Europhys. Lett.
Volume 75, Number 6, September 2006
Page(s) 992 - 998
Section Interdisciplinary physics and related areas of science and technology
Published online 23 August 2006
Europhys. Lett., 75 (6), pp. 992-998 (2006)
DOI: 10.1209/epl/i2006-10207-5

Elasticity of fluctuating charged membranes probed by X-ray grazing-incidence diffuse scattering

G. Brotons1, 2, 3, L. Belloni2, Th. Zemb2 and T. Salditt1

1  Institut für Röntgenphysik - Friedrich-Hund-Platz 1, 37077 Goettingen, Germany
2  LIONS-CEA Saclay, Bât. 125 - 91191 Gif-sur-Yvette, France
3  LPEC-Université du Maine - Av. O. Messiaen, 72085 Le Mans, France

received 14 April 2006; accepted in final form 26 July 2006
published online 23 August 2006

We have investigated the structure, fluctuations and elasticity parameters of charged fluid membranes composed of didodecyldimethylammonium halide bilayers (DDAX) with different counterions ( $\ab{X}=\ab{Cl}^-$ or Br-) by non-specular (diffuse) X-ray scattering. The samples have been prepared as highly aligned thick stacks on silicon support and have been swollen at controlled osmotic pressures. From line-shape analysis of the diffuse scattering, we can determine the fluctuation amplitudes of the membranes in the $\L _{\alpha}$ phase and the characteristic correlation lengths. According to the Caillé model, the measured structural parameters have been expressed in terms of the exponent $\eta$ and smectic penetration length $\lambda$, from which the smectic compression modulus B (J/m3) and the bending modulus K (J/m) have been determined, as a function of the swelling state. We show that the results are in quantitative agreement with the mean-field electrostatic Poisson-Boltzmann calculations, controlled by the bilayers surface charge density (surfactant head groups with free or condensed counterions).

87.16.Dg - Membranes, bilayers, and vesicles.
61.10.Kw - X-ray reflectometry (surfaces, interfaces, films).
62.20.Dc - Elasticity, elastic constants.

© EDP Sciences 2006