CORRELATIONS BETWEEN CHEMICAL-STRUCTURE AND CHAIN PACKING IN 2-DIMENSIONAL AND 3-DIMENSIONAL SYSTEMS

Phosphatidylcholines (PCs) containing a branched chain fatty acid at the sn-1 position of the glycerol backbone were characterized by calorimetric and X-ray diffraction studies with bilayers as well as by thermodynamic studies with monolayers. Branching leads to altered thermal properties which are connected with a modified structural polymorphism. Gel phases with interdigitated chains or nonbilayer liquid-crystalline phases could be found in dependence on the length of the side branches. The pressure-area isotherms of the monobranched PCs were measured over a wide range of temperature. The critical temperature T(c) of the transition between the liquid-expanded and condensed films were estimated and compared with the main transition temperatures T(m) of the bulk phase. T(c) was found to be either lower or higher or equal to T(m). The correlations between T(c) and T(m) are discussed in terms of chain packing in the two- and three dimensional systems.