Phase diagrams of binary fatty alcohol + fatty acid mixtures Mariana C. Costa a, Natália D. D. Carareto, Antonio J. A. Meirelles a

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1 Phase diagrams of binary fatty alcohol + fatty acid mixtures Mariana C. Costa a, Natália D. D. Carareto, Antonio J. A. Meirelles a a Department of Food Engineering FEA UNICAMP, Campinas, Brazil (mcdcosta@yahoo.com.br) ABSTRACT Fatty acids are known for their importance as major components of oils and fats, representing 96% of their total mass. Fatty alcohols are components of vegetal waxes and can also be found in smaller quantities in vegetable oils and fats. All play an important role in the food industrie and also can be used in the pharmaceutical and chemical industries. Solid-liquid equilibrium data of binary mixtures formed by fatty acid + fatty alcohol mixtures are scarce in the literature. It is possible that there are invariant points such as eutectic, peritectic and metatectic points in these mixtures, as well as solid transitions and solid solution formation at the extremities of the phase diagrams. Therefore, it is interesting to investigate the phase behavior of binary mixtures formed by such substances. Phase diagrams of the following binary mixtures were constructed using the DSC technique: 1-dodecanol + lauric acid, 1-tetradecanol + myristic acid, 1-hexadecanol + palmitic acid and 1-octadecanol + stearic acid. Based on the experimental runs, the uncertainty of the equilibrium data is estimated to not be greater than 0.3 K. Solid-liquid equilibrium; fatty acids; fatty alcohols; differential scanning calorimetry INTRODUCTION Currently, when considering the increase in vegetable oil production due to the increase in human consumption and increased interest in biodiesel production, it is important to understand the physical and chemical properties of its components. Among these components fatty acids may be cited, representing 96% of the total mass of vegetable oils and fats [1], as well as fatty alcohols which are components of vegetal waxes and also found in vegetable oils and fats but in smaller quantities [2]. With the impressive growth of the oil industry, finding better destinations for byproducts generated in all process, from oil extraction to acquisition of refined oil or biodiesel, has become essential to reduce agroindustrial losses, especially in the case of byproducts with high added value such as the fatty alcohols and acids. Fatty acids and fatty alcohols play an important role in food industries and also can be used in the pharmaceutical and chemical industries. Fatty alcohols are considered a class of compounds that may be used for structuring in the lipid phases present in food products [3-5]. Good results have been obtained in this field, especially when fatty alcohols were employed in synergy with fatty acids [4-5]. Fatty acidsare applied in the production of coatings, plastics, cleaning products [2], phase change materials for energy storage [6-7] and biodiesel [8-9]. Thus, thermal properties of these substances such as the melting point, enthalpy and heat capacity are normally required to design and/or development equipment, as well as to propose new applications for these substances or their mixtures. Phase diagrams of binary mixtures formed of fatty acids and phase diagrams of binary mixtures composed of fatty alcohols have been extensively studied in recent years [10-15], however data of binary mixtures consisting of one fatty acid plus one fatty alcohol are scarce in literature. It is also know that phase diagrams of fatty acids may exhibit invariant points such as eutectic points, peritectic points and metatectic points, solid-solid transitions and the presence of a complete miscibility region at the extremities of the phase diagram [13-15]. It is possible that these invariant points, solid-solid transitions and the formation of a solid solution can also occur in systems formed by a fatty acid plus a fatty alcohol. In this study, phase diagrams were determined of the mixtures 1-dodecanol + lauric acid, 1-tetradecanol + myristic acid, 1-hexadecanol + palmitic acid and 1-octadecanol + stearic acid using differencial scanning calorimetry (DSC). MATERIALS & METHODS Materials: Standards used for calibration of the DSC were indium (99.99%) certified by TA instruments, benzoic acid (min 99.9% - Metler) and naphtalene (99% - Merck). Commercial nitrogen (used for preparing binary samples) and high purity nitrogen (used in the calorimeter) were supplied by Air Liquide. The fatty acids and fatty alcohols used in this study, with no further purification, are presented in Table 1. Preparation of binary mixtures: 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

2 tube, were heated under stirring in a nitrogen atmosphere. The heating temperature did not exceed 15 degrees above the highest melting point of the components. Subsequently, the solutions were placed in a freezer and maintained under refrigeration until later use. Table 1. Reagents used in this study. Fatty Acids Purity Supplier Fatty Alcohols Purity Supplier Lauric acid % Sigma Aldrich 1-dodecanol 99 % Fluka Myristic acid % Sigma Aldrich 1-tetradecanol 98.4 % Sigma-Aldrich Palmitic acid Min 99% Sigma Aldrich 1-hexadecanol 99.9 % Sigma-Aldrich Stearic acid Min 97% Merck 1-octadecanol 99.6 % Sigma-Aldrich Differential scanning calorimetry (DSC): Melting temperatures of pure substances and binary mixtures were characterized by DSC, using a MDSC 2920, TA Instruments calorimeter. The calorimeter was equipped with a refrigerated cooling system which operated between 260 K and 380 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 ± g and placed in sealed aluminum pans. In order to erase previous thermal histories, each sample was submitted to an initial heating run at 5.0 K min -1 until reaching a temperature 15 K above the highest melting temperature of the components. After 20 min at this temperature, the samples were cooled to 40 degrees below the lowest melting point of the components at a cooling rate of 1.0 K min -1 and allowed to stay at this temperature for 30 min. After this pre-treatment each sample was then analyzed in a heating run, at a heating rate of 1.0 K min -1. Peak temperatures of each transition were considered. The experimental uncertainty was evaluated based on repeated DCS runs and estimated to no exceed 0.3 K. RESULTS & DISCUSSION In this study the phase diagrams of the following binary mixtures was presented: 1-dodecanol + lauric acid, 1-tetradecanol + myristic acid, 1-hexadecanol + palmitic acid and 1-octadecanol + stearic acid. These systems are presented in literature for the first time. As will be discussed below, some transitions were observed beneath the liquidus line of the systems. Unfortunately, a better characterization of such transitions requires the use of other analytical techniques that will be applied in future studies. Nevertheless, due to the originality of these data its publication is very important, principally concerning the liquidus line of the systems that was well described here. Thermograms of the system 1-dodecanol + lauric acid are presented in Figure 1. In the thermograms it is possible to observe the transitions that occur in the sample during heating. Complete melting of the sample is represented by peaks at the higher temperatures of each thermogram. Generally, according to the complexity of the systems, other peaks can be observed. In the case of fatty acid systems these other peaks are generally overlapped [13-15] as can be seen in Figure 1, except for pure lauric acid. In the case of pure 1-dodecanol it is possible note, from an insert of the thermogram, one more small peak indicated by the arrow in Figure 1. The maximum temperatures of all peaks in the thermograms were plotted versus the sample composition and are presented in Figure 2. For a composition of x three peaks can easily be observed in Figure 1, the first at the highest temperature, approximately 315 K, is attributed to the complete melting of the sample. The other two peaks, at approximately 292 and 296 K, respectively, can be attributed to a solid-solid transition and a peritectic reaction, respectively. It is possible to observe that the increase of 1-dodecanol in the mixture caused the peak of complete melting to become less intense and approach the peak observed at 296 K. At the composition of x only one peak is observed. For composition of x three peaks can again be seen observed.

3 x1 = 1.00 x x x Heat Flux (a.u.) x x x x x x1 = Figure 1. Thermograms of the 1-dodecanol + lauric acid system. Observing the phase diagram of the 1-dodecanol + lauric acid system, it may be considered that this phase diagram is similar to those observed for binary mixtures of saturated fatty acids except by the absence of the metatectic reaction [13]. In the other words, it is possible that the phase diagram of this system presents a peritectic reaction that occurs at approximately 269 K and an eutectic reaction at approximately 292 K. The Tamman plot of this system, presented in Figure 2b, also suggests the existence of peritectic and eutectic reactions. In the Tamman plot the increase of the enthalpy values for the transition observed at 296 K can be observed, as was expected for a peritectic or eutectic reaction. The same behavior is observed for the transition observed at 296 K, but only for compositions greater than x indicating that the transitions observed for roughly the same temperature for compositions less than x 0.60 is another type of transition, probably polymorphic Enthalpy (kj/mol) x 1-dodecanol x 1-dodecanol Figure 2. a) Phase diagram of the 1-dodecanol + lauric acid system; b) Tamman plot of the same system. complete melting of the sample (liquidus line); peritectic reaction; eutectic reaction; +, probable polymorphic transitions. Thermograms of the 1-octadecanol + stearic acid system are presented in Figure 2. These thermograms are very similar of those presented in Figure 1. Increase of the 1-octadecanol fraction in the sample implies emergence of the other peaks as can be seen in the thermograms for compositions lesser than x 0.60, at this composition only one peak can be observed and for compositions greater than

4 x 0.60, more than one peak can again be observed. The phase diagram of this system is presented in Figure 4. x1 = 1.00 x x x Heat Flux (a.u.) x x x x x x1 = Figure 3. Thermograms of the 1-octadecanol + stearic acid system. Different from the phase diagram of the 1-dodecanol + lauric acid system, the temperatures of the observed transitions are not constant, as can be seen in Figure 4 for transitions represented by the symbols and, presenting a larger variation than commonly observed for fatty acid systems [11-15]. It is therefore impossible to attribute these transitions to eutectic and peritectic reactions x 1-dodecanol Figure 4. Phase diagram of the system 1-octadecanol + stearic acid. This same behavior is observed for the other two systems, 1-tetradecanol + myristic acid and 1- hexadecanol + palmitic acid, presented in Figure 5. The liquidus lines of these systems were well established, but the transitions clearly observed under the liquidus lines in the thermograms of the systems cannot yet be identified due to the necessity of other techniques. These transitions will be the subject of future studies.

5 a) 340 b) x 1-tetradecanol 310 x 1-hexadecanol Figure 5. a) Phase diagram of the 1-tetradecanol + myristic acid system; b) phase diagram of the 1-hexadecanol + palmitic acid system. complete melting of the sample; transitions under the liquidus line. CONCLUSION The solid-liquid phase diagrams of the 1-dodecanol + lauric acid, 1-tetradecanol + myristic acid, 1- hexadecanol + palmitic acid and 1-octadecanol + stearic acid mixtures were determined by DSC and presented for the first time in literature. The system composed of 1-dodecanol + lauric acid appears to present the same behavior previously observed for binary systems of saturated fatty acids. The liquidus lines of the other three systems studied were well defined but the transitions observed under the liquidus lines require the use of other techniques for characterization. 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] Daniel J. & Rajasekharan R Organogelation of plant oils and hydrocarbons by long-chain saturated FA, fatty alcohols, wax esters, and dicarboxylic acids. J. Am. Oil Chem. Soc., 80, [4] Gandolfo F.G., Bot A. & Floter E Structuring of edible oils by long-chain FA, fatty alcohols, and their mixtures. J. Am. Oil Chem. Soc., 81, 1-6. [5] Schaink H.M., van Malssen K.F., Morgado-Alves S., Kalnin D. & van der Linden E Crystal network for edible oil organogels: Possibilities and limitations of the fatty acid and fatty alcohol systems. Food Res Int, 40, [6] Shilei L., Neng Z. & Feng G.H Eutectic mixtures of capric acid and lauric acid applied in building wallboards for heat energy storage. Energ Buildings, 38, [7] Zhang J.J., Zhang J.L., He S.M., Wu K.Z. & Liu X.D Thermal studies on the solid-liquid phase transition in binary systems of fatty acids. Thermochim. Acta, 369, [8] Falk O. & Meyer-Pittroff R The effect of fatty acid composition on biodiesel oxidative stability. Eur. J. Lipid Sci. Technol., 106, [9] Meher L.C., Sagar D.V. & Naik S.N Technical aspects of biodiesel production by transesterification - a review. Renew. Sust. Energ. Rev., 10, [10] Bailey A.E Melting and solidification of fats, New York: Interscience publishers. [11] Costa M.C., Boros L.A.D., Rolemberg M.P., Krahenbuhl M.A. & Meirelles A.J.A Solid-Liquid Equilibrium of Saturated Fatty Acids plus Triacylglycerols. J. Chem. Eng. Data, 55, [12] Costa M.C., Rolemberg M.P., Boros L.A.D., Krähenbühl M.A., Oliveira M.G. & Meirelles A.J.A Solidliquid equilibrium of binary fatty acids mixtures. J. Chem. Eng. Data, 52, [13] Costa M.C., Rolemberg M.P., Meirelles A.J.A., Coutinho J.A.P. & Krahenbuhl M.A The solid-liquid phase diagrams of binary mixtures of even saturated fatty acids differing by six carbon atoms. Thermochim. Acta, 496, 30-7.

6 [14] 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, [15] 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,