A fluid floating bilayer

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 T p and the chain melting temperature T m . 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 ( k B T ) 2 / A κ, where A is the Hamacker constant and κ the bending modulus.