Phase transition temperature chain-melting

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... 

Hydrated bilayers containing one or more lipid components are commonly employed as models for biological membranes. These model systems exhibit a multiplicity of structural phases that are not observed in biological membranes. In the state that is analogous to fluid biological membranes, the liquid crystal or La bilayer phase present above the main bilayer phase transition temperature, Ta, the lipid hydrocarbon chains are conforma-tionally disordered and fluid ( melted ), and the lipids diffuse in the plane of the bilayer. At temperatures well below Ta, hydrated bilayers exist in the gel, or Lp, state in which the mostly all-trans chains are collectively tilted and pack in a regular two-dimensional... 

The dependence of the chain-melting or main phase transition temperature in a lipid bilayer, Tm, on the nature of the lipid head group as well as the length of the acyl chains and the degree and type (cis- or trans-) of unsaturation of the acyl chains has been studied exhaustively for some diacyl phospholipid classes (21). Bulkier head groups result in lower values of... 

Fig. 1 Phase transition temperature as a function of chain length, n, for the [Cnmim][BF4] series where n - 0-16,18 [123]. open squares, melting point filled circles, glass transition filled triangles, clearing transition...

Figure 41.1 shows the gel-to-liquid crystalline phase transition temperatures (Tm) of DPPC-cholesterol mixtures as a function of the cholesterol-lipid molar ratio. The Tm of fully hydrated DPPC is 42°C (Crowe and Crowe, 1988 Vist and Davis, 1990 McMullen et al., 1993 Ohtake et al., 2004). Upon the addition of cholesterol, the transition enthalpy decreases continuously imtil it is no longer observable at 50 mol% cholesterol. The disappearance of the melting transition has been attributed to strong interactions between cholesterol and DPPC (McCoimell, 2003). Upon dehydration, the Tm for DPPC increases from 42 to 105°C (Crowe and Crowe, 1988 Ohtake et al., 2004). This Tm increase is caused by the reduction in the spacing between the phospholipids, which allows for increased van der Waals interactions between the lipid hydrocarbon chains (Koster et al., 1994). Between 10 and 70 mol% cholesterol, two endothermic transitions are observed, both lower than the Tm of the pure phospholipid (Figure 41.1). High-sensitivity DSC studies on fully hydrated DPPC-cholesterol systems reported endotherms consisting of two components, suggesting the existence of domains enriched/depleted in cholesterol (Vist and Davis, 1990 McMullen et al., 1993). The two peaks present in our freeze-dried systems also suggest the...

Brain phospholipid can exist in the presence of water in two different phases (15), the usual so-called lamellar phase and a hexagonal phase dependent upon temperature and concentration. The temperature at which a particular phase of this type can exist for a given phospholipid will also depend upon the transition temperature for melting of the hydrocarbon chains. 

Cevec, G. 1991. How membrane chain-melting phase-transition temperature is affected by the lipid chain asymmetry and degree of unsaturation an effective chain length model. Biochemistry, 30, 7186. 

Figure 96 Variation of the transition temperatures of complexes (213) with alkoxy and V-alkyl chain length. Tm melting, Ts-n SmC-to-N phase transition temperature, Tc clearing.

Figure 13 shows the variation of the phase transition temperature with concentration of dication artificial amphiphile. In case of dication artificial amphiphile, two mesomorphic phases were recognized. These phase transitions may arise from melting of the part of hydrocarbon chain and the whole part of hydrophobic portion. Figure 13... 

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