Responsive microgels allow the realization of smart emulsions, which can be broken on demand. However, their interfacial properties and demulsification mechanism are not fully understood. In our study we aim to elucidate the connection between their temperature-dependent swelling (i.e. softness) and properties at liquid-liquid interfaces. We synthesised thermo-responsive poly(N-isopropylacrylamide) p(NIPAM) microgels and measured their interfacial properties in swollen (10 °C) and collapsed (40 °C) state. At these temperatures microgels lower the interfacial tension of water-decane interfaces to values below 20 mN/m. A Langmuir trough was used to investigate the mechanical properties of the microgel monolayer, with subsequent deposition to a substrate. With respect to the swollen microgels, the compression isotherms of collapsed microgels show an unusual one-staged course, albeit similar surface pressures (Figure 1). Furthermore, the collapsed microgel monolayers possess a lower compression modulus. The deposited microgels were recorded ex situ with atomic force microscopy (AFM). By image analysis we found structurally identical phases for both states, yet the transitions take place at different compressions. The results of cryo-SEM and ellipsometry at liquid interfaces demonstrate that their interfacial radius and height of protrusion into the non-polar phase are temperature independent, but the microgel parts situated in the aqueous phase are still thermo-responsive. As a consequence, at elevated temperatures the microgels are collapsed and the monolayer becomes more brittle. Our findings emphasize the fundamental differences between rigid and soft colloids at liquid-liquid interfaces.

Thermo-responsive microgels at interfaces: Correlation between microgel softness and monolayer properties

255th National Meeting (2018)

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