This work applies a molecular theory to study the formation of lateral self-assembled aggregates in mixed brushes composed of polyanion and polycation chains. In order to overcome the well-known limitations of mean-field electrostatics to capture polyelectrolyte complexation, the formation of ion pairs between anionic and cationic groups in the polyelectrolytes is explicitly modeled in our theory as an association reaction. This feature is essential to capture the microphase separation of the mixed brush and the formation of lateral aggregates triggered by polyelectrolyte complexation. The effects of solution pH and ionic strength, surface coverage, and chain length on the morphology of the mixed brush are systematically explored. It is shown that increasing salt concentration leads to the rupture of polyelectrolyte complexes and the stabilization of the homogeneous, non-aggregated brush, providing that the formation of ion pairs between the polyelectrolytes and the salt ions in solution is explicitly accounted for by the theory. The inclusion of ion-pairing association reactions between oppositely charged polyelectrolytes within a mean-field description of electrostatics emerges from this work as a useful and simple theoretical approach to capture the formation of polyelectrolyte complexes and their responsiveness to solution ionic strength and pH.

Polyelectrolyte coacervates, obtained by mixing solutions of oppositely charged polyions, and layer-by-layer films, produced by sequential adsorption of polyelectrolytes on a surface, are two types of closely related soft materials. While both types of materials are produced by polyelectrolyte complexation, their theoretical description had so far followed divergent paths. This work reports a unifying theoretical treatment of polyelectrolyte complexation in solution and layer-by-layer self-assembled thin films using a molecular theory that describes polyelectrolyte complexation by using a chemical-equilibrium formalism. The theory is shown to predict both the phase diagrams of polyelectrolyte mixtures in solution and the formation of layer-by-layer thin films in good agreement with experimental evidence. In the latter case, the theory correctly captures the effects of solution pH and ionic strength on the mass of the deposited films as well as the possibility of layer-by-layer deposition without full charge reversal at extreme pHs. The theory is then used to revisit the "universal curve" for the effect of salt concentration on layer-by-layer deposition previously proposed on experimental grounds by Salehi et al. [Macromolecules 2015, 48, 400−409]. This universal curve makes predictions about the growth rate of a layer-by-layer film for a given polyanion/polycation pair by using only information obtained from a mixture of the same polyelectrolytes in solution, thereby linking both phenomena. Our theoretical results confirm the validity of the curve. This achievement demonstrates the practical importance of describing polyelectrolyte coacervates and multilayer films within a unified theoretical framework.