Universality in interacting membranes: The effect of cosurfactants on the interfacial rigidity

We report on synchrotron x-ray scattering studies of multimembrane systems in the fluid ${\mathrm{L}}_{\mathrm{\alpha }}$ phase. By embedding cosurfactant molecules in a model biological membrane (di-myristoyl-phosphoridyl-choline) system, we dramatically reduce the bending rigidity of the fluid bilayer. This effect leads to a crossover from a microscopically driven to a fluctuation-induced universal regime for intermembrane interactions. Studies of a homologous cosurfactant series, with varying hydrocarbon chain length between 5 and 12, allow us to attribute this reduction in ${\mathrm{k}}_{\mathrm{c}}$ to the thinning of the membrane.