Critical Fluctuations in Membranes

High resolution x-ray scattering measurements on model membranes consisting of fully hydrated lipid bilayers have been performed to evaluate the origin of the anomalous temperature dependence of the lamellar $D$ spacing. The data are well fit by our recent refinement of the theory of power law tails that occur in fluctuating smectic liquid crystals. The data show that the anomalous increase in $D$ is due to an increase in the thickness of the bilayer, caused by critical straightening of the hydrocarbon chains, and that little change in undulation force takes place.