Effect of the 3D swelling of microgels on their 2D phase behavior at the liquid-liquid interface*

We investigate soft, temperature-sensitive microgels at fluid interfaces and how changing temperature across the microgels' volume phase transition temperature, which leads to swelling/deswelling of the microgels in the aqueous phase, affects the phase behavior within the monolayer. We combine compression isotherms, AFM imaging, and ellipsometry.
At low compression, the interaction between microgels is dominated by their highly stretched corona and the phase behavior of the monolayers is the same. The polymer segments within the interface lose their temperature-sensitivity. At high compression, the portions of the microgels located in the aqueous side of the interface become relevant and prevail in their interactions. These portions are able to collapse and, consequently, the isostructural phase transition is altered. Thus, the temperature-dependent swelling perpendicular to the interface (3D) affects the compressibility parallel to the interface (2D). Our results highlight the distinctly different behavior of soft, stimuli-sensitive microgels as compared to rigid nanoparticles.