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

Abstract

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.

Surface Patterning with SiO2@PNiPAm Core–Shell Particles

“…As expected, the cross-linking swelling behavior decreases with increasing cross-linking density ( Figure 2 c). 54 , 56 The shell thickness, however, has no effect on the swelling behavior, which is in agreement with previous work. 58 …”

“…With decreasing cross-linking density, the particles are able to spread more at the air/water interface and thus occupy larger areas, in agreement with previous work. 54 , 56 At high surface pressure (>30 mN/m), all particles show a similar area per particle of 250 000 nm 2 , which corresponds to an interparticle distance of 564 nm. This is substantially larger than the silica core with a diameter d c = 160 nm.…”

“… 48 − 51 Due to their compressible nature, such microgel particles show a complex interfacial phase behavior as a function of the available interfacial area, which can be systematically varied using a Langmuir trough. 52 − 56 At low densities, hexagonal non close-packed phases with a corona–corona contact prevail. With decreasing available surface, the system undergoes a phase transition into a hexagonal close-packed phase, where the microgels are in a core–core contact.…”

“…With decreasing available surface, the system undergoes a phase transition into a hexagonal close-packed phase, where the microgels are in a core–core contact. 48 , 50 , 53 , 54 , 56 , 57 The surface pressure upon which the microgels undergo phase transition depends on the cross-linking density. 54 , 56 Softer, less cross-linked microgels undergo phase transition at higher surface pressures compared to more cross-linked microgels.…”

Synthesis of Janus and Patchy Particles Using Nanogels as Stabilizers in Emulsion Polymerization

“…Interestingly, 2D studies of ordering of colloidal particles on liquid-liquid interfaces, showed no dependence in spatial arrangement on pH or salt concentration. 57,58 A comparison with these systems is reasonable as up to high styrene monomer conversion the latex particle core will be swollen, and hence in a liquid state. When we plot the surface area per patch (calculated as the ratio of the latex surface area divided by the number of patches present) as a function of pH ( Figure 6C), a striking difference emerges.…”