  Understanding interactions between membranes requires measurements on
well-controlled systems close to natural conditions, in which fluctuations play
an important role. We have determined, by grazing incidence X-ray scattering,
the interaction potential between two lipid bilayers, one adsorbed on a solid
surface and the other floating close by. We find that interactions in this
highly hydrated model system are two orders of magnitude softer than in
previously reported work on multilayer stacks. This is attributed to the weak
electrostatic repulsion due to the small fraction of ionized lipids in
supported bilayers with a lower number of defects. Our data are consistent with
the Poisson-Boltzmann theory, in the regime where repulsion is dominated by the
entropy of counter ions. We also have unique access to very weak entropic
repulsion potentials, which allowed us to discriminate between the various
models proposed in the literature. We further demonstrate that the interaction
potential between supported bilayers can be tuned at will by applying osmotic
pressure, providing a way to manipulate these model membranes, thus
considerably enlarging the range of biological or physical problems that can be
addressed.
