The use of DSC in the determination of the vapor
pressure of fatty acids
Rafael M. Matricarde Falleiro a, Antonio J. A. Meirelles b, Maria A. Krähenbühl a, *
Laboratório de EXtração,
TeRmodinâmica Aplicada e Equilíbrio
a Laboratory
b Laboratory
of Thermodynamic Properties, LPT, School of Chemical Engineering, University of Campinas, Brazil
of Extraction, Applied Thermodynamics and Equilibrium, ExTrAE, School of Food Engineering, University of Campinas, Brazil
INTRODUCTION
The present work aims to determine the vapor pressure of longchain 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].
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].
(b)
(a)
Palmitic, stearic and 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.
(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
(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.
Fatty acids
A*
B*
C*
Validity range / ºC
Mean deviation / ºCa
Palmitic acid
5.7743
1111.1435
22.6771
210.14 to 260.24
0.27
Stearic acid
9.7847
4289.055
260.1239
228.12 to 280.09
0.37
Oleic acid
10.822
5380.57
324.261
223.52 to 274.25
0.32
* log
B
p (mmHg)  A T (º C)  C
N
Mean deviation / º C   Texperiment al  Tcalculated
a
i 1
1

N
, N7
1.0
(b)
log10 p / kPa
0.8
(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.
0.6
0.4
0.2
0.0
0.0035
0.0040
-1
T / ºC
0.0045
0.0050
-1
Figure 5 - Vapor pressure curves of the fatty acids. ()
tungsten ball
palmitic acid; (●) stearic acid; (○) oleic acid and (−) Antoine.
pinhole
Figure 2 – Small ball and the crucible with a
pinhole
CONCLUSION
Figure 3 – Tungsten ball being placed on the
pinhole
RESULTS AND DISCUSSION
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].
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.
REFERÊNCIAS BIBLIOGRÁFICAS
[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.