Phase Transitions in a Lipid Bilayer Embedded in a Crystalline Matrix

The hydrocarbon part of (${\mathrm{C}}_{10}$${\mathrm{H}}_{21}$N${\mathrm{H}}_{3}$)$_{2}\mathrm{Cd}$${\mathrm{Cl}}_{4}$ represents a smectic lipid bilayer exhibiting two structural phase transitions in analogy to biomembranes. Single-crystal $^{13}\mathrm{C}$ NMR and $^{14}\mathrm{N}$ nuclear quadrupole resonance and x-ray data show that the first transition is connected with a disordering of the polar heads whereas the second represents a partial melting of the lipid chains. A Landau theory describing the melting of bilayer membranes in terms of order parameters used in the theory of liquid crystals has been constructed to explain the obtained results.