Two Sides of the Same Coin: A Unified Theoretical Treatment of Polyelectrolyte Complexation in Solution and Layer-by-Layer Films

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.