Theory of Orientational Order in Chain Molecules –Phase Transitions in n-Alkanes and Lipid Membranes–

By assuming an attractive potential between molecules which depends on their orientations and configurations, an orientational phase transition accompanied with a configurational change of molecules are studied in mean field approximation. A “two state model”, in which only the state of “long” configuration causes the orientational order, is assumed. The phase transition temperature T c , the orientational order parameter S , the averaged extension of molecules, and the transition entropy are calculated as functions of the statistical weight of “long” configuration. The results are investigated with respect to the chain-length dependences. By analysing thermal data, the orientational order parameters adjacently below the transition temperature are estimated as S ∼0.95 for n-alkane crystals and as 0.7–0.8 for lipid membranes.