Breakdown of the Poisson-Boltzmann description for electrical double layers involving large multivalent ions

Langmuir films of amphiphiric molecules with ionizable head groups are very convenient model systems for basic electrostatic studies. Indeed their surface charge density can be continuously adjusted by a simple mechanical compression of the monolayer. Being planar, they also lend themselves to the direct observation of the ion distribution in the aqueous subphase by Brewster angle optical microscopy and by scattering techniques. Using X-ray reflectivity, we have measured the electrical double-layer profile for eicosylamine Langmuir monolayers in contact with a subphase containing large trivalent phosphotungstate ions. The profile observed with a vertical resolution of a few angstroms is markedly different from the diffuse exponential layer described by the Poisson-Boltzmann equation and valid for small monovalent ions. The trivalent anions appear to be condensed in a narrow layer of monomolecular width and of high density. The Brewster images are compatible with this finding and show in addition that their in-plane distribution is homogeneous. Their volume fraction is locally so large, typically 50-70%, that the total number of counter-ions in this layer exceeds what is required by electroneutrality. The interface seen by the X-ray technique appears thus as globally negative. We postulate that the electric compensation is provided by an accumulation of positive hydronium ions near the interface. (C) 1998 Elsevier Science S.A. All rights reserved.