(71d) Intermolecular Forces Between Adsorbed Polyelectrolytes: A Simulation Study

Polyelectrolytes are important to a number of industrial applications, such as selective filtering, separations, and sensing, and as electrochemical components, for example, solid electrolytes in batteries and fuel cells. Understanding their interactions is the key to controlling polyelectrolyte aggregation and adsorption behavior. In polar or ionic solvents, electrostatic forces are a sensitive factor governing the interaction; Typically, polyelectrolyte adsorption is controlled by solution variables such as salt concentration and pH, is spontaneous, and saturates due to interfacial charge build-up (often to the point of overcompensation). As a result, polyelectrolyte films are usually grown in oppositely charged layers as a stepwise process. In recent experiments, we have uncovered conditions where polyelectrolyte adsorption to a conducting surface may become continuous in the sense of scaling linearly with time over hours [1,2]. This discovery of continuous layer growth offers an enticing possibility of nanoscale thin film growth in a single step process, but also brings forth questions of the underlying mechanisms.

Based on our experimental observations, we suspect that the three key features to understanding the process are polymer charge regulation, the dielectric discontinuity at the adsorbing surface, and short-range attractive interactions between the polymers. Here, we present a molecular Monte Carlo simulation study aimed at understanding mechanistically the continuous adsorption process and, more broadly, the quantitative significance of these three features in polyelectrolyte adsorption. Our system consists of charged tangent spheres above a surface of variable dielectric discontinuity between the substrate and the solution, and spherical counterions and salt ions. We find that counter ion correlations act to enable the formation of stable polymer-polymer binding and aggregation. We discuss the sensitivity of the attractive regime to charge distribution both in the polyelectrolytes and in the ions, and implications to experimental observations.

[1] A. P. Ngankam and P. R. Van Tassel, Proc. Nac. Acad. Sci. 104, 1140-1145 (2007). [2] C. Olsen and P. R. Van Tassel, J. Colloid and Interface Science 329, 222-227 (2009).