Adsorption of large inorganic polyanions under a charged Langmuir monolayer: an ellipsometric study

Abstract

In previous studies of Langmuir films made of the dimethyldioctadecylammonium bromide the exchange of small counter-ions against large inorganic anions type polyoxometalates induced a densification of the monolayer. However, when the polyanions concentration in the subphase exceeded a critical value the monolayer re-expanded. In this work, we demonstrate, by means of ellipsometric measurements, that those effects can be explained by the adsorption of a first layer of inorganic anion under the organic layer then by the formation of a second diffuse layer when the polyanion concentration continues to increase. This process can be detected and analyzed in our system because of the large size and relatively high refractive index of the inorganic anions.