Experimental phase diagrams of binary fatty acid mixtures containing oleic acid

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1 Experimental phase diagrams of binary fatty acid mixtures containing Mariana C. Costa a, Marlus P. Rolemberg b, Natália D. D. Carareto a, Cecilia Y. C. S. Kimura a, Maria A. Krahenbühl c, Antonio J. A. Meirelles a a Department of Food Engineering FEA UNICAMP, Campina s- SP, Brazil (mcdcosta@yahoo.com.br) b Department of Chemical Technology - UFMA, São Luís - MA, Brazil (marlus@ufma.br) c Department of Chemical Processes FEQ UNICAMP, Campinas - SP,Brazil (mak@fea.unicamp.br) ABSTRACT Currently, knowledge on the solid-liquid equilibrium of fatty acids is of great importance for the food and chemical industries. Due to the increase interest of industries in the separation processes to obtain pure fatty acids or a vegetable oil with low acidity, for example to produce biodiesel, the knowledge of equilibrium properties is an important tool that can make it easier to develop new methods to separate such compounds. The purpose of this study was to investigate the influence of the molecule structure and/or size on the phase diagrams of binary mixtures of saturated and unsaturated fatty acids with. Differential Scanning Calorimetry was used to measure the following binary mixtures: + capric acid, + stearic acid, + elaidic acid and +. Based of the experimental runs, the uncertainty of the equilibrium data is estimated to not be higher than 0.3 K. Moreover, this study presents a better characterization of the solidus lines of such systems and shows that the Margules-3-suffix and NRTL models can be used to accurately describe the liquidus line. Keywords: Solid-liquid equilibrium; fatty acids; Differential Scanning Calorimetry DSC; thermodynamic models; INTRODUCTION Fatty acids play an important role as the major components of oils and fats, representing 96% of their total mass [1]. Oleic acid is one of the most common fatty acids found in vegetable oils representing, in some cases 50 % of oil composition, as for example in olive oil and is an important raw material for the oleochemical industry. Moreover, fatty acids have been used for different applications in the textile, food and pharmaceutical industries [2]. Phase science of lipids and lipid mixtures is a key topic in making connections between bio-science the fundamental science field, as well of great importance for the food and chemical industries since it provides basic information on the molecular interaction between acyl chains, being an important factor in determining physical properties and phase behavior of lipid mixtures [3]. Physical properties of bulk lipid assemblies, particularly phase behavior, may be responsible for their biological function [4]. Particularly in the food industry, the phase behavior and polymorphism of the fatty acid mixtures influence the characteristics of consumer products such as confectionary fats [5]. The polymorphism of triglycerides and fatty acids has been well understood for some time, but the study of the crystal forms of pure fatty acids dates back to the 1950 s and still is a challenging task [5-6]. Additionally, due to the increased interest of industries in the separation processes to obtain pure fatty acids or a vegetable oil with low acidity, in biodiesel production for example, the knowledge of equilibrium properties is an important tool that can facilitate development of new methods for separation of such compounds [6]. It is also know that phase diagrams of fatty substances can exhibit invariant points that have important consequences in separation processes. On the other hand, it is possible that the molecule structure and/or size can contribute to the occurrence of invariant reactions such as peritectic or metatectic reactions. The complex phase diagram for some binary fatty acid systems makes the use of crystallization processes for these compounds impracticable [5]. It is important to continue the study of systems formed by fatty acids and then verify if it is possible to use such separation processes. The aim of this study was investigate the influence of molecule structure and/or size on the phase diagrams of binary mixtures of some pure saturated and unsaturated fatty acids with. Differential Scanning Calorimetry was used to measure the following binary mixtures: + capric acid, + stearic acid, + elaidic acid and +.

2 MATERIALS & METHODS Reagents. The DSC equipment was calibrated using the following standards: indium ( molar fraction, TA Instruments), benzoic acid (min molar fraction, Metler) and deionized water (Milli-Q, Millipore). The suppliers and purity of the saturated and unsaturated fatty acids are shown in Table 1. All reagents were used with no further purification. Table 1. Fatty acids used in this study, respective purity and suppliers. Fatty Acids Purity Supplier Oleic acid 99 % Merck Capric acid 99% Sigma Aldrich Stearic acid 99% Sigma Aldrich Lin 99% Sigma Aldrich Elaidic acid 99% Sigma Aldrich Sample preparation. The mixtures were prepared on an analytical balance (Adam AAA/L) with an accuracy of 0.2 mg. The weighed quantities of the binary mixture components, placed in a glass tube, were heated while stirring under a nitrogen atmosphere. The heating temperature did not exceed K above the highest melting point of the components. Subsequently, the solutions were placed in a freezer and maintained under refrigeration until later use. DSC analyses. Temperature-driven melting of each binary mixture was characterized by DSC, using a MDSC 2920 TA Instruments calorimeter. The calorimeter was equipped with a refrigerated cooling system which operated between 230 K and 365 K in this study. Samples (2 to 5 mg) of each mixture were weighed on a microanalytical balance (Perkin Elmer AD6) with an accuracy of 0.2 x10-5 g and placed in sealed aluminium pans. A heating run was performed in the DSC for each sample at a rate of 1 K min -1 as described in previous studies [7-10], after utilizing a thermal treatment to erase thermal memories. Peak top temperatures were measured for pure fatty acids and for binary fatty acid mixtures. Experimental uncertainty was estimated to be no greater than 0.3 K based on repeated runs performed with indium and some selected fatty acid samples. RESULTS & DISCUSSION This study presents the solid-liquid equilibrium data of the following systems: + capric acid, oleic acid + stearic acid, + and + elaidic acid. Although these systems have already been studied in other published works just for the oleic + capric acid [6] and oleic + stearic acid systems [12], there are detailed studies of the solid phase. In this study it is presented, for the first time, the transitions under the liquidus line of the oleic + and oleic + elaidic acid systems, moreover in the oleic + capric acid system other transitions situated above the eutectic temperature and under the liquidus line were observed. Comparing our experimental data with data from literature, except for the oleic + stearic acid system that does not have a data set adequate for a good comparison, the average absolute deviation (AAD), exposed as in Eq. 1, was less than 0.60 % between the values. This small deviation indicates a very good agreement between experimental data of this study and that found in literature. n 1 T T i,lit i,exp AAD =. 100 (1) n i 1 T = i, lit where n is the total number of literature data, T indicates the melting point and exp and lit stand for the experimental and literature values, respectively. Thermograms of the oleic + capric acids are presented in Figure 1. For pure capric acid ( x = ), it appears that there is only one peak, but there are actually two peaks overlapped. For pure ( x = 1.00 ) two peaks can be observed, one at a lower temperature, approximately 270 K, which is attributed to a polymorphic form where there is a transition from the γ to α form of, and the other at a greater temperature, approximately 287 K, represent complete melting of the [3,6,12].

3 0.95 = Heat Flux (a. u.) = (K) Figure 1. Thermograms of the + capric acid system. Increase if the fraction of the sample implies a decrease of the sample melting temperature and appearance of more two peaks, indicated by the arrows in Figure 1 (arrows 1 and 2); peak 1 is attributed to the eutectic reaction and is confirmed by the Tamman plot, as will be discussed further. Peak 2 also in Figure 1 can be attributed to a polymorphic transition. In the thermograms presented in Figure 1, it is easy observe that the peak attributed to the eutectic reaction, at approximately 270 K, disappeared for compositions equal to or greater than x 0. 80, indicating that for greater compositions the system formed a solid solution as proposed by Inoue and co-workers [6]. In this system two other transitions were also observed above the eutectic reaction that are represented in the phase diagram of the system + capric acid, presented in Figure 2 by the symbols ( ) and (+). Unfortunately, to correctly characterize these transitions it is necessary the use other techniques such as FT-IR, for example. This is the subject of future studies, but analyzing the Tamman plot can provide information regarding these observed transitions relating enthalpy values with the system composition. The Tamman plot of the oleic + capric acid system is presented in Figure 2b. In this diagram the previously mentioned transitions are represented at approximately 270 K, 279 K and 292 K. According to literature, for a eutectic reaction, enthalpies values should increase linearly to the eutectic point and also decrease linearly. This behavior is observed in Figure 2b for the transition at approximately 270 K, confirming that this transition is really a eutectic reaction. It is also reaffirmed that the system exhibits a different behavior for compositions greater than x whereas the enthalpy values decrease significantly for compositions greater than x The enthalpies values of the transition observed at 278 K are constant, as can be observed in Figure 2b, indicating that this transition is probably a polymorphic transition [13]. The enthalpy of the other transition, observed at 292 K, also increased with the composition, indicating that this transition can be another reaction as observed for other fatty acid systems.

4 310 a) 25 b) Enthalpy (kj/mol) Figure 2. a) Phase diagram of the + capric acid system; b) Tamman plot of the same system, ( ) enthalpy of the eutectic reaction, ( ) and (+) enthalpy of transition under the liquidus line. The phase diagram of the + system is presented in Figure 3. In this phase diagram a transition is observed at roughly 263 K until reaching the composition of x 0. 80, for compositions greater than this two other transitions are observed, but cannot be related to the transition observed at 263 K. The Tamman plot of the observed transitions is presented in Figure 3b. This plot indicates that the transition observed at approximately 263 K is not the same transition over the entire range of compositions observed due to the change in the slope of the points between x and x This behavior was observed for binary mixtures of saturated fatty acids [9], but it is not sufficient to affirm that the eutectic reaction occurs only in the interval comprised between 0.60 x The phase diagram of this system appears to present the same behavior observed in the oleic + capric acid system as the solid solution formation for compositions greater than x x Enthalpy (kj/mol) x Figure 3. a) Phase diagram of the + system; b) Tamman plot of the + system, ( ) enthalpy of the eutectic reaction, ( ) and (+) enthalpy of transition under the liquidus line. The phase diagrams of the + elaidic acid and + stearic acid systems are presented in Figure 4(a,b). Both systems exhibit a phase diagram with a simple eutectic point. The difference lies in the fact that in the mixture with stearic acid the polymorphic transition of remains while in the mixture with elaidic acid the polymorphic transition it is not observed.

5 320 a) b) Figure 4. Phase diagram of the systems: a) oleic + elaidic acids; b) oleic + stearic acids. In order to calculate the liquid phase activity coefficients for the four binary fatty acid mixtures, the Margules-3-suffix and NRTL models were used. The interaction parameters were adjusted from the experimental equilibrium data using the Simplex Downhill method [14], as suggested in the study of Costa and co-workers [15]. Table 2 shows the AAD for each experimental run compared to both the Margules-3-suffix and NRTL models. It is observed that, on average both thermodynamic models presented the same AAD and fitting of the results presented a good correlation for all binary mixtures. The best results were obtained for the oleic acid + system using the Margules-3-suffix model. Table 2. AAD between experimental data and NRTL or Margules-three-suffix models. Fatty Acids binary system AAD Margules-three-suffix + capric acid stearic acid elaidic acid NRTL CONCLUSION The phase diagrams of the + capric acid, + stearic acid, + elaidic acid and oleic acid + binary mixtures were reported in this study. It was shown that the phase diagrams of these systems exhibited a eutectic point as well as transitions above and below the eutectic temperature. The Margules-3-suffix and NRTL models presented good fit of the phase diagrams. REFERENCES [1] Naudet, M Oils & Fats Manual: A Comprehensive Treatise, Properties, Production, Applications, Paris Lavoisier. [2] Johnson R.W. & Fritz E Fatty Acids in Industry: Processes, Properties, Derivatives, Applications, Nova York: Marcel Dekker. [3] Inoue T., Hisatsugu Y., Ishikawa R. & Suzuki, M Solid liquid phase behavior of binary fatty acid mixtures 2. Mixtures of with lauric acid, myristic acid, and palmitic acid. Chemistry and Physics of Lipids, 127(2) [4] Bloom M., Evans E. & Mouritsen O.G Physical-Properties of the Fluid Lipid-Bilayer Component of Cell- Membranes - a Perspective. Q Rev Biophys, 24,

6 [5] Costa M.C., Sardo M., Rolemberg M.P., Coutinho J.A.P., Meirelles A.J.A., Ribeiro-Claro P. & Krähenbühl M.A The solid liquid phase diagrams of binary mixtures of consecutive, even saturated fatty acids: differing by four carbon atoms. Chem. Phys. Lip., 157, [6] Inoue T., Hisatsugu Y., Suzuki M., Wang Z. & Zheng L Solid-liquid phase behavior of binary mixtures 3. Mixtures of with capric acid (decanoic acid) and caprylic acid (octanoic acid). Chem. Phys. Lipids, 132, [7] Costa M.C., Rolemberg M.P., Boros L.A.D., Krähenbühl M.A., Oliveira M.G. & Meirelles A.J.A Solid-liquid equilibrium of binary fatty acids mixtures. J. Chem. Eng. Data, 52, [8] Boros L., Batista M.L.S., Vaz R.V., Figueiredo B.R., Fernandes V.F.S., Costa M.C., Krahenbuhl M.A., Meirelles A.J.A. & Coutinho J.A.P Crystallization Behavior of Mixtures of Fatty Acid Ethyl Esters with Ethyl Stearate. Energy & Fuels, 23, [9] Costa MC, Rolemberg MP, Meirelles AJA, Coutinho JAP, Krahenbuhl MA. The solid-liquid phase diagrams of binary mixtures of even saturated fatty acids differing by six carbon atoms. Thermochim. Acta 2009;496:30-7. [10] Costa M.C., Sardo M., Rolemberg M.P., Coutinho J.A.P., Meirelles A.J.A., Ribeiro-Claro P. & Krähenbühl M.A The solid-liquid phase diagrams of binary mixtures of consecutive, even saturated fatty acids Chem. Phys. Lipids, 160, [11] Bailey A.E., Melting and solidification of fats, New York: Interscience publishers. [12] Inoue T., Hisatsugu Y., Yamamoto R. & Suzuki M Solid-liquid phase behavior of binary fatty acid mixtures 1. Oleic acid stearic acid and behenic acid mixtures. Chem. Phys. Lipids, 127, [13] Chernik G.G Phase-Equilibria in Phospholipid Water-Systems. Adv. Colloid Interface Sci., 61, [14] Press W.H., Flannery B.P., Teulosky S.A. & Vetterling W.T Numerical Recipes. The art of scientific computing, Cambridge: Cambridge University Press. [15] Costa M.C., Rolemberg M.P., Boros L.A.D., Krahenbuhl M.A., Oliveira M.G. & Meirelles A.J.A Solid-liquid equilibrium of binary fatty acid mixtures. Journal of Chemical and Engineering Data, 52,