Experimental phase diagrams of binary fatty acid mixtures containing oleic acid
Mariana C. Costaa, Marlus P. Rolembergb, Natália D. D. Cararetoa, Cecilia Y. C. S. Kimuraa, Maria A.
Krahenbühlc, Antonio J. A. Meirellesa
a Department of Food Engineering – FEA – UNICAMP, Campina s- SP, Brazil ([email protected])
b Department of Chemical Technology - UFMA, São Luís - MA, Brazil ([email protected])
c Department of Chemical Processes – FEQ – UNICAMP, Campinas - SP ,Brazil ([email protected])
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 oleic acid. Differential Scanning
Calorimetry was used to measure the following binary mixtures: oleic acid + capric acid, oleic acid + stearic
acid, oleic acid + elaidic acid and oleic acid + linoleic acid. 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 oleic acid. Differential Scanning
Calorimetry was used to measure the following binary mixtures: oleic acid + capric acid, oleic acid + stearic
acid, oleic acid + elaidic acid and oleic acid + linoleic acid.
MATERIALS & METHODS
Reagents. The DSC equipment was calibrated using the following standards: indium (0.9999 molar fraction,
TA Instruments), benzoic acid (min 0.999 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
Linoleic acid
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 10 - 15 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: oleic acid + capric acid, oleic
acid + stearic acid, oleic acid + linoleic acid and oleic acid + 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 + linoleic acid 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.
 1 n Ti ,lit − Ti ,exp
AAD =  ∑
 n i=1
Ti ,lit


.100


(1)
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 ( xoleic acid = 0.00 ), it
appears that there is only one peak, but there are actually two peaks overlapped. For pure oleic acid (
xoleic acid = 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 oleic acid, and the other at
a greater temperature, approximately 287 K, represent complete melting of the oleic acid [3,6,12].
xoleic acid = 1.00
xoleic acid ≅ 0.95
xoleic acid ≅ 0.90
Heat Flux (a. u.)
xoleic acid ≅ 0.80
xoleic acid ≅ 0.70
xoleic acid ≅ 0.50
xoleic acid ≅ 0.40
xoleic acid ≅ 0.30
xoleic acid ≅ 0.20
xoleic acid = 0.00
1
260
270
2
280
290
Temperature
Temperature(K)
(K)
300
310
320
Figure 1. Thermograms of the oleic acid + capric acid system.
Increase if the oleic acid 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 oleic acid ≅ 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 oleic acid + 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 oleic acid ≅ 0.80 whereas the enthalpy values decrease significantly for
compositions greater than xoleic acid ≅ 0.60 . 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 oleic acid composition,
indicating that this transition can be another reaction as observed for other fatty acid systems.
310
25
a)
20
Enthalpy (kJ/mol)
Temperature (K)
300
290
280
270
260
0.0
b)
15
10
5
0.2
0.4
0.6
0.8
0
0.0
1.0
0.2
xoleic acid
0.4
0.6
0.8
1.0
xoleic acid
Figure 2. a) Phase diagram of the oleic acid + 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 oleic acid + linoleic acid system is presented in Figure 3. In this phase diagram a
transition is observed at roughly 263 K until reaching the composition of xlinoleic acid ≅ 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 xlinoleic acid ≅ 0.50 and xlinoleic acid ≅ 0.60 . 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 ≤ xlinoleic acid ≤ 1.00 . 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 xlinoleic acid ≅ 0.80 .
290
20
18
16
14
280
Enthalpy (kJ/mol)
Temperature (K)
285
275
270
12
10
8
6
4
265
2
260
0.0
0.2
0.4
0.6
xlinoleic acid
0.8
1.0
0
0.0
0.2
0.4
0.6
0.8
1.0
xlinoleic acid
Figure 3. a) Phase diagram of the oleic acid + linoleic acid system; b) Tamman plot of the oleic acid + linoleic acid
system, (●) enthalpy of the eutectic reaction, (×) and (+) enthalpy of transition under the liquidus line.
The phase diagrams of the oleic acid + elaidic acid and oleic acid + 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 oleic acid remains while in the mixture
with elaidic acid the polymorphic transition it is not observed.
320
350
a)
b)
340
310
330
Temperature (K)
Temperature (K)
320
300
290
310
300
290
280
280
270
0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
xoleic acid
0.4
0.6
0.8
1.0
xoleic acid
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 + linoleic acid system using the Margules-3-suffix model.
Table 2. AAD between experimental data and NRTL or Margules-three-suffix models.
AAD
Fatty Acids binary system
Margules-three-suffix
NRTL
oleic acid + capric acid
0.84
0.84
oleic acid + stearic acid
0.34
0.34
oleic acid + elaidic acid
0.58
0.70
oleic acid + linoleic acid
0.12
0.13
CONCLUSION
The phase diagrams of the oleic acid + capric acid, oleic acid + stearic acid, oleic acid + elaidic acid and oleic
acid + linoleic 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.
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