A Method for Direct Determination of the Prevailing Counterion Distribution at a Charged Surface

The surface charge of an anionic or cationic monolayer at the air−water interface is compensated by the
oppositely charged counterions that are either located within the diffuse layer or directly bound to the interface.
In this paper we demonstrate that ellipsometric measurements allow a direct determination of the prevailing
ion distribution. A simple analytical expression is derived, which can be used to retrieve the number of ions
within the compact layer. This formalism is applied to the experimental data gained on aqueous alkyl pyridinium
bromide solutions. The analysis of the experimental ellipsometric isotherm provides evidence for the existence
of an ion condensation just before the cmc. The derived formalism relates ellipsometric signal with the surface
charge and should be of utmost importance to discard or proove the recently introduced concept of fluctuation
driven ion condensation at a charged surface.