Inter cross-linking microgels by superchaotropic nano-ions at interface: Controlled stabilization of emulsions

Abstract

Hypothesis

Poly(N-isopropylacrylamide) (pNIPAM) microgels are soft particles that adsorb at liquid interfaces and confer emulsion stability against coalescence. Their conformation and interactions at the interface greatly impact the mechanical properties of the interface. In particular, the interfacial elastic modulus increases as the microgel cross-linking density decreases, as a consequence of microgel ability to deform and entangle with neighbors. The purpose of this work is to investigate how these features can be tuned by physical interactions between superchaotropic Keggin nano-ions (POMs) and pNIPAM microgels.

Experiments

Interactions between polyoxometalates (POMs) and pNIPAM microgels of varying cross-linking densities and sizes are investigated in aqueous suspensions and at liquid/liquid interfaces. The ability of microgels to stabilize oil-in-water emulsions is assessed by evaluating their kinetic stability and flow characteristics, with POMs introduced either before or after emulsification. Cryogenic electron microscopy (cryo-EM) is employed to directly visualize the microgel-stabilized emulsions. The adsorption of microgels, at a model interface and the resulting interfacial elasticity with various POM concentrations, are also studied using the oscillating pendant drop method.

Findings

POMs act as physical cross-linkers that promotes microgel deswelling. For large microgels, this effect increases their stiffness. and thus adding POMs reduces the stability of the emulsions. In contrast, small POM-loaded microgels produce highly stable emulsions that resist coalescence under mechanical stress or temperature increase. The same is true for large microgels supplemented in POMs after emulsification. Indeed, POMs enhance interfacial elasticity by promoting both intra-particle and inter-particle crosslinking at the interface. Lastly, by connecting microgel monolayers between neighboring droplet surfaces, POMs promote adhesion between droplets. All levels of the multiscale structure within Pickering emulsions are controlled by interactions between POMs and pNIPAM.