Unraveling the 3D Localization and Deformation of Responsive Microgels at Oil/Water Interfaces: A Step Forward in Understanding Soft Emulsion Stabilizers

Abstract

Responsive microgels are deformable submicrometer cross-linked polymeric hydrogel particles that are used as a novel class of emulsion stabilizers. Their flexibility and the triggering of conformational changes by external stimuli lead to several advantages compared to rigid particles used in conventional Pickering emulsions. Despite their rapidly increasing use, several key aspects relating to microgel microstructure and localization at liquid interfaces are still unexplored. We present here a novel characterization that employs freeze-fracture shadow-casting cryo-SEM to disclose quantitative 3D information on the deformation and protrusion of microgels at water/oil interfaces. Despite the bulk pH response (swelling), we report here the unexpected absence of size and vertical position changes as a function of pH at liquid interfaces and interpret the results using simple arguments that link the particle interfacial activity, solvation, and internal deformation. These results pave the way to a deeper understanding of a novel class of soft materials.

Adsorption Energies of Poly(ethylene oxide)-Based Surfactants and Nanoparticles on an Air–Water Surface

“…Similar morphologies have been directly observed for larger soft particles. 40 The desorption processes measured here thus involve the interaction of significantly stretched interfacial shells. As with homopolymer chains, entire NPs desorb from the particulate monolayer once further compression of interfacial PEO chains/shells would cost more energy than expelling a particle from the interface.…”

Interactions and Stress Relaxation in Monolayers of Soft Nanoparticles at Fluid-Fluid Interfaces

“…Soft colloids at fluid-fluid interfaces, for instance star copolymers [1], microgels [2,3], and polymer-or ligandgrafted nanoparticles [4][5][6][7][8][9], stretch and deform, adopting shapes that are dictated by the interplay of surface tension and deformability, much like in the wetting of membranes, vesicles [10], and soft solids [11,12]. Ligand-grafted metal or semiconductor nanocrystals at fluid-fluid interfaces are widely used in nanomaterials synthesis [13][14][15] and in catalytic processes [16].…”

Isostructural solid–solid phase transition in monolayers of soft core–shell particles at fluid interfaces: structure and mechanics

“…The use of microgels as emulsion stabilizers is driven by their strong propensity to adsorb spontaneously at oil-water (o/w) interfaces. As opposed to the case of hard particles, which adsorb and interact at an o/w interface without any deformation 35 , it has been extensively reported that microgels can be significantly deformed upon adsorption at a fluid interface 34,[36][37][38][39][40] . Adsorption is driven by a reduction of the fluid-interface free energy when the particles sit at the interface, and deformation is driven by the fact that microgels tend to spread to maximize the amount of surface-active polymer chains at the interface.…”

“…Adsorption is driven by a reduction of the fluid-interface free energy when the particles sit at the interface, and deformation is driven by the fact that microgels tend to spread to maximize the amount of surface-active polymer chains at the interface. Deformation proceeds until the free energy gain is balanced by internal elasticity 41 ; microgels reach therefore effective diameters at the interface that are significantly larger than their size in bulk, depending on the cross-linking ratios 36,37 , but largely independent of other parameters that affect bulk dimensions, such as pH for PNiPAm-co-MAA microgels 38 . In particular, previous observations showed that the presence of the aforementioned radial gradients of cross-linking density, as well as of some dangling chains at the particle periphery, lead to the exasperation of the core-shell (or core-corona) morphology of the particles after adsorption at a fluid interface 37,38 .…”