Soft Colloidal Particles at Fluid Interfaces

Abstract

The assembly of soft colloidal particles at fluid interfaces is reviewed in the present paper, with emphasis on the particular case of microgels formed by cross-linked polymer networks. The dual polymer/colloid character as well as the stimulus responsiveness of microgel particles pose a challenge in their experimental characterization and theoretical description when adsorbed to fluid interfaces. This has led to a controversial and, in some cases, contradictory picture that cannot be rationalized by considering microgels as simple colloids. Therefore, it is necessary to take into consideration the microgel polymer/colloid duality for a physically reliable description of the behavior of the microgel-laden interface. In fact, different aspects related to the above-mentioned duality control the organization of microgels at the fluid interface, and the properties and responsiveness of the obtained microgel-laden interfaces. This works present a critical revision of different physicochemical aspects involving the behavior of individual microgels confined at fluid interfaces, as well as the collective behaviors emerging in dense microgel assemblies.

Keywords: deformability; interfaces; interfacial tension; microgels; reconfigurable; softness.

Figure 1

(a) Sketch of the microgel deformation upon adsorption to a fluid interface. Adapted from Deshmukh et al. [28] with permission from the Royal Chemical Society, Copyright (2014). (b) Conformation and dimension of microgel particles (characterized by the microgel radius at the contact length, r, and height, h) at water/oil interfaces with different interfacial tensions (γ). Reprinted from Vialetto et al. [41] under the Creative Common Attribution License 4.0, 2022. (c) Sketch of the conformation of the PNIPAM microgel at a water/vapor interface at 25 °C and 50 °C. Reprinted from Harrer et al. [42] with permission from the American Chemical Society, Copyright (2019).

Figure 2

Sketch of the conformation of microgels adsorbed at a fluid interface from temperatures below (a) and above (b) the phase volume transition (VPTT: volume phase transition temperature). Adapted from Wu et al. [52] with permission from the American Chemical Society, Copyright (2019).

Figure 3

(a) Surface pressure (П) vs. number of microgels per area unit isotherms for PNIPAM microgels at a water/decane interface. The blue and red curves correspond to the isotherms obtained at 20 °C (below the volume phase transition temperature) and 40 °C (above the volume phase transition), respectively. The dashed lines indicate the onset of the different regimes, which are indicated by roman numerals in the corresponding colors. (b) Atomic Force Microscopy images of Langmuir–Blodgett deposits obtained for monolayers of PNIPAM microgels at a water/decane interface below the volume phase transition temperature (at 20 °C). (c) Atomic Force Microscopy images of Langmuir–Blodgett deposits obtained for monolayers of PNIPAM microgels at a water/decane interface above the volume phase transition temperature (at 40 °C). Adapted from Bochenek et al. [45] with permission from the American Chemical Society, Copyright (2019).

Figure 4

Atomic Force Microscopy images of Langmuir–Blodgett deposits obtained from low cross-linked (a–d) and high cross-linked (e–h) PNIPAM microgels at the water/vapor interface, obtained at increasing values of the interfacial coverage (from left to right). Reprinted from Picard et al. [75] with permission from the American Chemical Society, Copyright (2017).

Figure 5

Effect of the size and charge of microgels adsorbed at a water/oil interface on the in-plane organization of the microgel film as a function of the diameter of the projection of the particle at the interface (D_interface). Reproduced from Schmidt et al. [76] with permission from the Royal Society of Chemistry, Copyright (2020).

Figure 6

Stabilizing efficiency of PNIPAM microgel particles for oil-in-water Pickering emulsions as a function of pH and temperature. (■) Stable emulsions, (●) unstable emulsions and (◆) phase separation. Reproduced from Ngai et al. [84] with permission from the Royal Society of Chemistry, Copyright (2005).

Figure 7

Effect of the different parameters affecting the emulsion properties on the organization of microgel-laden interfaces. (a,b) Microgel size. (c,d) Cross-linking density. (e,f) Processing temperature (VPTT: volume phase transition temperature). Adapted from Destribats et al. [85] with permission from the American Chemical Society, Copyright (2014).

Figure 8

Effect of the shear rate on the dispersion of Mickering oil-in-water emulsions. Reprinted from Destribats et al. [79] with permission from the American Chemical Society, Copyright (2013).