A fluid floating bilayer

A highly hydrated lipid bilayer, floating a few A above another one adsorbe d on a smooth solid substrate, was prepared at room temperature where lipid s (DSPC) are in gel phase, then heated at several temperatures including th e pretransitional temperature T-p and the chain melting temperature T-m. A precise in situ characterization by neutron reflectivity led to two new res ults. First, even when in fluid phase the floating bilayer was structured a nd stable; it thus provides a flexible model system for physical and biophy sical studies of membranes. Second, at intermediate temperatures, a spectac ular maximum in both inter-bilayer distance and bilayer roughness was simul taneously observed. It might be a, direct observation of the balance betwee n energy minimization and entropic repulsion, leading to an estimation of t he dimensionless parameter (k(B)T)(2)/A(kappa), where A is the Hamacker con stant; and kappa the bending modulus.