Measuring the counterion cloud of soft microgels using SANS with contrast variation

The behavior of microgels and other soft and compressible colloidal particles depends on particle concentration in ways that are absent in their hard-particulate counterparts. For instance, poly-N-isopropylacrylamide (pNIPAM) microgels can spontanously deswell and reduce suspension polydispersity at high concentrations. Despite the pNIPAM network in these microgels is uncharged, the key to understand this distinct behavior relies on the existence of peripheric charged groups, which provide stability when deswollen, and the associated counterion cloud. When in close proximity, clouds of different particles overlap, effectively freeing the associated counterions, which are then able to exert an osmotic pressure that can potentially cause the microgels to change size. Up to to now, however, no direct measurement of such an ionic cloud exists, perhaps even for hard colloids, where it is referred to as electric double layer. Here, we use small-angle neutron scattering with contrast variation with different ions to isolate the change in the form factor directly related to the counterion cloud and obtain its radius and width. Our results highlight that modeling of microgel suspensions must unavoidably and explicitly consider the presence of this cloud, which is present for nearly all microgel particles synthesized today.