Mixed bilayers melting

The phase transition of lipid bilayers which comprise phospholipid mixtures, or phospholipids with different lengths of acyl chains, are of great importance due to their similarities with biomembranes or with lipid drug carriers such as liposomes, it is important to study the phase transitions and to detennine the exact crystalline mesophases of the mixed system. Thermal analysis studies indicated that the mi.xed lipids provide higher melting temperatures compared to those of pure lipids. This phenomenon occurrs w hen the lengths of the acyl chains are substantially different. 

The interaction of cholesterol with membrane phospholipids is further complicated by findings that cholesterol is not necessarily uniformly dispersed in mixed phospholipid bilayers. In DSC studies on bilayers composed of two lipids of the same class, but with melting points sufficiently different to give two peaks in the calorimeter, cholesterol associates with the lower melting point lipid. That is, it appears to be frozen out of solid crystalline regions of the bilayer. In addition, however, cholesterol has a preference for specific lipid classes that overides its preference for lower melting point lipids... 

Flow also induces the formation of micro-emulsions, as it can detach the pinch-offs from the interface. As shown by Kim et al. [76] for a bilayer of reactive PS-end-carboxylic and PMMA-ran-epoxy, micro-emulsions could be observed in a shear fiow under a shear rate of 100 s after less than 50 min, while for static conditions they would form after 15 h [57[. In the case of melt blends prepared in internal mixers, micro-emulsions could be detected after less than 5 min of mixing, thus demonstrating again the importance of fiow [68[. 

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