Electrostatic adsorption and orientational order of inhomogeneously charged particles on flat surfaces

Electrolyte solutions often contain globular macroions such as certain proteins, colloids, and lipid vesicles that carry an asymmetric, non-uniform surface charge distribution. For the electrostatic adsorption of these macroions onto an oppositely charged planar surface, orientational entropy contributions becomes relevant and thus influence the structure of the electric double layer. We consider a simple model electrolyte that contains besides small salt ions also spatially extended spherical particles with one single surface charge or two surface charges attached on opposite sides and employ both a modified Poisson-Boltzmann approach and Monte Carlo simulations to study how these particles adsorb on a flat surface. Our Poisson-Boltzmann approach accurately accounts for the shape of the spherical particles and their charge distribution but neglects correlations and steric interactions between different particles. We characterize the degree of adsorption and how that is influenced by the particles' charge distribution and their orientational entropy loss. We generally find our mean-field approach in good agreement with Monte Carlo simulation results.