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

The behavior of microgels and other soft, compressible colloids depends on particle concentration in ways that are absent in their hard-particulate counterparts. For instance, poly-N-isopropylacrylamide (pNIPAM) microgels can spontaneously deswell and reduce suspension polydispersity when concentrated enough. Despite the pNIPAM network in these microgels is neutral, the key to understanding this distinct 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, effectively freeing the associated counterions, which are then able to exert an osmotic pressure that can potentially cause the microgels to decrease their size. Up to now, however, no direct measurement of such an ionic cloud exists, perhaps even also for hard colloids, where it is referred to as an 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 the modeling of microgel suspensions must unavoidably and explicitly consider the presence of this cloud, which exists for nearly all microgels synthesized today. The existence of peripheric charged groups on poly-N-isopropylacrylamide (pNIPAM) microgels together with the corresponding counter ion clouds has been proposed to be responsible for the spontaneous deswelling of pNIPAM microgels in concentrated suspensions but no direct measurement of such an ionic cloud exists. Here, the authors 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.

Automated summary

The behavior of microgels depends on particle concentration, which is different from hard particles. The presence of charged groups on microgels and the associated counterion cloud are responsible for the spontaneous deswelling of microgels in concentrated suspensions. However, there is no direct measurement of this counterion cloud.