Buildup of Ultrathin Multilayer Films by a Self‐Assembly Process: II. Consecutive Adsorption of Anionic and Cationic Bipolar Amphiphiles and Polyelectrolytes on Charged Surfaces

Abstract

We have recently reported on the consecutive physisorption of anionic and cationic bipolar amphiphiles onto charged surfaces, adsorbed out of aqueous solutions [1]. Here, we extend our previous concept to multipolar compounds such as polyelectrolytes. In contrast to the bipolar amphiphile system, it is not necessary to separate single charges by a rigid unit, when the polyelectrolyte is adsorbed from sufficiently concentrated solutions. In this case the physisorbed layer does not bind with all ionic groups to the surface and exposes free ionic groups at the new film/solution interface. Therefore a polyelectrolyte layer can replace a layer of bipolar amphiphiles in the consecutive buildup of a multilayered assembly. The buildup of multilayers can then be described as follows: A solid substrate with a positively charged planar surface is immersed in the solution containing the negatively charged bipolar amphiphile, a monolayer of the amphiphile is adsorbed and due to its bipolar structure the surface charge is reversed. After rinsing in pure water the substrate is immersed in the solution containing the positively charged polyelectrolyte. Again a monolayer is adsorbed but now the original surface charge is restored. By repeating both steps in a cyclic fashion alternating multilayer assemblies of both compounds are obtained. In the same way multilayer assemblies can be prepared by using negatively charged polyelectrolytes and positively charged bipolar amphiphiles. It is demonstrated that multilayer films composed of at least 39 consecutively alternating layers, which corresponds to a total film thickness of 151 nm, can be assembled.