Elsevier

Thermochimica Acta

Volume 369, Issues 1–2, 22 March 2001, Pages 39-50
Thermochimica Acta

Measurements of temperature and melting heat of some pure fatty acids and their binary and ternary mixtures by differential scanning calorimetry

https://doi.org/10.1016/S0040-6031(00)00752-8Get rights and content

Abstract

Differential scanning calorimetry (DSC) is used to study the melting process of three pure fatty acids (palmitic, stearic and oleic) and both their binary and ternary mixtures. The results obtained for palmitic (T=337 K, ΔH=208.2 J g−1) and stearic (T=344 K, ΔH=210.8 J g−1) acids indicate the existence of a simple process. However, oleic acid presents a reversible solid–solid phase transition (T=225 K, ΔH=3.6 J g−1) prior to melting (T=278 K, ΔH=75.5 J g−1). The solidification of this acid is a complex process, since a liquid–liquid transformation (T=272 K, ΔH=−10.5 J g−1) takes place prior to the change of the liquid–solid aggregation state (T=260 K, ΔH=−61.0 J g−1). The presence of the oleic acid in the binary and ternary mixtures produces melting points at different levels of temperature. The eutectic temperature is determined for the palmitic/stearic mixture (T=331 K), its non-ideal behavior being demonstrated. The melting heat in the binary mixtures hardly changes, while in the ternary mixtures a lineal decrease is observed when the oleic composition is increased.

Introduction

The rubber, textile, food and pharmaceutical industries have been some of the chemical industries that have used fatty acids for diverse applications [1]. In raw and edible soya, sunflower and olive oils, the majority compounds are hexadecanoic (palmitic), octadecanoic (stearic), cis-9-octadecenoic (oleic) and 9,12-octadecadienoic acids (linoleic) [2], [3]. In the deodorization process, some of the distilled compounds formed are composed of a mixture of fatty acids and the stripping gas used (steam or nitrogen) [3]. These distilled compounds are sent to a fatty acid condenser and then to a barometric condenser. However, the separation of the fatty acids condensed in the barometric condenser is very difficult. For this reason, the substitution of the above equipment by tubes-and-shell exchangers is currently being studied [4]. For design purposes, it is necessary to study the melting point (Tmp) and the melting enthalpy (ΔHmp) of both the pure fatty acids, as well as their mixtures.

Differential scanning calorimetry (DSC) is one of the most widely used techniques for studying the thermal behavior of compounds. This technique has been employed with satisfactory results in the determination of the heat capacities (Cp) of organic compounds and fatty acids [5], [6] and the measurement of Tmp in vegetables oils and triglycerides [7], [8], [9].

This study presents the results of the Tmp and ΔHmp measurements in palmitic, stearic and oleic fatty acids, their binary mixtures (palmitic/stearic, palmitic/oleic, and oleic/stearic), and their ternary mixtures (palmitic/stearic/oleic). The presence of oleic acid was increased within the range of compositions so as to reproduce those obtained in raw and deodorized oils. The measurements are compared with published values found by other authors for both pure compounds and binary mixtures.

Section snippets

Experimental

Measurements of Tmp and ΔHmp were carried out for the palmitic, stearic and oleic fatty acids, and both their binary and ternary mixtures.

The samples were prepared in a Mettler AE240 analytical balance, accurate to ±0.1 mg. The binary mixtures were weighed in three different mass ratios, approximately 1:1, 1:3 and 3:1, where 1:3 means 25% of mass weight from the first and 75% of mass weight from the second. The first selected component corresponded to the compound with the lower molecular mass.

Results and discussion

For the measurements, a wide temperature range was chosen to identify the phase transitions. The DSC curve provides the flow heat by mass versus temperature. When the phase change occurs, an abrupt change in the base line is detected, and a peak is obtained. The approach used to measure Tmp was the temperature of the peak, though other approaches also usually consider criteria such as the onset temperature or the end of the thermal process [7], [8], [9]. The heat of the phase transition, ΔHmp,

Acknowledgements

This work was financially supported by the Spanish Interdepartmental Commission for Science and Technology (CICYT) for the Project “Design and experimental validation of a methodology for design and control of shell and tubes freezing exchangers for mixtures arising from the deodorization of edible fats under high vacuum conditions” (ALI 95 0517) and a Study Grant from the Spanish Agency of Latin-American Cooperation (AECI).

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