Charge Inversion and Layer-by-Layer Assembly of Flexible Polyelectrolytes

Qiang Wang

Department of Chemical and Biological Engineering

Colorado State University

Proposed in early 90's, the polyelectrolyte (PE) layer-by-layer assembly technique has attracted exponentially increasing interest due to its simplicity, versatility, and potential applications. Through alternating exposure of a charged substrate to solutions of polyanions and polycations, hundreds of thin adsorbed layers of polyions can be readily built-up on the substrate. In addition to charged polymers, charged nano-particles and platelets have also been used. This technique has a wide variety of potential applications, including surface modification, sensors, conducting or light-emitting devices, drug delivery, nano-reactors, etc.

Our understanding on the formation mechanism, internal structure, and molecular property of PE multilayer, however, is still at an early stage. In great contrast to thousands of experimental papers on the layer-by-layer assembly, very few theoretical or simulation studies have been reported. In general, the build-up of PE multilayers is based on the charge inversion, i.e., the charge of newly adsorbed PE inverts (overcompensates) the existing charge in the film (including the bare substrate charge). In this work, we have applied a self-consistent field (SCF) theory to flexible PE on flat surfaces either uncharged or carrying opposite charges to the PE. We examined in detail the effects of various parameters on the PE adsorption and surface charge compensation by the adsorbed PE. Strong charge inversion is found for relatively long PE on oppositely charged, attractive surfaces in poor solvent at high salt concentrations. At the mean-field level, the adsorption behavior of PE at high salt concentrations can be understood by that of neutral polymers in good solvent.

Based on the above results, we have further modelled the process of layer-by-layer assembly of flexible PE on flat surfaces. The multilayer has a three-zone structure. An exponential growth is found for the first several layers, followed by a linear growth for subsequent layers evolving towards a steady state. While adjacent layers are highly interpenetrating, stratification can be seen for every four or more layers. We have also examined the effects of surface charge density, bulk salt concentration, and solvent quality on the thickness and internal structure of the multilayer. Our results agree with most experimental findings on PE layer-by-layer assembly.