Characterization of the counter-ion cloud of pNipam microgel via SANS

Microgels as soft colloids are mesoscopic particles suspended in a solvent. In contrast to hard colloids, the phase behavior of soft, deformable particles is not well understood. Although pNipam is uncharged, pNipam microgels contain charged groups because of particle synthesis. The counterions are expected to be mostly located at the periphery of the microgels. A fraction of the counterions if free sets the osmotic pressure of the microgel suspension and causes the spontaneous microgel deswelling reported in previous work.

Using SANS, we find pNIPAM microgels to deswell at a true volume fraction well below random close packing. This suggests that microgels are effectively larger due to their counterion cloud. To obtain the form factor of the ion cloud, we compare SANS measurement of pNIPAM suspension with Ammonia and Sodium ion clouds at the same concentration. We find a signal reminiscent of the counter-ion cloud, but it is at the detection limit of SANS. While more measurements are on the way, the analysis of our current data indicates the cloud could present at the fuzzy shell with width at around 50nm.