Counterion distribution near a monolayer of variable charge density

We study the bromide counterion distribution near a solid-supported monolayer in the case of vanishing bulk electrolyte concentration by resonant X-ray reflectivity. The surface charge density of the monolayer was varied by using different molar ratios of the cationic Di-Octadecyl-Di-methyl-Ammonium-Bromide (DODAB) and the neutral Di-Palmitoyl-Glycero-Phosphocholine (DPPC). The analysis, either based on a conventional box model with an additional counterion contribution, or based on an independent unbiased global optimization approach, yields a good agreement with the classical Poisson- Boltzmann theory for the salt-free case. Copyright (c) EPLA, 2007.