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.