Microgels, their ionic cloud and some neutron scattering experiments

This talk will focus on colloidal hydrogels -microgels- and emphasize their significance and what new ingredients they bring compared to more conventional colloids. For instance, colloidal hydrogels can spontaneously deswell to allow crystallization of binary mixtures without defects. The key to this behavior relies on the existence of peripheric charged groups, responsible for providing colloidal stability when deswollen, and the associated counterion cloud. When in close proximity, clouds of different particles overlap respectively freeing the associated counterions, which are then able to exert an osmotic pressure that can potentially shrink the particles. Up to now, however, no direct measurement of this ionic cloud exists, perhaps even also for hard colloids, where it is referred to as an electric double layer. We will present recent small-angle neutron scattering experiments with contrast variation with different ions to isolate the change in the form factor directly related to this counterion cloud, to further obtain its geometric properties. Our results highlight that the modeling of colloidal hydrogel suspensions must unavoidably and explicitly consider the presence of this cloud, which exists for nearly all colloidal hydrogels synthesized today.