Enhancement of Steric Repulsion with Temperature in Oriented Lipid Multilayers

We have studied the temperature dependence of the stacking periodicity, $d$, of oriented phospholipid multilayers using grazing angle neutron scattering techniques. $d$ is found to increase substantially at higher temperatures, just before the bilayers peel off from the substrate. Although we do not observe thermal unbinding, our results are consistent with the notion that the unbinding transition is driven by steric repulsion arising from thermal fluctuations of the membranes, in contrast to those of a recent study by Vogel et al. [Phys. Rev. Lett. 84, 390 (2000)].