Fatty acid systems, phase transitions

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... 

Kaneko E, Polymorphism and phase transitions of fatty acids and acylglycerols, in Crystallization Processes in Fats and Lipid Systems, Garti N. and Sato K., eds., Marcel Dekker, New York, 2001, 53-97. 

Hand-shaken dispersions of phospholipids thus may be a more reasonable choice for comparison with membranes, and except in rare cases of highly unsaturated systems the high resolution NMR spectra of such dispersions do not exhibit high resolution proton absorption. Hence, for most membranes, NMR can indicate immobilization of fatty acid chains but not whether chains are immobilized by binding onto proteins or by tight packing into bilayers. Broad line NMR may be useful, especially when a phase transition is present. 

The most important phase for the membranes of living cells is the lamellar fluid pliase, In this phase, the conformational order of the fatty acid chains is low and the lipid lateral mobility is substantial. The lempcrature at which the liquid crystal phase alters to gel phase and, as a result, the fatty acid chains obtain a high degree of conformational order with low mobility, is called the main transition temperature (Tm). This temperature is considered as an important physicochemical characteristic at isobaric conditions, indicating that the system exists in equilibrium and the AG at the Tm point is zero (AG=0). 

Monolayers of distearoyl lecithin at hydrocarbon/water interfaces undergo temperature and fatty acid chain length dependent phase separation. In addition to these variables, it is shown here that the area and surface pressure at which phase separation begins also depend upon the structure of the hydrocarbon solvent of the hydrocarbon oil/aqueous solution interfacial system. Although the two-dimensional heats of transition for these phase separations depend little on the structure of the hydrocarbon solvent, the work of compression required to bring the monomolecular film to the state at which phase separation begins depends markedly upon the hydrocarbon solvent. Clearly any model for the behavior of phospholipid monolayers at hydrocarbon/water interfaces must account not only for the structure of the phospholipid but also for the influence of the medium in which the phospholipid hydrocarbon chains are immersed. 

Transition Metai-Cataiyzed Epoxidation with Hydrogen Peroxide. There are few transition metal-catalyzed epoxidations of unsaturated fatty acids with hydrogen peroxide. By using a procediue developed by Ishii et al. (26), Bavaj (27) in oiu group epoxidized various fatty acid esters in a two-phase system with a combination of a tungsten heteropoly acid and a phase-transfer catalyst, using 30% H2O2 as the oxidant. (Table 1). 

Figure 14 Phase behavior transitions as the concentration of sodium hydroxide is increased in a system containing a fatty acid, oil, and water.

Fig. 1. The fatty acid soap-water phase diagram of McBain (58) modified (1) to show the molecular arrangement in relation to aqueous concentration (abscissa) and temperature (ordinate). Ideal solution, i.e., true molecular solution, is to the left of the vertical dashed line, indicating the critical micellar concentration (CMC), which varies little with temperature. At concentrations above the CMC, provided that the temperature is above the critical micellar temperature (CMT), a micellar phase is present. At high concentrations, the soap exists in a liquid crystalline arrangement, provided that the solution is above the transition temperature of the system, i.e., the temperature at which a crystalline phase becomes liquid crystalline. The Krafft point is best defined (D. M. Small, personal communication) as the triple point, i.e., the concentration and temperature at which the three phases (true solution, micelles, and solid crystals) coexist, but in the past the Krafft point has been equated with the CMT. The diagram emphasizes the requirement for micelle formation (a) a concentration above the CMC, (b) temperature above the CMT, and (c) a concentration below that at which the transition from micelles to liquid crystals occurs. Modified from Hofmann and Small (1).

The melting properties are of crucial importance to the technical functionality of emulsifiers, in addition to their amphiphilic properties. Most food and feed emulsifiers are based on natural fat sources, thus giving different melting properties. The consequences of the melting properties can be expressed as the Krafft temperature (i.e. the temperature at which the solubility is above the critical micelle concentration) or as the transition temperature (chain melting temperature, i.e. the melting temperature of the fatty acids in a semicrystaline bilayer). The transition temperature in an emulsifier water system forming a lamellar liquid crystalline phase... 

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