MEASUREMENT OF THE COMPRESSIBILITY MODULUS IN A LYOTROPIC LAMELLAR PHASE STABILIZED BY UNDULATION FORCES

Lamellar mesophases of the anionic double-chain surfactant AOT in brin e were oriented homeotropically between two smooth silica surfaces of a surface force apparatus (SFA). The force-distance profile was measur ed at different membrane volume fractions (Phi = 0.09-0.18) all close to the sponge/lamellar phase boundary in the phase diagram. Oscillator y profiles were consistently observed with the same characteristic fea tures. The periodicity of the oscillations was twice the reticular spa cing of the bulk lamellar phase, indicating that the lamellar stack re sponds to the applied strains by creating edge dislocations of Burgers vector 2. Such a defect is topologically equivalent to a handle joini ng two membranes and arises naturally from the topology of the neighbo ring sponge phase. The compressibility modulus at constant chemical po tential, (B) over bar, was extracted from the parabolic shape of the f orce oscillations and was found to be consistent with an intermembrane interaction dominated by undulation forces.