Polyelectrolytes I: Polyanion/Polycation Multilayers at the Air/Monolayer/Water Interface as Elements for Quantitative Polymer Adsorption Studies and Preparation of Hetero-superlattices on Solid Surfaces

Abstract

Polyanion/polycation multilayers floating at the air/water interface were prepared by consecutive adsorption of polyelectrolyte layers onto a Langmuir monolayer from aqueous polyelectrolyte subphase solutions. With the positively charged Langmuir monolayer headgroups of the lipid dimethyldioctadecylammonium bromide, the layer sequence starts with the negatively charged polyelectrolyte polystyrene sulfonate. With the negatively charged monolayer dimyristoylphosphatidic acid, it starts with the positively charged polyallylamine. Equally charged monolayer headgroups and polyelectrolytes do not bind to each other. Consecutive subphase exchange cycles of polyelectrolyte solutions with alternating charges and pure solvent in between lead to the formation of floating multilayers consisting of an alternating polyelectrolyte sequence. Ellipsometric measurements show that the thickness (adsorbed amount per unit area) of the multilayer film at the air/water interface grows in proportion to the number of adsorbed polyelectrolyte layers. The thickness of individual layers increases with increasing polyelectrolyte and/or ion subphase concentration, respectively. The floating multilayers can be deposited as a sequence of layers of monomeric lipid and polyelectrolytes (hetero-superlattices) onto solid substrates via Langmuir−Blodgett transfer. UV-absorbance studies corroborate the quantitative interpretation of the ellipsometric data in terms of the polymer concentration in the layers, the individual layer thicknesses, and the adsorbed amounts per unit area.