Thermal undulations in salt-free charged lamellar phases: Theory versus experiment

The interplay between thermal undulations and electrostatic repulsion is investigated theoretically for salt-free lamellar phases of charged fluid membranes. The electrostatics of the undulating charged membranes is dealt with in the nonlinear Poisson-Boltzmann approximation. A harmonic undulation theory is formulated that accounts quantitatively for the shift of the Bragg peak that has recently been observed in light-scattering experiments on the dilute lamellar phase of the nonionic surfactant n-dodecyl pentaethylene glycol ether, to which small amounts of the ionic surfactant sodium dodecyl sulphate were added.