Polyampholyte model of ion clusters: Double-layer interactions in the presence of dissociated simple salt

We explore interactions between equally charged surfaces in the presence of simple salt and either neutral or monovalently charged polyampholytes. We consider the possibility of using these charged polymers as crude models of ion clusters. The latter have been hypothesized to form in concentrated aqueous salt solutions and are possibly related to anomalous underscreening. This phenomenon usually manifests itself in unexpectedly strong and long-ranged effective forces at very high ionic strengths. If ion clusters are formed, they are expected to carry at most a weak net charge. Keeping this in mind, we investigate how polyampholyte chains mediate interactions between charged surfaces. A significant amount of simple salt is also present in most cases. We highlight that if the charges of the polyampholytes are unevenly distributed, there is a polarization response that, in turn, can generate very strong and long-ranged surface forces, even at rather high concentrations of simple salt. Aside from their possible relevance to ion clusters and underscreening phenomena, these results also suggest the possibility of tailoring synthetic polyampholytes in order to regulate colloidal stability.