The use of DSC in the determination of the vapor pressure of fatty acids

1. The use of DSC in the determination of the vapor pressure of fatty acids Rafael M. Matricarde Falleiroa, Antonio J. A. Meirelles b, Maria A. Krähenbühl a, * a Laboratory of Thermodynamic Properties, LPT, School of Chemical Engineering, University of Campinas, Brazil b Laboratory of Extraction, Applied Thermodynamics and Equilibrium, ExTrAE, School of Food Engineering, University of Campinas, Brazil INTRODUCTION According to this work, the methodology involving the DSC is an appropriate technique to obtain the vapor pressure data of fatty acids, since the data determined in this study showed a mean deviation of 0.70 ºC from the data of the literature [5, 6]. The present work aims to determine the vapor pressure of long-chain fatty acids by Differential Scanning Calorimetry (DSC). The boiling temperature of the fatty acids was measured as a function of previous established pressure, within a dynamically heated environment [1]. Palmitic, stearicand oleic acids are the major components of soybean triacylglycerols. These, through the transesterification reaction with ethanol, give rise to the principal constituents of soybean biodiesel [2]. All the studied fatty compounds are still poorly characterized, since data of their physicochemical properties are scarce in the literature. Therefore, these data are important in order to improve the biodiesel production processes. (b) (a) (c) METHODOLOGY Palmitic and stearic acids with purity greater than 99% were acquired from Sigma, and oleic acid, with 99% purity, was acquired from Fluka. The experimental apparatus (Figure 1) was especially projected based on the ASTM and 1782-03 guidelines [3], consisting of a Differential Scanning Calorimeter (DSC – Model 2920) with a vacuum system fitted to it. For the DSC analysis, the vapor pressure was set within the range from 1.33 to 9.33 kPa. Samples of 3 to 5 mg were used in the analysis, with a heating rate of 25 °C.min−1 and a small ball (Figure 2) placed over the pinhole (Figure 3), in order to avoid the pre-vaporization of the sample, since it behaves as an exhaust valve, releasing the vapor phase in a controlled manner [4]. Figure 4 – Boiling endotherms measured between 1.33 and 9.33 kPa. (a) palmitic acid; (b) stearic acid; (c) oleic acid. From the nonlinear regression of data obtained in this study, the Antoine constants (Table 1) were determined using the DDB Software Package [7]. Table 1 – Antoine constants. * (a) a (b) (c) Figure 1 – (a) General view of experimental apparatus room; (b) Expanded perspective of the DSC furnace; (c) Perspective in more details of some accessories under of the bench. Figure 5 - Vapor pressure curves of the fatty acids. () palmitic acid; (●) stearic acid; (○) oleic acid and (−) Antoine. CONCLUSION The results proved that the DSC technique is reliable, since the data obtained showed precision similar to those obtained using the conventional techniques. Differential Scanning Calorimetry was shown to be capable of determining vapor pressure data with efficacy, in a short time and requiring low amount of chemicals. Figure 3 – Tungsten ball being placed on the pinhole Figure 2 – Small ball and the crucible with a pinhole REFERÊNCIASBIBLIOGRÁFICAS RESULTSANDDISCUSSION [1] R. J. Seyler, Thermochim. Acta, 17, 129 - 136 (1976). [2] G. Knothe, J. Van Gerpen, J. Krahl and P. L. Ramos, Biodiesel Guideline, 1st edition (2006). [3] ASTM (American Society for Testing and Materials) E 1782-03 (2003). [4] R. F. Farritor and L.C. Tao, Thermochim. Acta, 1, 297 (1970). [5] D. R. Stull, Ind. Eng.Chem, 39 (4), 517 – 540, (1947). [6] Texas A&M University, Thermodynamics Research Center, (1980). [7] DDBST - Dortmund Data Bank Software Package - Educational version - 2003, www.ddbst.de. For each established pressure, the boiling temperature was determined by the Differential Thermal Curves (Figure 4), and the experimental values ​​obtained were compared to the literature data obtained by conventional techniques [5] and [6].