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

TL;DR

This study explores how Keggin-type polyoxometalates influence the bulk and 2D interfacial behavior of thermoresponsive microgels, revealing charge inversion, structural changes, and enhanced etching resistance, advancing smart material control.

Contribution

It demonstrates the specific effects of POM on microgel behavior at different temperatures and interfaces, introducing new insights into tuning microgel properties via ionic interactions.

Findings

Charge inversion occurs at low POM concentrations.

POM induces deswelling-swelling-deswelling cycles below VPTT.

Microgel resistance to plasma etching is improved with POM.

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 charged pNIPAM microgels with the Keggin-type polyoxometalate $Na_{3}PW_{12}O_{40}$ (POM). In bulk, we observed charge inversions below and above the volume phase transition temperature (VPTT) at significantly low POM concentrations as $5\cdot10^{-5}$ M. In the presence of POM, the microgels exhibited a deswelling-swelling-deswelling behaviour below the VPTT, and a two-step further deswelling above the VPTT. When microgels were confined at 2D water/air interfaces, adding $10^{-5}$ M of POM below the VPTT was equivalent to heat above the VPTT and compress the monolayer from $5$ to $20\,\text{mN m$^{-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 POM 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 POM was characterized by EDS mapping and XPS. The microgel monolayers with POM improved their resistance to plasma etching, which could be useful for soft colloidal lithography.