Ultrathin Layers of Polyelectrolytes on Mica: Preparation, Characterization, and Electrokinetic Surface Potential

We report on the preparation of ultrathin polyelectrolyte network films on solid substrates, particularly on mica. This supramolecular construct serves as a useful model of ionogenic surfaces in materials science or biological contexts. The polyelectrolyte network films were prepared from photo-cross-linkable copolymers which in turn were synthesized from 3-(dimethyl-maleinimidopropyl) methacrylate (DMIMA) and tertbutyl methacrylate (tBMA) via radical copolymerization. DMIMA gives rise to photo-cross-linkable sites in the copolymer. Ultrathin multilayered films are prepared with P[tBMA-co-DMIMA] by the Langmuir-Blodgett (LB) technique with mica as the substrate. The mica surface was hydrophobized by a monolayer of cetyltrimethylammonium bromide self-assembled from aqueous solution. Two subsequent reactions were performed with the LB films: cross-linking by irradiation with UV light to fix the morphology of the film; subsequent acidolysis by gaseous HCl to convert the construct into a polyelectrolyte film. Its thickness was determined by small-angle X-ray reflectometry, and the surface structure by AFM. The electrokinetic potential (zeta potential) at the polyelectrolyte/aqueous solution interface was determined by measuring the electroosmotic mobility of the solution near the surface. Following the formalism of the Einstein-Smoluchowski equation, a zeta potential of-50mVwasdetermined at zero concentration of added electrolyte and neutral pH.