Long-chain fatty acid systems, phase

Effect of Thermal History and Impurities on Phase Transitions in Long-Chain Fatty Acid Systems... 

Another interesting class of phase transitions is that of internal transitions within amphiphilic monolayers or bilayers. In particular, monolayers of amphiphiles at the air/water interface (Langmuir monolayers) have been intensively studied in the past as experimentally fairly accessible model systems [16,17]. A schematic phase diagram for long chain fatty acids, alcohols, or lipids is shown in Fig. 4. On increasing the area per molecule, one observes two distinct coexistence regions between fluid phases a transition from a highly diluted, gas -like phase into a more condensed liquid expanded phase, and a second transition into an even denser... 

In the ebb phase, there is increased activity of the sympathetic nervous system and increased plasma levels of adrenaline and glucocorticoids but a decreased level of insulin. This results in mobilisation of glycogen in the liver and triacylglycerol in adipose tissue, so that the levels of two major fuels in the blood, glucose and long-chain fatty acids, are increased. This is, effectively, the stress response to trauma. These changes continue and are extended into the flow phase as the immune cells are activated and secrete the proinflammatory cytokines that further stimulate the mobilisation of fuel stores (Table 18.2). Thus the sequence is trauma increased endocrine hormone levels increased immune response increased levels of cytokines metabolic responses. 

Miwa et al. (26) have demonstrated that both short- and long-chain fatty acids can also be converted into their 2-nitrophenylhydrazides and separated bv RP-HPLC with acetonitrile-water as the eluent. They have described a method for the direct derivatization without an extraction step and the simultaneous microanalysis of 14 kinds of C 0 0-C22 6 fatty acid hydrazides in a reverse-phase HPLC system (27). 

As with conventional emulsions the nature of the oil can affect the behaviour of the system. For pharmaceutical uses, oils used include the refined hydrocarbon oils such as light liquid paraffin and esters of long-chain fatty acids including vegetable oils, for example, ethyl oleate and isopropyl myristate, olive oil and sesame oil. Frankenfeld et al (SO used mixtures of Solvent 100 Neutral (an isoparaffinic, dewaxed oil of high viscosity) and Norpar 13 (a non-viscous, normal paraffinic solvent) to vary the viscosity of the oil phase in attempts to control the transfer of solutes across the oil membrane. 

The most important part of all chromatographic systems is the separator column. The choice of a suitable stationary phase (see Section 1.5) and the chromatographic conditions determine the quality of the analysis. The column tubes are manufactured from inert material such as Tefzec or epoxy resins. In general, separation is achieved at room temperature. Elevated temperatures are required only in very few cases, such as the analysis of carbohydrates or long-chain fatty acids. 

Fat-soluble vitamins such as retinol and )3-carotene are readily dissolved in mixed lipid-bile salt micelles in vitro. Retinol is approximately 10 times more readily dissolved in such micelles than /3-carotene [105]. It is likely that these two substances occupy different regions of the micelle. The difference in solubility, therefore, may reflect the limited capacity of the nonpolar core of the micelle for the relatively bulky /3-carotene molecule. Retinol, on the other hand, may occupy a more hydrophilic region of the micelle. In a mixed oil/micellar system, a-tocopherol distributes between the two phases, its concentration in the micellar phase being enhanced by expansion of the micelles with monoglycerides and lecithin of long-chain fatty acids. However, lipids containing medium-chain fatty acids do not expand the micelles as effectively as their long-chain counterparts such that there is less solubilization of a-tocopherol in the micellar phase [106]. 

The main functional groups on paper are hydroxyls with acids and aldehydes next in amount. These, along with the adsorbed water, make the system polar and not particularly good to separate nonpolar compounds. To obtain a nonpolar stationary phase (reversed phase), the paper is first dried thoroughly. It is then dipped in a nonpolar liquid and dabbed dry with paper towels. This produces a "reversed phase" suitable for separating water-insoluble substances like steroids, long chain fatty acids, or chlorinated insecticides. Figure... 

For long-chain fatty acids. Pa can be in the 100-1000 range. In other words, neglecting the amount of acid present in the oil phase would be equivalent to neglecting most of it. It is not known for sure how the dissociation and partitioning equilibria are related to the composition of the interfacially adsorbed surfactant mixture layer, because such a system... 

A micromembrane suppressor for ion-exclusion chromatography has been introduced under the trade name AMMS-ICE. Its structure corresponds to the systems developed for anion and cation exchange chromatography (see Sections 3.6.3 and 4.3.3). However, in its mode of operation, it corresponds to the AFS-2 hollow fiber suppressor. An AMMS-ICE micromembrane suppressor also contains membranes that are compatible with water-miscible organic solvents. Therefore, it is used for the analysis of long-chain fatty acids, which are separated on a non polar stationary phase in a weakly acidic medium with methanol or acetonitrile as mobile phase components. In this case, a dilute potassium hydroxide solution is used as the regenerant. With respect to the ion-exchange... 

The long-chain fatty acid moieties contribute hydrophobic properties those properties are counterbalanced by the polar or hydrophilic character of the phosphate moiety. In an oil-water system the phospholipid components concentrate at the oil/water interface. The polar, hydrophilic parts of the molecules are directed toward the aqueous phase, and the nonpolar, hydrophobic (or lipophilic) parts are directed toward the oil phase. Concentration of phosphatides at the oil/ water interface lowers the surface tension and makes it possible for emulsions to form. Once the emulsion is formed, the phosphatide molecules at the surface of the oil or water droplets act as barriers to prevent the droplets from coalescing and thus stabilize the emulsion. 

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