Weakly charged lamellar bilayer system: Interplay between thermal undulations and electrostatic repulsion

We study the interplay between thermal undulations and electrostatic intera ctions for weakly; charged surfactant bilayers by measuring the backscatter ing of light from very dilute lamellar phases of the non-ionic surfactant t riethylene glycol monodecyl ether (C10E3) doped with small amounts of the a nionic surfactant sodium dodeeyl sulphate (SDS), both with and without adde d electrolyte. Upon charging, the lamellar phases show a transition from un dulation to electrostatic Stabilization. Non-lamellar structures develop if the molar mixing ratio R = [SDS]/[C10E3] exceeds approximate to 2.5 x 10(- 3). Deviations from ideal swelling, (D) over bar proportional to l/Phi, whe re (D) over bar is the lamellar repeating distance and Phi, the membrane vo lume fraction, were detected for all lamellar phases. Salt-free lamellar ph ases with charge densities below sigma (c) = 1.47 x 10(-4) e/nm(2), as well as more highly charged lamellar phases at high ionic strength show a unive rsal logarithmic deviation fr om ideal swelling that was analyzed using the ories for undulation stabilized lamellar phases. Deviations from ideal swel ling for electrostatically stabilized lamellar phases were analyzed using t heories recently developed fur undulations in charged lamellar phases. The fits to the various theories yield a value of k(c,0) approximate to 1.8k(B) T for the bending modulus of the C10E3 bilayers.