Solid-liquid phase equilibrium diagrams of binary mixtures containing fatty acids, fatty alcohol compounds and tripalmitin using differential scanning calorimetry
Introduction
Lipids are a chemically diverse group of compounds whose common feature is their water insolubility. The biological functions of lipids are as wide as their chemical composition. Fats and oils are derived from triacylglycerols, fatty acids, and to a lesser extent diacylglycerol, monoacylglycerol, fatty alcohol compounds and liposoluble components like vitamins and natural antioxidants. Complex lipid mixtures of triacylglycerols (TAGs), fatty acids or fatty alcohol compounds among others, are raw materials used to prepare several products in food, chemical and pharmaceutical industries [1,2].
Triacylglycerols (TAGs) are major constituents of fats and oils. They are formed by three fatty acids linked to a triester of glycerol, whose three hydroxyls (–OH) bind to the carboxylic radicals (–COOH) of fatty acids with the release of three molecules of water. Oils and fats may contain more than 500 different molecules of TAGs, so they do not have a distinct melting point, but a melting temperature range [[3], [4], [5]].
Fatty acids are carboxylic acids with hydrocarbon chains ranging from 4 to 36 carbons atoms (C4 to C36), formed through glycerol hydrolysis reaction. The fatty acids physical properties are strongly influenced by length and hydrocarbon chain unsaturation, like water solubility and melting point [1]. Fatty alcohol is a generic name used for a series of aliphatic hydrocarbons containing a hydroxyl group (-OH); They are truly found in vegetable and animal oils and fats and can be applied in the production of food and emulsions, surfactants and as thickeners in the food and pharmaceutical industry [6,7].
Fatty acids, fatty alcohols and TAGs play a significant role in industrial applications. For example, fatty acids are widely used in chemical industries to produce coatings, plastics, cleaning products and paints. Fatty alcohol compounds are used to produce cosmetics and some pharmaceuticals, as well as in food industries as structuring agents of surfactants and co-surfactants, emulsifiers, gelling agents and coating. TAGs are used in all the applications cited above, besides representing the main components used in food industry employed to produce cream, margarine and confectionery fats [4,8,9].
Texture, stability, spreadability and mouth sensation are properties that help to control the quality of the final products made with oils and fats [10] such as ice cream, chocolate and so on. One of the most important physicochemical properties of oils and fats is their melting temperature, because of their complex melting behavior due to the existence of a crystalline form variety, i.e. different polymorphic forms. Thus, a critical thermodynamic evaluation of fatty systems is required for design and optimization of process and also product development [9].
Due to these applications and the importance of the solid phase on the characteristic of the final products, the solid-liquid equilibrium (SLE) study of fatty mixtures can contribute to a better understanding of the ways in which fat blends interact. Since these studies can provide basic information regarding the interaction among different carbon chains of complex fatty mixtures melting behavior, fat compounds or a mixture of them are capable of crystallize under several different phases (polymorphism) according to heat treatment or pressure conditions to which they are submitted [[11], [12], [13], [14]]. Furthermore, determination of SLE is also important to develop new thermodynamic models or check the ability of the existent ones to predict the phase behavior under selected operating conditions and also to determine physical properties of lipids mixture [8,15].
SLE phase diagrams of fatty mixtures can be obtained using different equipment and techniques but the most used equipment is the Differential Scanning Calorimeter (DSC). The DSC technique allows more accurate data determination due to its precision, speed, sensibility, reliability, and also because it requires small amount of sample. Through calorimetric technique using a DSC equipment it is possible to determine phase transition temperatures, the enthalpy of fusion including information about polymorphic transformations [16,17]. Moreover, it is also possible to show heat behavior during phase transitions, determine technological parameters of edible fats, data for fat content prediction and occurrence of eutectic and monotectic points [2,[18], [19], [20], [21], [22], [23]].
The eutectic point is located in phase diagram at the place where two solid phases A and B are in equilibrium with a liquid solution of specific composition [24,25]. It occurs at the intersection between a solute A melting curve and a solvent B melting curve when the melting temperature of A decreases with the addition of B, and the melting temperature of B decreases with the addition of A. According to Gamsjäger (2008) the eutectic reaction is a reversible isothermal reaction in which a liquid phase is transformed into two (or more) different solid phases during the cooling of a system [26]. It is also common in fat systems the occurrence of a monotectic reaction generally observed when the melting points of different compositions’ solutions are not lower than the melting points of either of the compounds. In this case, the compounds have similar melting points, molecular volumes, and polymorphic forms [27].
This study investigated the SLE behavior of eight binary fatty mixtures: tripalmitin (1) + fatty acids: tripalmitin (1) + capric acid (2), tripalmitin (1) + lauric acid (3), tripalmitin + (1) myristic acid (4), tripalmitin (1) + palmitic acid (5), tripalmitin (1) + stearic acid (6), and tripalmitin (1) + fatty alcohol compounds: tripalmitin (1) + 1-decanol (7), tripalmitin (1) + 1-dodecanol (8); tripalmitin (1) + 1-tetradecanol (9) through their phase diagrams using the DSC equipment. To better understand and evaluate some solid transitions, an optical microscope was employed coupled with a temperature controller and also an X-ray diffraction analysis was done. Ideal assumption, 3-suffix Margules and NRTL models were used to adjust the liquidus line of the proposed systems.
Section snippets
Materials
Table 1 presents the highly pure components used in this study without further purification to prepare the binary samples. The DSC calibration was performed using indium (≥0.99 M fraction, CAS number 7440-74-6), naphthalene (≥0.99 M fraction, CAS number 91-20-3) and decane standards (≥0.99 M fraction, CAS number 124-18-5) from Fluka Analytical (Germany) and cyclohexane (≥0.99 M fraction, CAS number 110-82-7) from Sigma-Aldrich (USA), at a 1 K min−1 heating rate.
Preparation of binary mixture samples
The binary mixtures were prepared
Pure compounds thermal properties
Table 2 presents transition temperatures, melting temperatures and the molar enthalpies of fusion of pure compounds determined in this study. In this table, the respective experimental standard deviations are also presented in parenthesis. The average relative deviation (ARD) was calculated between melting temperatures and molar enthalpies of fusion determined in this study and those data from literature, according to Equation S(4) presented in Supplementary Material. The ARD calculated to the
Conclusion
The analysis and experimental behavior of the solid-liquid equilibrium of eight binary mixtures formed by tripalmitin (1) + fatty acids (capric acid (2), lauric acid (3), myristic acid (4), palmitic acid (5) and stearic acid (6)) and tripalmitin (1) + fatty alcohol compounds (1-decanol (7), 1-dodecanol (8) and 1-tetradecanol (9)) were measured using the DSC equipment.
The melting temperature and enthalpy of each pure fat compound were determined and compared with literature data showing a good
Acknowledgements
This study was financed partly by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brazil (CAPES) - Finance Code 001, Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)- Brazil (2014/21252-0) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)- Brazil (305870/2014-9, 406963/2014-4, 310272/2017-3, 309780/2014-4 and 132149/2017-6).
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