A theoretical model of the lamellar (Lα) to inverted hexagonal (HII) phase transition is developed for mixtures of water and biological lipids. A free energy per lipid molecule is calculated for each phase as the sum of four lattice-specific terms: a local elastic term and global terms involving the packing of hydrocarbon chains, Debye-shielded electrostatics, and hydration effects. A critical lipid concentration for the transition is indicated by the concentration where the net lamellar and hexagonal free energies are equal. The relevance of the model to membrane interactions in biological systems is discussed. Suggestions are made for introducing temperature dependence into the model, and experiments are proposed to investigate the principal opposing forces in the model.
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