Renormalization of Helfrich's interactions between fluid membranes in a lyotropic lamellar phase by addition of amphiphilic

We study the effect of an amphiphilic block copolymer on the thermodynamic properties of a lyotropic lamellar phase into which it is incorporated. A q uantitative analysis of the small angle neutron scattering spectra of these samples shows that the polymer strongly enhances the stabilizing Helfrich' s interactions between membranes, by increasing the effective bilayer thick ness. The smectic compressibility of the lamellar phase becomes insensitive to the addition of salt, as soon as the Debye length is smaller than the t hickness of the layer of copolymer adsorbed onto the membrane. This effect is similar to the "steric repulsion" of anchored polymer layers in colloida l dispersions. [S0031-9007(98)08129-0].