The Compressibility of pH‐Sensitive Microgels at the Oil–Water Interface: Higher Charge Leads to Less Repulsion

Geisel, Karen; Isa, Lucio; Richtering, Walter

doi:10.1002/ange.201402254

Cited by 54 publications

(90 citation statements)

References 37 publications

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“…Several studies have focused on the spontaneous adsorption of microgels in model geometries, using particle tracking techniques at a flat interface 21,22 , the pendant drop method 5,[22][23][24][25][26] or Langmuir trough experiments [25][26][27] . Interestingly, particle tracking studies performed recently showed that the particles eventually form clusters 20,21 , the origin of which is still unknown.…”

Section: Introductionmentioning

confidence: 99%

Drainage dynamics of thin liquid foam films containing soft PNiPAM microgels: influence of the cross-linking density and concentration

et al. 2017

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We investigate the drainage dynamics of thin liquid foam films containing PNiPAM microgel suspensions with two cross-linking densities (1.5 and 5 mol% BIS) and at two microgel concentrations (0.1 and 1% wt). For this purpose, we use a thin-film pressure balance apparatus that can apply a controlled and sudden hydrostatic pressure on a film, and record the subsequent film thinning as a function of time. Once the film thickness has reached a stationary value, we test the adhesion between the interfaces of the film by reducing the pressure and measuring the angle between the film and the meniscus. This angle increases on reduction of pressure for adhesive films, which resists the separation of their interfaces. Non-adhesive films separate easily, and the meniscus angle stays constant. At a low microgel concentration, the more densely cross-linked microgels (5 mol% BIS) tend to drain into more adhesive films than the more loosely cross-linked particles (1.5 mol% BIS). The adhesion results from particles that bridge the two air-water interfaces of the film and are shared between them. In these cases, the film, which is initially stabilized by a bilayer of microgel particles, rearrange to a state where the microgels bridge the interfaces. These results are discussed and compared with previous studies at a low concentration of microgels, which have shown that emulsions stabilized with densely cross-linked microgels are more adhesive and less resistant to mechanical stresses than those obtained with lower cross-linking densities. In addition, micron-scale depleted zones with no microgels are observed in the films stabilized with the 5 mol% BIS particles, which eventually lead to the rupture of the films. At 1% wt, the films drain slowly, are not adhesive and have the thickness of a bilayer of microgel; while at 0.1% wt, the films have the thickness of a monolayer of microgel, are adhesive and show bridging. From the thin liquid foam film thicknesses we extract a rough estimation of the radii of adsorbed particles in the thick films before applying the pressure. Our results are consistent with particles being adsorbed in a spread conformation for the 0.1% wt sample and in a compressed conformation for the 1% wt sample. In line with previous studies on emulsions, we conclude that a larger surface coverage may reduce rearrangements, thus preventing bridging.

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Section: Introductionmentioning

confidence: 99%

Drainage dynamics of thin liquid foam films containing soft PNiPAM microgels: influence of the cross-linking density and concentration

et al. 2017

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“…The surface activity of the co-polymers in the microgel is in fact a very weakly varying function of pH between 3 and 9, which is the experimentally relevant window. Conversely, the mechanical response of microgel monolayers upon compression varies at different values of pH within the same window, highlighting the complex interplay between the different particle properties 43 . To fix the experimental conditions, all the experiments reported in this paper were performed using microgel dispersions in Milli-Q water.…”

Section: Two-dimensional Phase Diagrams: Compression and Depositionmentioning

confidence: 99%

“…In particular, the existence of both shell-shell and core-core contacts has been assumed, but to date, a systematic visualization of these structures and a thorough study on the transitions between them is still lacking. Additionally, measurements of compression isotherms showed a counterintuitive response in the presence of bulk charges in the microgels, with uncharged particles interacting via the interface at larger inter-particle separations compared to charged systems 43 . Furthermore, dilatational elasticity showed an unexpected non-monotonic behavior of the elastic modulus with compression of interfacial microgel monolayers, suggesting that the presence of coreshell interactions couples non-trivially to the interface structure and mechanical properties 42,44,45 .…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2016

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We have studied the complete two-dimensional phase diagram of a core-shell microgel-laden fluid interface by synchronizing its compression with the deposition of the interfacial monolayer. Applying a new protocol, different positions on the substrate correspond to different values of the monolayer surface pressure and specific area. Analyzing the microstructure of the deposited monolayers, we discovered an isostructural solid-solid phase transition between two crystalline phases with the same hexagonal symmetry, but with two different lattice constants. The two phases corresponded to shell-shell and core-core inter-particle contacts, respectively; with increasing surface pressure the former mechanically failed enabling the particle cores to come into contact. In the phase-transition region, clusters of particles in core-core contacts nucleate, melting the surrounding shell-shell crystal, until the whole monolayer moves into the second phase. We furthermore measured the interfacial rheology of the monolayers as a function of the surface pressure using an interfacial microdisk rheometer. The interfaces always show a strong elastic response, with a dip in the shear elastic modulus in correspondence with the melting of the shell-shell phase, followed by a steep increase upon the formation of a percolating network of the core-core contacts. These results demonstrate that the core-shell nature of the particles leads to a rich mechanical and structural behavior that can be externally tuned by compressing the interface, indicating new routes for applications, e.g. in surface patterning or emulsion stabilization.

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“…Microgels exhibit interesting properties that lie between linear polymers and three-dimensional polymer networks (hydrogels). [1][2][3][4][5][6] Stimuli-responsive microgels can undergo changes in size, softness and surface charge by varying pH, light, solvent or temperature. [7][8][9][10][11] The modification of microgels with metal or metal oxide nanoparticles, ions or proteins results in new materials with enhanced properties.…”

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confidence: 99%

Reversible Switching and Recycling of Adaptable Organic Microgel Catalysts (Microgelzymes) for Asymmetric Organocatalytic Desymmetrization

et al. 2018

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Adaptable enzyme-mimetic catalysts based on temperature-responsive polymer microgels (microgelzymes) have been developed. By a simple change in the temperature, a microgel catalyst can be reversibly switched into its soluble or precipitated form, thus combining the advantages of homogeneous and heterogeneous catalysis. The responsive microgel reactors show high reactivity and selectivity, as well as good recycling properties in the alcoholysis of cis-tetrahydrophthalic anhydride.

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The Compressibility of pH‐Sensitive Microgels at the Oil–Water Interface: Higher Charge Leads to Less Repulsion

Cited by 54 publications

References 37 publications

Drainage dynamics of thin liquid foam films containing soft PNiPAM microgels: influence of the cross-linking density and concentration

Drainage dynamics of thin liquid foam films containing soft PNiPAM microgels: influence of the cross-linking density and concentration

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

Reversible Switching and Recycling of Adaptable Organic Microgel Catalysts (Microgelzymes) for Asymmetric Organocatalytic Desymmetrization

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