Methyl palmitate system

Figure 2. Mixed monolayers of cholesterol- methyl palmitate system at 27° C. and pH 6...

We are now in a position to examine the various systems listed in Table I which follow Type I behavior. Experimentally (Figure 2) the behavior of the cholesterol-methyl palmitate system shows excellent agreement with the generalized system depicted in Figures 12 and 13,... 

As for the ethane/palmitic acid system, only a single sonrce contains data for the ethane/methyl palmitate system. The data is outlined in Table 8 and the phase behavionr for the system ethane/methyl palmitate is shown in Figure 12. 

The data for the ethane/ethyl pahnititate system is veiy similar to that of the ethane/methyl palmitate system and the same comments thus apply. 

As for the propane/methyl palmitate system, total solubility can be attained at very low pressures (< 8 MPa) for the propane/ethyl palmitate system. Similar to the observed linear relationship between temperature and the phase transition pressure, Schwarz et al. [30] also foimd a linear relationship between the phase transition pressure and temperature. They also did not observe any three phase behaviour or indications thereof. 

Eight binary systems are reported. Six include cholesterol as one component of the mixed film. The second components in these six films were myristic acid, methyl palmitate, ethyl palmitate, 1,2-dimyristin, 1,2-dimyristoyl-3-lecithin, and l,2-didecanoyl-3-lecithin. In addition, the systems trilaurin-dimyristoyl lecithin and triolein-dimyristoyl lecithin... 

Methyl Esters. This fraction can be examined first by its behavior on unidimensional thin-layer chromatography. In a solvent system of petroleum ether (30-60°)-diethyl ether-glacial acetic acid (90 10 1, v/v) and using silica gel G (250 p-m) plates, there was only a single spot at Rf 0.65-0.67. This compared exactly with a standard synthetic methyl palmitate. As described in Chapter 4, the chemical nature of the methyl esters can be obtained by analysis on gas-liquid chromatography coupled with mass spectrometry. 

Lockemann CA, Schliinder EU. High-pressure mass transfer coefficients in the liquid phase of the binary systems carbon dioxide-methyl myristate and carbon dioxide-methyl palmitate. Chem Eng Proc 1996 35 121-129. 

SCF processing is no different. In fact, a phase behaviour analysis is vital as it provides an estimation of the operating pressitfes required and also indicates whether separation fi om other components will be possible. This section will focus on the phase behavioiu of palmitic acid, methyl palmitate, ethyl pahnitate and tripalmitin in SC CO2, ethane and propane and concentrate on the data available and trends observed therein. In addition, the three solvents will be eompared and the effect of co-solvents, often used to decrease the operating pressiu e, will be eonsidered. In particular this section will focus on the phase transition pressures of the systems studied. The phase transition pressure indieates the pressitfe required for total solubility at the said temperature and composition. For the type of systems studied here, a higher phase transition pressure leads to a lower solubility, therefore lower phase transition pressures indieate improved solubility. 

Outlined in Table 4 is a summary of data for the system C02/methyl palmitate. Due to the low melting point of the data (See Table 1), all data is of a vapom-liquid equilibrimn (VLE) nature. 

Figme 5 shows the phase equilibrium data of the system C02/methyl palmitate of both Inomata et al. [48] and Lockemarm [49]. Figure 5 hints towards significant differences between the data sets, yet insufficient information is available to determine which data set us superior. However, the data clearly shows that an increase in temperature results in an increase in phase transition pressure. Due to the scatter in the data and slight inconsistencies between the two sources the exact nature of the relationship between temperature and the phase transition pressure can not be determined. Additional measurements would be required therefore. Both sets of data do, however, indicate that total solubility can be achieved at moderate pressures (less than 25 MPa at temperatures below 343 K). 

Table 4. Literature data for the CO2 (l)/methyl palmitate (2) system...

Figure 9. Comparison of the pressure - composition (W2) for the systems CO2 (l)/palmitic acid (2) [34,43], CO2 (l)/methyl palmitate (2) [48,49], COj (l)/ethyl palmitate (2) [26,51] andC02 (l)/tripalmitin (2) [32,43,54] systems at 313.15 K (a) entire composition range and (b) detail of low molecular mass composition range.

Schwarz et al. [29] found that total solubihty can be attained for the system ethane/methyl palmitate at low pressnres (< 15 MPa) and that, as for the ethane/pahnitic acid system, a linear relationship exists between the temperature and the phase transition pressure at constant composition in the temperature range studied. 

In general similar phase behaviour trends were observed between the systems. Tripalmitin is the least soluble in ethane followed by palmitic acid. Methyl palmitate and ethyl palmitate show high solubility in ethane and show total miscibility at pressures below 15 MPa in the temperature range studied. As observed by Schwarz et al. [30] very little difference exists between the phase behaviour of ethane/methyl palmitate and ethane/ethyl palmitate systems, most probably due to the similarity in the nature of their structure. 

Rovetto et al. [59] published phase equilibrium data for the system propane/methyl palmitate. The data is summarised in Table 11 and Figure 17 shows the phase behaviour. 

As for the propane/methyl palmitate data, the propane/tripalmitin data presented was generated using a linear relationship fitted to the experimentally published data of Coorens et al. [31]. Once again an exeellent fit was obtained (R > 0.997 in all cases), therefore illustrating the linear relationship between temperature and the phase transition pressure. Coorens et al. [31] also published vapoiu -liquid-liquid equilibrium data showing that the system has a three phase region between 349 and 370 K. However, eompositions were not ineluded. 

Many of the comments made for the phase behaviour of palmitic acid and its derivatives in CO2 and in ethane are also valid for propane as SC solvent. Figure 21 shows a comparison of the propane/tripalmitin, propane/palmitie acid, propane/methyl palmitate and propane/ethyl palmitate systems. 

While it is believed that solute-solute interactions are not as large as solute-solvent interactions, there is clear evidence that some type of solute-solute interaction is present in SCF/high molecular mass systems. Lockemann [49] studied the phase behaviour of the ternary system C02/methyl myristate/methyl palmitate and found that these two components can be separated using SC CO2. However, the separation factor, which dictates the degree or difficulty of separation, is dependent on the composition of the feed to be separated and the operating pressure and temperature. They found that while the composition does not significantly affect the separation factor, better separation can be achieved at lower composition of the component to be extracted. 

Schlunder, E.-U. and Lockemann, C.A. (1995) Liquid-phase viscosities of the binary systems carbon dioxide-oleic acid, carbon dioxide-methyl myristate, and carbon dioxide-methyl palmitate at high pressure. Chem. Eng. Process., 34, 487-493. 

Fig. 25 Surface shear viscosity vs. film composition for the palmitic acid/stearoylserine methyl ester film system at 25°C.

Fig. 26 Hysteresis isotherms for the 1/5 palmitic acid/(A) racemic and (B) enantiomeric stearoylserine methyl ester (17% palmitic acid) monolayer system at 25°C. Arrows indicate direction of expansion and compression. Reprinted with permission from Arnett et al., 1989. Copyright 1989 American Chemical Society.

In plant systems, de novo synthesis occurs in the plastid and results mainly in the conversion of acetate to palmitate. All 16 carbon atoms in palmitic acid are derived from acetate— half from the methyl carbon and half from the acyl carbon. Two of the carbon atoms (C-15 and C-16) come directly from acetate, and the other 14 come from acetate via the more reactive malonate. Production of malonate requires the incorporation of an additional carbon atom into the acetyl group. This is supplied as bicarbonate, and this same carbon atom is subsequently lost as carbon dioxide. The acyl groups are attached to co-enzyme A (CoASH) during part of the cycle and to acyl carrier protein (ACPSH) during another part. The abbreviated symbols used for these co-enzymes emphasize the thiol groups (SH) to which the acyl chains are attached. 

During the inhibited self-initiated autoxidation of methyl linoleate by a-Toc in solution, Niki and coworkers made the interesting observation that a-Toc acts as an antioxidant at low concentrations, but high concentrations (up to 18.3 mM) actually increased hydroperoxide formation due to a pro-oxidant effect. The pro-oxidant effect of a-Toc was observed earlier by Cillard and coworkers in aqueous micellar systems and they found that the presence of co-antioxidants such as cysteine, BHT, hydroquinone or ascor-byl palmitate inverted the reaction into antioxidant activity, apparently by reduction of a-To" to a-Toc . Liu and coworkers ° found that a mixture of linoleic acid and linoleate hydroperoxides and a-Toc in SDS micelles exhibited oxygen uptake after the addition of a-Toc. The typical ESR spectrum of the a-To" radical was observed from the mixture. They attributed the rapid oxidation to decomposition of linoleate hydroperoxides, resulting in the formation of linoleate oxy radicals which initiated reactions on the lipid in the high concentration of the micellar micro-environment. Niki and coworkers reported pro-oxidant activity of a-Toc when it was added with metal ions, Fe3+25i Qj. jjj (jjg oxidation of phosphatidyl choline liposomes. a-Toc was found... 

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