Harnessing the polymer-particle duality of ultra-soft nanogels to stabilise smart emulsions

Micro- and nanogels are widely used to stabilise emulsions and simultaneously implement their responsiveness to the external stimuli. One of the factors that improves the emulsion stability is the nanogel softness. Here, we study how the softest nanogels that can be synthesised with precipitation polymerisation of N-isopropylacrylamide (NIPAM), the ultra-low crosslinked (ULC) nanogels, stabilise oil-in-water emulsions. We show that ULC nanogels can efficiently stabilise emulsions already at low mass concentrations. These emulsions are resistant to droplet flocculation, stable against coalescence, and can be easily broken upon an increase in temperature. The resistance to flocculation of the ULC-stabilised emulsion droplets is similar to the one of emulsions stabilised by linear pNIPAM. In contrast, the stability against coalescence and the temperature-responsiveness closely resemble those of emulsions stabilised by regularly crosslinked pNIPAM nanogels. The reason for this combination of properties is that ULC nanogels can be thought of as colloids in between flexible macromolecules and particles. As a polymer, ULC nanogels can efficiently stretch at the interface and cover it uniformly. As a regularly crosslinked nanogel particle, ULC nanogels protect emulsion droplets against coalescence by providing a steric barrier and rapidly respond to changes in external stimuli thus breaking the emulsion. This polymer-particle duality of ULC nanogels can be exploited to improve the properties of emulsions for various applications, for example in heterogeneous catalysis or in food science.

Adsorption of soft NIPAM nanogels at hydrophobic and hydrophilic interfaces: Conformation of the interfacial layers determined by neutron reflectivity

The application of stimuli-responsive microgels and nanogels in drug delivery, catalysis, sensing, and coatings is restricted currently by the limited understanding of the factors influencing their adsorption dynamics and structural changes at interfaces. We have used neutron reflectivity to resolve, on the Ångström scale, the structure of 5% crosslinked N-isopropylacrylamide nanogels at both hydrophobic and hydrophilic interfaces in situ, as a function of temperature and bulk nanogel concentration. Our results show that the higher flexibility given by the low crosslinker content allows for a more ordered structure and packing. The adsorption of the thermoresponsive nanogels is primarily driven by temperature, more specifically its proximity to its volume phase transition temperature, while concentration plays a secondary role. Hydrophobic interactions drive the conformation of the first layer at the interface, which plays a key role in influencing the overall nanogel structure. The mobility of the first layer at the air-water interface as opposed to the interfacial confinement at the solid (SiC8)-liquid interface, results in a different conformation, a more compact and less deformed packing structure, which ultimately drives the structure of the subsequent layers. The evidence for the different structural conformations determined by the degree of hydrophobicity of the interface provides new knowledge, which is essential for the development of further applications. The key role of hydrophobic interactions in driving adsorption and interfacial behavior was also confirmed by fluid AFM experiments which visualized adherence of the nanogels to SiC8 modified surfaces.

Engineering hybrid microgels as particulate emulsifiers for reversible Pickering emulsions

Thermo-responsive microgels are unique stabilizers for stimuli-sensitive Pickering emulsions that can be switched between the state of emulsification and demulsification by changing the temperature. However, directly temperature-triggering the phase inversion of microgel-stabilized emulsions remains a great challenge. Here, a hybrid poly(N-isopropylacrylamide)-based microgel has now been successfully fabricated with tunable wettability from hydrophilicity to hydrophobicity in a controlled manner. Engineered microgels are synthesized from an inverse emulsion stabilized with hydrophobic silica nanoparticles, and the swelling-induced feature can make the resultant microgel behave like either hydrophilic or hydrophobic colloids. Remarkably, the phase inversion of such microgel-stabilized Pickering emulsions can be in situ regulated by temperature change. Moreover, the engineered microgels were capable of stabilizing water-in-oil Pickering emulsions and encapsulation of enzymes for interfacial bio-catalysis, as well as rapid cargo release triggered by phase inversion.

Characterization of the volume fraction of soft deformable microgels by means of small-angle neutron scattering with contrast variation

The volume occupied by colloids in a suspension - namely the volume fraction - is the thermodynamic variable that determines the phase behavior of these systems. While for hard incompressible spheres this quantity is well defined, for soft compressible colloids such as microgels - polymeric crosslinked networks swollen in a good solvent - the determination of the real volume occupied by these particles in solution is particularly challenging. This fact depends on two aspects: first the surface and, therefore, the volume of the microgels is hard to define properly given their external fuzziness; second, microgels can osmotically deswell, deform or interpenetrate their neighbors, i.e. change their shape and size depending on the solution concentration. Here, the form factors of few hydrogenated microgels embedded in a matrix of deuterated but otherwise identical microgels are measured using small-angle neutron scattering with contrast variation. From the analysis of the scattering data, the variation of the volume of the microgels as a function of concentration is obtained and used to calculate the real microgel volume fraction in solution. Soft neutral microgels are shown to facet already at low concentrations while in contrast, harder microgels maintain their shape and change their volume.