Organization of Microgels at the Air-Water Interface under Compression: Role of Electrostatics and Cross-Linking Density.

Poly(N-isopropylacrylamide) (pNIPAM) microgels are soft and deformable particles, which can adsorb at liquid interfaces. In the present paper, we study the two-dimensional organization of charged and quasi-neutral microgels with different cross-linking densities, under compression at the air-water interface and the transfer of the microgel monolayer onto a solid substrate at different surface pressures. At low cross-linking densities, the microgels form highly ordered hexagonal lattices on the solid substrate over large areas, with a unique lattice parameter that decreases continuously as the surface pressure increases. We thus prove that the microgel conformation evolves at the air-water interface. The microgels undergo a continuous transition from a highly flattened state at low surface coverage, where the maximal polymer segments are adsorbed at the interface, to entangled flattened microgels, and finally the thickening of the layer up to a dense hydrogel layer of compacted microgels. Moreover, two batches of microgels, with and without charges, are compared. The contribution of electrostatic interactions is assessed via changing the charge density of the microgels or modulating the Debye length. In both cases, electrostatics does not change the lattice parameter, meaning that, despite the microgel different swelling ratio, charges do not affect neither interactions between particles at the interface nor microgels adsorption. Conversely, the cross-linking density has a strong impact on microgel packing at the interface: increasing the cross-linking density strongly decreases the extent of microgel flattening and promotes the occurrence of coexisting hexagonally ordered domains with different lattice parameters.

Interfacial rheology of model water-air microgels laden interfaces: Effect of cross-linking.

HYPOTHESIS: The mechanical properties of model air/water interfaces covered by poly(N-isopropylacrylamide) microgels depend on the microgels deformability or in other words on the amount of cross-linker added during synthesis. EXPERIMENTS: The study is carried out by measuring the apparent dilational, the compression and the shear moduli using three complementary methods: (1) the pendant drop method with perturbative areas, (2) the Langmuir trough compression, and (3) shear rheology using a double wall ring cell mounted onto a Langmuir through. FINDINGS: In the range of surface coverages studied, the interfaces exhibit a solid-like behavior and elasticity goes through a maximum as a function of the surface pressure. This is observable whatever the investigation method. This maximum elasticity depends on the microgel deformability: the softer the microgels the higher the value of the moduli. The mechanical behavior of model interfaces is discussed, taking into account the core-shell structure of the particles and their packing at the interface.

Thermo-induced inversion of water-in-water emulsion stability by bis-hydrophilic microgels.

HYPOTHESIS: Stabilization of water-in-water (W/W) emulsions resulting from the separation of polymeric phases such as dextran (DEX) and poly(ethyleneoxide) (PEO) is highly challenging, because of the very low interfacial tensions between the two phases and because of the interface thickness extending over several nanometers. In the present work, we present a new type of stabilizers, based on bis-hydrophilic, thermoresponsive microgels, incorporating in the same structure poly(N-isopropylacrylamide) (pNIPAM) chains having an affinity for the PEO phase and dextran moieties. We hypothesize that these particles allow better control of the stability of the W/W emulsions. EXPERIMENTS: The microgels were synthesized by copolymerizing the NIPAM monomer with a multifunctional methacrylated dextran. They were characterized by dynamic light scattering, zeta potential measurements and nuclear magnetic resonance as a function of temperature. Microgels with different compositions were tested as stabilizers of droplets of the PEO phase dispersed in the DEX phase (P/D) or vice-versa (D/P), at different concentrations and temperatures. FINDINGS: Only microgels with the highest DEX content revealed excellent stabilizing properties for the emulsions by adsorbing at the droplet surface, thus demonstrating the fundamental role of bis-hydrophilicity. At room temperature, both pNIPAM and DEX chains were swollen by water and stabilized better D/P emulsions. However, above the volume phase transition temperature (VPTT ≈ 32 °C) of pNIPAM the microgels shrunk and stabilized better P/D emulsions. At all temperatures, excess microgels partitioned more to the PEO phase. The change in structure and interparticle interaction induced by heating can be exploited to control the W/W emulsion stability.

Pickering emulsions stabilized by thermoresponsive oligo(ethylene glycol)-based microgels: Effect of temperature-sensitivity on emulsion stability.

HYPOTHESIS: The stability of emulsions stabilized by soft and responsive microgels and their macroscopic properties are governed by the microstructure of microgels, in particular their deformability. However, little is known about the role of the microgel chemistry, though it is expected that polymeric backbone with an amphiphilic structure is a requirement for their adsorption at the oil-water interface. EXPERIMENTS: A series of biocompatible, thermoresponsive and amphiphilic poly(oligoethylene glycol)methacrylate (pOEMA) microgels is synthesized, with varying hydrophobic-hydrophilic balance, or equivalent varying volume phase transition temperature (VPTT). Their behavior in the bulk phase and at solid interfaces is compared to their behavior at liquid interfaces, studied on flat and model interfaces by the pendant drop method, and on drops, in microgel-stabilized emulsions. FINDINGS: Controlling the composition of microgels by simply changing the number of ethylene oxide groups in the hydrophilic side chain allows a precise tuning of their VPTT in the range of 20-60 °C. Simultaneously, the swelling ratio and the deformability of the microgels increase by increasing the hydrophilicity, as a result of the polymerization process. Regardless of their hydrophilicity, all the swollen pOEMA microgels adsorb at the liquid interface and stabilize emulsions, whose flocculation state and mechanical stability depends on the microgel deformability. Unexpectedly, most emulsions remain stable upon heating above the VPTT of the microgels. Such feature highlights their extreme robustness, whose origin is discussed. This study opens new opportunities for the use of biocompatible Pickering emulsifiers.

Sugar-responsive Pickering emulsions mediated by switching hydrophobicity in microgels.

HYPOTHESIS: Pickering emulsions stabilized by soft and responsive microgels can demulsify on demand upon microgel collapse. The concept has been explored with simple model microgels such as poly(N-isopropylacrylamide) (pNIPAM) and their derivatives, but the role of functionalization is largely unexplored. EXPERIMENTS: Saccharide-responsive phenylboronic-modified microgels are used as Pickering emulsion stabilizers. Emulsion stability and microgel organization at drop surface are studied as a function of saccharide concentration. Better insight into their behavior at interfaces is gained through adsorption kinetics and Langmuir film studies at air-water interface. FINDINGS: The functionalization of water-swollen microgels by phenylboronic functions imparts some hydrophobicity to the structure, at the origin of additional internal cross-links analogous which rigidify the structure compared to non-functionalized microgels, as proved by their slow adsorption kinetics and poor interfacial compressibility. Upon boronate ester formation with diol groups of the saccharide, the hydrophobic character of the phenylboronic acid decreases, increasing the adsorption kinetics and their interfacial compressibility. Emulsions are stable in the presence of saccharide, given the high deformability of the yet-hydrophilic microgels, and mechanically unstable with less deformable particles in low saccharide concentration. The hydrophobic-hydrophilic switch acts as a trigger to tune the microgel stabilizing properties.

Kinetics of spontaneous microgels adsorption and stabilization of emulsions produced using microfluidics.

The aim of the paper is to examine the adsorption kinetics of soft microgels and to understand the role of fundamental parameters such as electrostatics and deformability on the process. This knowledge is further exploited to produce microgel-stabilized emulsions using a co-flow microfluidic device. Uncharged microgels made of poly(N-isopropylacrylamide) are synthesized with variable cross-linker contents, and charged ones are produced by introducing pH sensitive co-monomers during the synthesis. The study is carried out by measuring the microgels adsorption kinetics by means of the pendant drop method. The surface pressure is derived from the previous results as a function of time and is measured as a function of the area compression using a Langmuir trough. Emulsions are produced using a microfluidic device varying the microgels concentration and their stability is visually assessed. The microgels deformability as well as higher particle concentrations favour their adsorption. The adsorption is not governed by diffusion, it is cooperative and irreversible. Conversely, the kinetics is slowed down for increasing cross-linking density. The presence of charges slows down the kinetics of adsorption. In the presence of electrolyte, the kinetics accelerates and becomes similar to the one of neutral microgels. The original features of microgel adsorption is highlighted and the differences with adsorption of polymers, star polymers, proteins, and polyelectrolytes are emphasized. Taking benefit from the adsorption kinetics, the required formulation conditions for producing microgel-stabilized emulsions using a co-flow microfluidic device are derived. There exists a critical concentration above which microgels spontaneously adsorb in a sufficient way to decrease the interfacial tension. This critical microgel concentration increases with the cross-linking density and is higher for charged microgels. Whatever the kinetics, the same surface pressure is finally reached. This peculiar behaviour is likely a consequence of the presence of dangling chains in the as-prepared microgels. Consequently, a microgel excess is required to produce emulsions using microfluidics where adsorption has to be spontaneous.