Effects of anions on the aggregation of charged microgels

In the present study, we have prepared poly(N-isopropylacrylamide-co-vinylbenzyl trimethylammonium chloride) (NIPAM-co-VT) and poly(N-isopropylacrylamide-co-sodium acrylate) (NIPAM-co-SA) thermally sensitive microgels, which are positively and negatively charged, respectively. By use of laser light scattering (LLS), we have investigated the temperature-induced aggregation of microgels in the presence of salts with the same cation but different anions. The studies demonstrate that neither kosmotropic nor chaotropic anions can lead to the aggregation of NIPAM-co-VT microgels. No obvious specific anion effects can be observed in the phase transition of NIPAM-co-VT microgels. However, kosmotropic and chaotropic anions lead to the aggregation of NIPAM-co-SA microgels. The ordering of anions based on the aggregation temperature at the same salt concentration is consistent with the typical Hofmeister series. The aggregation is thought to be determined by the indirect interfacial effects of anions near the microgel surface. Additionally, the aggregate size of microgels increases with salt concentration for the kosmotropic anions.