  The equilibrium distance between two lipid bilayers stable in bulk water and
in proximity of a substrate was investigated. Samples consisted of a
homogeneous lipid bilayer, floating near an identical bilayer deposited on the
hydrophilic surface of a silicon single crystal. Lipids were saturated di-acyl
phosphocholines, with the number of carbon atoms per chain, n, varying from 16
to 20. The average and r.m.s. positions of the floating bilayer were determined
by means of neutron specular reflectivity. Samples were prepared at room
temperature (i.e. with the lipids in the gel phase) and measurements performed
at various temperatures so that the whole region of transition from gel to
fluid phase was explored. Data have been interpreted in terms of competition
between the interbilayer potential and membrane fluctuations and used to
estimate the bending rigidity of the bilayer.
