Counterion Distribution in Aligned Lamellar Phases and on Monolayers at the air/water Interface

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We propose an investigation of the counterion distribution and the associated interactions in charged lamellar phases composed of (i) charged phospholipids and (ii) charged surfactant systems. Interaction forces and fluctuations are fundamentally different in such phases from the neutral or zwitterionic counterparts, often leading to novel self-assembled structures and mesophases [Zemb et al 1999, Rädler et al. 1996, Koltover et al. 1998, Salditt et al 1997]. The counterion distribution rCs(z) is of particular interest, not only for the phase diagram and the interactions in lamellar phases but in many colloidal and biomolecular systems as well, and shall be determined from anomalous scattering techniques (counterion replacement by Bromide, Caesium, Rubidium). Previous studies of the counterion distribution near charged and soft interfaces such as membranes have been restricted to isotropic bulk suspensions (SAXS) and have suffered from weak resolution [Richardson et al 1996]. The present approach is to use highly oriented systems amenable to interface-sensitive diffraction techniques. We aim for a resolution in rCs(z) which is high enough to observe possible deviations from the Poisson-Boltzmann approximation. The subtle interplay between electrostatic forces and interface fluctuations shall be elucidated. Since it has been demonstrated that the counterion size has a pronounced effect on surface potential for highly negatively charged monolayers at the air-water interface, two-dimensional monolayers will be studied by surface potential measurements, grazing incidence x-ray diffraction, x-ray reflectivity and IR reflection-absorption spectroscopy. We plan to study the competition between univalent (alkali metals) as well as divalent (alkali-earth metals) cations with different size in dependence on the charge density of the monolayer. Infrared spectroscopy will be used to measure the effect of different counterions on the ionization behavior of carboxylic acids, which depends on the pH gap between the interface and the subphase and therefore on electrical double layer (EDL) potential. These results will be compared with results obtained in three-dimensional systems (highly aligned multilamellar systems and micelles). On the basis of the experimental results, a development of new theoretical descriptions of EDL is expected.