Abstract
A highly hydrated lipid bilayer, floating a few Å above another one adsorbed on a smooth solid substrate, was prepared at room temperature where lipids (DSPC) are in gel phase, then heated at several temperatures including the pretransitional temperature Tp and the chain melting temperature Tm. A precise in situ characterization by neutron reflectivity led to two new results. First, even when in fluid phase the floating bilayer was structured and stable; it thus provides a flexible model system for physical and biophysical studies of membranes. Second, at intermediate temperatures, a spectacular maximum in both inter-bilayer distance and bilayer roughness was simultaneously observed. It might be a direct observation of the balance between energy minimization and entropic repulsion, leading to an estimation of the dimensionless parameter (kBT)2/Aκ, where A is the Hamacker constant and κ the bending modulus.