CRITICAL ADSORPTION OF POLYELECTROLYTES ONTO HETEROGENEOUSLY CHARGED SURFACES

The adsorption of polyelectrolytes on charged surfaces has several technological applications and is of utmost importance in a variety of biological processes. Its study provides the fundamentals for understanding interactions involving proteins. This adsorption presents a phase transition-like behavior in which discontinuous transitions between adsorbed and desorbed states are observed. The critical conditions obey a scaling relation between critical surface charge density and Debye length, where the scaling exponent is a geometry-dependent constant. Employing Metropolis Monte Carlo simulations, we investigate the adsorption of polyelectrolytes onto spherical particles with two oppositely charged regions. We also explore the effect of a dielectric discontinuity between the particle and the solvent. We observe two regimes in the adsorption of polyelectrolytes onto particles with a net charge of the same sign. At low ionic strength, the repulsion of the particle net charge results in the desorption of the polyelectrolyte. As the ionic concentration increases, the electrostatic screening causes the dominance of the local attraction promoting the adsorption, and we have discontinuous transitions between the desorbed and adsorbed states. The highest affinity occurs when the Debye parameter reaches the particle radius. With an even higher increase in the ionic concentration, the local attraction is not enough to ensure the adsorption, leading to transitions in conditions that obey the scaling relation mentioned earlier. Notably, the dielectric discontinuity amplifies the potential generated by the particle with heterogeneous charge distribution, resulting in a stronger affinity with the polyelectrolyte. However, as the particle's charge distribution approaches homogeneity, the affinity diminishes due to the emergence of image charges on the surface.