Surface characterisation of thermoresponsive microgels based on oligo(ethylene glycol): Adsorption kinetics, dilatational rheology and monolayers

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HYPOTHESIS: Biocompatible microgels based on oligo(ethylene glycol) (OEG-based microgels) represent a new generation of microgels designed for biomedical applications. OEG-based microgels undergo swelling or collapse in bulk in response to external stimuli such as temperature or pH. Gaining a deeper understanding of the surface properties of OEG-based microgels is essential for the proper development of stimuli-responsive emulsions and foams. METHODOLOGY: Combination of colloidal and surface experimental techniques; hydrodynamic diameter, electrokinetic, analysis of Langmuir and Gibbs monolayers with in-situ microscopy, Atomic Force Microscopy and dilatational rheology in linear and non-linear regime provide new insights into the surface conformation of OEG-based microgels. RESULTS: Swelling and electrokinetic response of OEG-based microgels are initially examined as a function of pH and temperature to stablish the Volume Phase Transition Temperature (VPTT) at various pHs. Langmuir monolayers of OEG-based microgels show a "fried-egg" structure typical of thermoresponsive microgels, below and above the VPTT, exhibiting similar surface coverage but protruding slightly further into the subphase in collapsed state. Gibbs monolayers of OEG-based microgels reveal that collapsed OEG-based microgels diffuse and adsorb faster onto the surface but rearrange similarly to swollen OEG-based microgels. Finally, the slightly greater protrusion of collapsed OEG-based microgels ultimately determines their dilatational behaviour in both linear and non-linear regimes. CONCLUSIONS: Surface conformation of OEG-based microgels show very subtle stimuli responsiveness, which in principle, should not compromise their use as emulsion stabilizers. However, they may need to be considered for fine applications of OEG-based microgels as stabilizers in the rational development of stimuli-responsive foams and emulsions.