Tuning the bulk behavior and 2D interfacial self-assembly of microgels by keggin-type polyoxometalate ionic specificity

Abstract

Finding new ways to tune the behavior of thermoresponsive microgels in bulk and confined at 2D liquid interfaces is key to achieve a deeper understanding and control of these smart materials. We studied the interaction of positively and negatively charged pNIPAM microgels with the Keggin-type polyoxometalates Na3PW12O40 (POM3−) and H4SiW12O40 (POM4−). In bulk, we observed charge inversions of the positively charged microgels below and above the volume phase transition temperature (VPTT) at significantly low POM concentrations as 5 ⋅ 10−5 M. In the presence of POM, both microgels exhibited a deswelling-swelling-deswelling behavior below the VPTT, and a two-step further deswelling above the VPTT for the positively charged microgels. When the later were confined at 2D water/air interfaces, adding 10−5 M of POM3− below the VPTT was equivalent to heating above the VPTT and compressing the monolayer from 5 to 20 mN m−1. Above the VPTT, the diameter at the interface did not change while the portion immersed in the subphase further deswelled, in agreement with the behavior in bulk. Adding more POM3− did not change the diameter at the interface nor the height of the microgels, showing a saturation effect in 2D. The restructuring of the pNIPAM polymeric network by the POM3− was characterized by EDS mapping and XPS. The microgel monolayers with POM3− improved their resistance to plasma etching, which could be useful for soft colloidal lithography.