We have demonstrated that charged glass surfaces induce anomalous attractions between nearby charge-stabilized spheres, even when account is taken of all known experimental artifacts. Such attractions can be masked by long-ranged electrostatic repulsions at the lowest ionic strengths. This poses the additional challenge for simulations that anomalous attractions may not be apparent in systems without added salt. Replacing charged glass surfaces with conducting metal surfaces eliminates the anomalous attractions. This suggests a primary role for the surface charge in mediating confinement-induced like-charge attractions.
We speculate that charged surfaces induce like-charge attractions by inducing non-monotonic correlations in the distribution of simple ions surrounding the macroions. In a crude sense, particles are attracted to counterions that the surface forces between them. Theories for macroionic interactions incorporating such correlations indeed predict like-charge attractions in confined (43,44) and crowded (45,52,46,47) dispersions, but are controversial (5,55,54,53). Recent large-scale simulations (56,57,7,58) strengthen the case for correlation-induced like-charge attractions, having revealed high-order correlations in the distribution of simples ions that are not captured by the mean-field approximation. These simulations show the onset of many-body cohesion that may be related to the wall-induced pair attraction that we observe.
This work was supported by the donors of the Petroleum Research Fund of the American Chemical Society.