Stable in Bulk and Aggregating at the Interface: Comparing Core–Shell Nanoparticles in Suspension and at Fluid Interfaces

Abstract

Colloidal particles are extensively used to assemble materials from bulk suspensions or after adsorption and confinement at fluid interfaces (e.g., oil-water interfaces). Interestingly, and often underestimated, optimizing interactions for bulk assembly may not lead to the same behavior at fluid interfaces. In this work, we compare model composite nanoparticles with a silica core coated with a poly-N-isopropylacrylamide hydrogel shell in bulk aqueous suspensions and after adsorption at an oil-water interface. Bulk properties are analyzed by confocal differential dynamic microscopy, a recently developed technique that allows one to simultaneously obtain structural and dynamical information up to high volume fractions. The results demonstrate excellent colloidal stability and the absence of aggregation in all cases. The behavior at the interface, investigated by a range of complementary approaches, is instead different. The same hydrogel shells that stabilize the particles in the bulk deform at the interface and induce attractive capillary interactions that lead to aggregation even at very low area fractions (surface coverage). Upon further compression of a particle-laden interface, a structural transition is observed where closely packed particle aggregates form. These findings emphasize the manifestation of different, and possibly unexpected, responses for sterically stabilized nanoparticles in the bulk and upon interfacial confinement.

Dynamics and Wetting Behavior of Core–Shell Soft Particles at a Fluid–Fluid Interface

“…8a, shows that, upon adsorption, the shell induces attractive capillary forces stemming from heterogeneities of the three-phase contact line. 30,31 These heterogeneities may be responsible for the increased drag at the interface, as already hypothesized for non-deformable spheres. 37 Moreover, unlike their hard-sphere counterparts, the drag experienced by the CSNPs appears to be independent of their position relative to the plane of the interface and to simply scale with the interfacial radius.…”

Microgels Adsorbed at Liquid–Liquid Interfaces: A Joint Numerical and Experimental Study

“…Our N-isopropylacrylamide (NiPAm, TCI 98.0%) based microgels were synthesized by a conventional method of precipitation polymerization in water. 74 First, MilliQ water was heated to 80 • C, NiPAm and the cross-linker N-N'-Methylenebisacrylamide (BIS, Fluka 99.0%),…”

Building Reconfigurable Devices Using Complex Liquid–Fluid Interfaces

“…This deformation of the particles can increase adsorption energies by orders of magnitude relative to rigid particles. These particles form a 2D network of percolating aggregates with large voids at low surface coverage, reflecting the presence of attractive capillary forces between the particles (Figure h) …”

“…h) Deformation of particles adsorbed to the liquid–fluid interface leads to strong capillary interactions, causing their aggregation. Reproduced with permission . Copyright 2018, American Chemical Society.…”

“…Observations of the size dependence of the bending modulus of particle assemblies in this size regime would prove invaluable in understanding the physics of these systems. A study of the bending modulus of rough nanoparticle assemblies such as those investigated by Isa and co‐workers, which generate strong capillary interactions regardless of their size, would prove insightful to the role capillary interactions in governing the bending modulus of the monolayers. The nanoparticle surfactant system of Cui et al, in which particles are bound to the interface by polymer ligands of arbitrary size and structure, may prove an insightful system in helping us to understand the physical origins and magnitude of the bending moduli of particle monolayers.…”