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.