [13], resveratrol [14], epicatechin [15], quercetin [16] and trans-cinnamic acid [17] are reported in literature. Moreover, thermodynamic models commonly used to describe systems containing supercritical fluids are derived from the cubic equation proposed by van der Waals in 1873, such as the Soave-Redlich-Kwong [18] and Peng-Robinson [19] equations.
In this context, the present paper provides a phase equilibrium study of the {scCO2 (1) + ethanol (2) + water (3) + ferulic acid (4)} system and sets out the thermodynamic modelling using the Peng-Robinson equation by fit of the binary systems and the ability of this equation to predict multicomponent system values.
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Materials
Suppliers and purity of the materials used in this work are listed in Table 1. None of them was subjected to further purification. Ferulic acid purchased from Sigma-Aldrich is the trans-4-hydroxy-3-methoxycinnamic acid, which has CAS number 537-98-4.
Measurements of solubility in ethanol and water
FA solubility in ethanol and water was evaluated at atmospheric pressure (94.3 kPa) using a 50 mL glass cell surrounded by a jacket with water circulation from a thermostatic bath to maintain the temperature. The system was prepared by adding a
FA solubility in ethanol and water
FA solubilities in ethanol and water were measured at nine different temperatures (293–333) K and the results are shown in Table 3. The values of solubility in ethanol ranged from 0.023 to 0.058 (mole fraction), therefore an increase of approximately 150% was observed when increasing the temperature from 293 K to 333 K. FA solubility values in ethanol are scarce in the literature. Buranov and Mazza [27] reported FA solubility in ethanol at room temperature of 65 mg·mL−1 (≈0.0195 mol fraction). Zhuo
Conclusion
Experimental values of ferulic acid solubility in scCO2 in the presence of ethanol and water are reported in this work. Small amounts of ethanol or the ethanol/water mixture as a co-solvent (≈10%) produced a considerable increase in FA solubility compared to pure scCO2. Furthermore, FA solubility reached higher values when pure ethanol was used as a co-solvent.
Although the fitting of the binary interaction parameter present low deviations between experimental and calculated values, prediction
Acknowledgments
The authors wish to thank CAPES and CNPq (Project number 483340/2012-0, 406856/2013-3, 140345/2014-0 and 305870/2014-9) for the financial support.
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