| In this dissertation, a macroscopic theory has been developed for lipid membrane phase transitions, after the classic example of van der Waals theory for liquid-gas phase transitions. The theory gives Gibbs potential of lipid membrane, based on molecular mean field calculations of inter-chain van der Waals and excluded volume interactions, and on statistical mechanical calculations of gauche bond entropy. Our fundamental hypothesis is that lipid membrane phase transitions can be characterized by a single variable, the molecular orientation order parameter. With chain length and asymmetry incorporated, a single equation gives accurate descriptions of a class of phosphatidylcholines, which form non-/partial-interdigitated gel phase. The theory gives transition temperature and enthalpy, van der Waals energy, number of gauche bonds, molecular orientation order parameter, gauche bond and excluded volume entropies, achieving excellent agreements with all available experiment data in literature. In addition, the theory establishes the foundation for weakly first-order phase transitions of lipid membrane regarding pseudo-critical exponents of heat capacity, relaxation time, lateral and bulk isothermal compressibility. Classical and non-classical exponents have been derived. Another part of this work is to rebuild a volume-perturbation calorimeter, which works by forcing a sample of lipid dispersion to undergo small adiabatic changes and monitoring its relaxations to yield kinetic information on the transition. The data analysis is performed in frequency space based on linear response theory, yielding multiexponential relaxation amplitudes and times. Phenomena observed by our restored volume perturbation calorimetry, as well as those observed a decade ago, are interpreted by our theory as pseudo-critical phenomena due to pseudo-Critical slowing down. |
