Compressibility studies of some vegetable oils up to 1 GPa

Transcription

1 High Pressure Research Vol. 29, No. 4, December 2009, Compressibility studies of some vegetable oils up to 1 GPa A.J. Rostocki a *, D.B. Tefelski a and S. Ptasznik b a Faculty of Physics, Warsaw University of Technology, ul. Koszykowa 75, Warszawa, Poland; b Meat and Fat Research Institute, Jubilerska 4, Warszawa, Poland (Received 9 September 2009; final version received 31 October 2009 ) The paper presents compressibility studies of some vegetable oils within the range of 1 GPa. The measurements of the volume were done for rapeseed oil, soy oil, sunflower oil and linseed oil at constant temperature. On this basis, the modified Tait equation coefficients were evaluated. In the case of rapeseed oil, soy oil and sunflower oil, the discontinuity of P V characteristics for phase transition has been observed. The value of Tait coefficients for the pressure above the phase transition suggests that the high-pressure phase is solidified. The experiment was conducted with a computer-based data acquisition system with software written in National Instruments LabVIEW graphical programming language. Piston displacement was measured by electronic digital calliper with RS232 interface. Keywords: compressibility; vegetable oils; Tait equation; high pressure; phase transitions 1. Introduction Vegetable oils are one of the most important materials in food industry. High-pressure food processing turned the attention of laboratories to the high-pressure properties of those oils. The area requires more advanced research because of a possible modification of triglycerides, which are main components of these oils. A possibility of irreversible changes caused by phase transitions observed in both fatty acids and their triglycerides is bringing the risk of such modifications, which can be dangerous for human health. The high-pressure properties of vegetable oils have been the subject of intensive research in our laboratory since 2004 [1 6] and are the continuation of earlier works on castor oil [7 10]. In all investigated oils, the first-order phase transition has been observed due to the solidification of those oils. One of the main interesting features of those experiments was to find a proper equation of state which describes both liquid and solid phases of the investigated oils. In this work, an attempt of an application of the modified Tait equation is presented. *Corresponding author. arostock@if.pw.edu.pl ISSN print/issn online 2009 Taylor & Francis DOI: /

2 722 A.J. Rostocki et al. Figure 1. Table 1. Structure of triglycerides: (a) schematic and (b) spatial. Content of fatty acids in investigated oils. Oil C14:0 C16:0 C16:1 C18:0 C18:1 C18:2 C18:3 C20:0 Others Castor oil Rapeseed oil Soybean oil Sunflower oil Linseed oil Triolein Structure and properties of vegetable oils Vegetable oils consist of a mixture of various triglycerides that have a structure as shown in Figure 1. The long chains shown in Figure 1 consist of residuals of fatty acids. The content of particular fatty acids in the investigated oils is shown in Table 1. The numerical symbols in the first row represent fatty acids that are shown in Table 2. With an increase in the pressure, the intermolecular distances are gradually reduced, causing an increase in the van der Waals attractive forces. This leads to the packing of particles into a chair structure. The molecular order of close-packed triglycerides was investigated by Ferguson and Lutton [11]. They discovered the crystal structure of triolein by X-ray diffraction. A more detailed description of the triglyceride structure has been presented in [12 14]. Within those papers, the crystal structure was achieved by cooling, whereas in our paper it has been obtained by a high-pressure process. 3. The experiment The experiments were carried out in a 17 mm chamber, as described in [1 4]. The chamber was connected to an automatic measuring multichannel system. All data were simultaneously Table 2. Fatty acids symbols. Fatty acid Symbol Miristic acid C14:0 Palmitic acid C16:0 Palmitoleic acid C16:1 Stearic acid C18:0 Oleic acid C18:1 Linoleic acid C18:2 α-linolenic acid C18:3 Arachidic acid C20:0

3 High Pressure Research 723 Figure 2. Experiment setup. collected by a system designed in National Instruments LabVIEW for the PC. For the pressure measurement, the typical manganin transducers have been used. The transducers were calibrated using a 0.05% accuracy dead weight piston gauge and high-resolution multimeter HP 3458A. Its resistance was measured by a precise HP multimeter also working in this system as analog multiplexer. The measurements of compressibility were done using a digital calliper of 0.01 mm resolution for the measurement of the piston position. All experiments were carried out at a temperature of 293 K. The experimental setup is presented in Figure Results and discussion In most of the experiments, the pressure was elevated by 20 MPa steps and stopped for few minutes to approach the thermal equilibrium. In some cases, other parameters, such as dielectric permeability, viscosity and sound velocity, which are not a subject of this paper, were also measured [1 7]. Careful observation of those parameters was helpful in the detection of the beginning of the phase transition. The increase in pressure was stopped for the whole process when the phase transition was about to begin. During the phase transition, the pressure was kept constant. In such cases, the volume decreased significantly. An example of such a process in shown in Figure 3. In some experiments, during the phase transition the position of the piston was blocked, assuring constant volume condition. In such cases, during transition, the pressure was decreasing even up to 100 MPa. An example of such a process is shown in Figure 4. The new phase has the volume smaller than the liquid phase. Volume changes and phase transitions are shown in Figures 3 and 4. For an analysis of the results, the Tait equation was adopted following the works of Malhotra and Woolf [15]. The modified Tait equation can be presented in the form: V V 0 = 1 C ln ( 1 + P B where V is the volume, V 0 the initial volume, P the pressure and C and B the fitting factors. Fitting factors for particular oils are presented in Table 3. The first observation of the volume changes of castor oil due to the phase transition has already been reported in [8]. It was noticed in that paper that at a certain pressure the volume of castor oil ),

4 724 A.J. Rostocki et al. V/V ΔV Pressure [GPa] Figure 3. Volume changes in triolein. Pluss sign shows experimental data. Curve presents the fitted Tait equation. Breakdown explains phase transition at constant pressure V/V ΔP Pressure [GPa] Figure 4. Volume changes in castor oil. Plus signs show experimental data. Curve presents the fitted Tait equation. Breakdown explains phase transition at constant volume. Table 3. Obtained parameters in the Tait equation for vegetable oils. Sample C C B (GPa) B (GPa) C a C a B a (GPa) B a (GPa) Triolein Castor oil Sunflower oil Soya oil Rapeseed oil Linen oil b Notes: a After phase transition; b Temperature stabilised at 40 C.

5 High Pressure Research 725 was shrinking even when the pressure was kept constant. Alternatively, at a constant position of the piston, the pressure was going down as long as the phase transition was going on. The Tait equation estimates the bulk modulus of investigated liquids [16] as a ratio B/C. It can also be interpreted within the free volume theory [16 18]. The changes of volume may result from the following effects: (1) close packing of the molecules into the β and β molecular crystal ordering; (2) reduction of the free volume; (3) reduction of the molecular oscillation. 5. Conclusions (1) All investigated oils have shown first-order phase transition with significant changes of volume. (2) During the phase transition, the change of the bulk modulus took place. References [1] R. Kościesza, A.J. Rostocki, A. Kos, D.B. Tefelski, R.M. Siegoczyński, and Ł. Zych, Observation of pressure induced phase transitions in the rapeseed oil with methyl alcohol mixtures, High Press. Res. 27 (2007), pp [2] A.J. Rostocki, R. Kościesza, D.B. Tefelski, A. Kos, R.M. Siegoczyński, and Ł. Chruściński, Pressure induced phase transition in soy oil, High Press. Res. 27 (2007), pp [3] A.J. Rostocki, R. Wiśniewski, and T. Wilczyńska, High pressure phase transition in rapeseed oil, J. Mol. Liq. 135 (2007), pp [4] A.J. Rostocki, R.M. Siegoczyński, J. Galas, R. Kościesza, and D.B. Tefelski, Pressure induced volume changes in sunflower oil, Paper Presented on 5ISCEPP Conference, Segovia, [5] A. Kos, D.B. Tefelski, R. Kościesza, A.J. Rostocki, A. Roszkiewicz, W. Ejchart, C. Jastrzȩbski, and R.M. Siegoczyński, Certain physico chemical properties of triolein and methyl alcohol triolein mixture under pressure, High Press. Res. 27 (2007), pp [6] D.B. Tefelski, A.J. Rostocki, A. Kos, R. Kościesza, and R.M. Siegoczyński, Pressure induced volume changes in triolein with addition of methyl alcohol, High Press. Res. 27 (2007), pp [7] R.M. Siegoczyński, J. Jȩdrzejewski, and R. Wiśniewski, Long time relaxation effect of liquid castor oil under high pressure condition, High Press. Res. 1 (1989), pp [8] R. Wiśniewski, J. Jȩdrzejewski, R.M. Siegoczyński, and A. Tkacz, Volume changes of castor oil during its transformation to the high pressure phase, High Press. Res. 11 (1994), p [9] R.M. Siegoczyński, Optyczne badania przejść fazowych indukowanych ciśnieniem w pewnej klasie cieczy lepkich, Reports of the Institute of Physics, Publishing House of the Warsaw University of Technology, No. 46, 1998 (in Polish). [10] R. Wiśniewski, R.M. Siegoczyński, A. Długosz, M. Przewłocki, M. Szymański, and M. Trzeciecki, Investigations of triolein under high pressure, High Temp. High Press. 33 (2001), pp [11] R.H. Ferguson and E.S. Lutton, The polymorphism of triolein, J. Am. Chem. Soc. 69(6) (1947), pp [12] K. Sato, Solidification and phase transformation behaviour of food fats a review, Fett/Lipid 101(12) (1999), pp [13] C.Akita, T. Kawaguchi, and F. Kaneko, Structural study on polymorphism of cis unsaturated triacylglycerol: triolein, J. Phys. Chem. B 110 (2006), pp [14] E. Da Silva and D. Rousseau, Molecular order and thermodynamics of the solid liquid transition in triglycerides via Raman spectroscopy, Phys. Chem. Chem. Phys. 10 (2008), pp [15] R. Malhotra and L.A. Woolf, Extrapolation of (p,v,t) data for liquids, HighTemp. High Press. 23 (1991), pp [16] G.A. Neece and D.R. Squire, On the Tait and related empirical equations of state, J. Phys. Chem. 72 (1968), pp [17] S.M. Oversteegen and R. Roth, General methods for free volume theory, J. Chem. Phys. 122 (2005), pp [18] K. Arakawa and K. Kojima, Investigation of the concept of free volume by means of a correlation function method, Bull. Chem. Soc. Jpn 48 (1975), pp

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