Compression and Ordering of Microgels in Monolayers Formed at Liquid-Liquid Interfaces: Computer Simulation Studies

Soft gliadin nanoparticles at air/water interfaces: The transition from a particle-laden layer to a thick protein film

HYPOTHESIS

Protein-based soft particles possess a unique interfacial deformation behavior, which is difficult to capture and characterize. This complicates the analysis of their interfacial properties. Here, we aim to establish how the particle deformation affects their interfacial structural and mechanical properties.

EXPERIMENTS

Gliadin nanoparticles (GNPs) were selected as a model particle. We studied their adsorption behavior, the time-evolution of their morphology, and rheological behavior at the air/water interface by combining dilatational rheology and microstructure imaging. The rheology results were analyzed using Lissajous plots and quantified using the recently developed general stress decomposition (GSD) method.

FINDING

Three distinct stages were revealed in the adsorption and rearrangement process. First, spherical GNPs (∼105 nm) adsorbed to the interface. Then, these gradually deformed along the interface direction to a flattened shape, and formed a firm viscoelastic 2D solid film. Finally, further stretching and merging of GNPs at the interface resulted in rearrangement of their internal structure to form a thick film with lower stiffness than the initial film. These results demonstrate that the structure of GNPs confined at the interface is controlled by their deformability, and the latter can be used to tune the properties of prolamin particle-based multiphase systems.

Computer Simulations of Responsive Nanogels at Lipid Membrane

The swelling and collapse of responsive nanogels on a planar lipid bilayer are studied by means of mesoscopic computer simulations. The effects of molecular weight, cross-linking density, and adhesion strength are examined. The conditions for collapse-mediated engulfing by the bilayer are found. In particular, the results show that at low hydrophobicity level the increase in the nanogel softness decreases the engulfing rate. On the contrary, for stronger hydrophobicity level the trend changes to the opposite one. At the same time, when the cross-linking density is too low or the adhesion strength is too high the nanogel deformation at the membrane suppresses the engulfing regardless of the network swelling ratio. Finally, for comparative reasons, the behavior of the nanogels is also studied at the solid surface. These results may be useful in the design of soft particles capable of tuning of their elasticity and porosity for successful intracellular drug delivery.

Relationship between π-A isotherms and single microgel/microgel array structures revealed via the direct visualization of microgels at the air/water interface

The structures of single microgels and microgel arrays formed at the air/water interface were visualized directly, and their structures correlated with π-A isotherms in order to understand the compression behavior of soft and deformable microgels at this interface. Large microgels (ca. 4 μm) were synthesized so that these can be clearly visualized at the air/water interface, even under high compression, and a series of microgel compression experiments were directly evaluated using a Langmuir trough equipped with a fluorescence microscope. The experiments revealed that upon compressing the microgel arrays at the interface voids disappeared and colloidal crystallinity increased. However, the colloidal crystallinity decreased when the microgel arrays were strongly compressed. In addition, when the structures were observed at higher magnification, it became clear that the single microgel structures, when visualized from above, changed from circular to polygonal upon compressing the microgel array. The results of this study can be expected to improve the understanding of the compression behavior of microgel arrays adsorbed at the air/water interface and will thus be useful for the creation of new functional microgel stabilizers with potential applications in e.g., bubbles and emulsions.

Peculiarities of Emulsions Stabilized by Stimuli-Responsive Interpenetrating Polymeric Network Microgels

Emulsions have become a crucial product form in various industries in modern times. Expanding the class of substances used to stabilize emulsions can improve their stability or introduce new properties. Particularly, the use of stimuli-responsive microgels makes it possible to create "smart" emulsions whose stability can be controlled by changing any of the specified stimuli. Thus, finding new ways to stabilize emulsions may broaden their application. In this work, for the first time, we applied microgels based on interpenetrating polymeric networks (IPNs) of poly(N-isopropylacrylamide) (PNIPAM) and poly(acrylic acid) (PAA) as stabilizing agents for "oil-in-water" emulsions. We have demonstrated that emulsions stabilized by such soft particles can remain colloidally stable for an extended period, even after being heated up to 40 °C, which is above the lower critical solution temperature (LCST) of PNIPAM. On the contrary, the emulsions stabilized by PNIPAM homopolymer microgels were broken upon heating. To understand the stabilization mechanism of the emulsions, mesoscopic computer simulations were performed to study the IPN microgels at the liquid-liquid interface. The simulations demonstrated that when the first subnetwork (PNIPAM) collapses, the particle adopts a flattened core-shell morphology with a highly swollen PAA-rich shell and a collapsed PNIPAM-rich core. Unlike its PNIPAM homopolymer counterpart, the IPN microgel maintains its three-dimensional shape, which provides stability to the microgel-based emulsions over a wide range of temperatures. Our combined findings could be useful in developing new approaches to emulsions' storage, biphasic catalysis, and lubrication of mechanisms in various operating and climatic conditions.

Compression and Ordering of Hollow Microgels in Monolayers Formed at Liquid-Liquid Interfaces

Monolayers of polymer microgels with a spherical cavity adsorbed at the liquid-liquid interface were studied using mesoscopic computer simulations. One liquid, named water, was always considered as a good solvent, while the microgel solubility in the second liquid, named oil, was varied. The symmetric and asymmetric cases of vanishing and the strong differences in solubility between the network particles and the liquids were considered. The simulations provided us with an insight into the shape and volume changes of the microgels upon compression, making it possible to relate the response of the individual network with the collective order and structure of the monolayer. Similar to regular microgels, the compression of the monolayer of hollow particles led to a decrease in lateral sizes accompanied by shape transformation from a flattened to a nearly spherical shape. However, the presence of a cavity filled with solvent caused some unique differences in the behavior of the system. The adsorption pathway of hollow microgels at the liquid interface predefines: (a) the position of the particles with respect to the interface and (b) the structure of the monolayer. A striking discovery is that in the symmetric case of similar solubility of the microgel in both liquids, it is possible to produce a monolayer in which one part of the network faces the aqueous phase and the other part faces the oil phase. The polymer concentration profiles plotted along the normal to the interface reveal a redistribution of polymeric mass of the microgels relative to the interface, distinguishing between the microgels whose cavities are filled with water and oil, respectively. Moreover, the ratio between the microgels faced in water and oil does not change upon compression and predetermines the response and order of the monolayer.

Swelling, collapse and ordering of rod-like microgels in solution: Computer simulation studies

Polymer microgels have proven to be highly promising macromolecular objects for a wide variety of applications. In particular, the soft particles of an anisotropic (rod-like) shape are of special interest because of their potential use in tissue engineering or materials design. However, a little is known about the physical behavior of such microgels in solution, which inspired us to study them using mesoscopic computer simulations. For single networks, depending on the solvent quality, the dimensional characteristics were obtained for microgels of different molecular weight, crosslinking density and aspect ratio. In particular, the conditions for the rod-to-rod (preserving the nonspherical shape) and rod-to-sphere collapse were found. In addition, the effect of the liquid-crystalline (LC) ordering was demonstrated for the ensemble of rod-like microgels at different swelling ratios, and the influence of microgel aspect ratio on the volume fraction of the LC transition was shown.

Effect of network topology and crosslinker reactivity on microgel structure and ordering at liquid-liquid interface

Polymer microgels synthesized in silico were studied at a liquid-liquid interface via mesoscopic computer simulations and compared to microgels with ideal (diamond-like) structure. The effect of crosslinkers reactivity ratio on the single particle morphology at the interface and monolayer behavior was examined. It was demonstrated that single particles deform into an explicit core-corona morphology when adsorbed at the interface. An increase in the crosslinker reactivity ratio decreased both the deformation ratio and the ratio between the core and corona sizes. Meanwhile, the compression of microgel monolayers revealed the existence of five distinct interparticle contact regimes, which have been observed experimentally in the literature. The crosslinker reactivity ratio appeared to define the compression range in these regimes and the sharpness of the transition between them. In particular, the higher the crosslinker reactivity ratio, the smaller the corona, and in turn, the narrower the range of the intermediate regime comprising both core-core and corona-corona contacts. The obtained results demonstrate that the more realistic model of microgels synthesized via precipitation polymerization allows for a more accurate prediction of the properties of the microgels at a liquid-liquid interface in comparison to the conventional diamond-like lattice model.

Anisotropic Microgels Show Their Soft Side

Anisotropic, submicrometer-sized particles are versatile systems providing interesting features in creating ordering in two-dimensional systems. Combining hard ellipsoids with a soft shell further enhances the opportunities to trigger and control order and alignment. In this work, we report rich 2D phase behavior and show how softness affects the ordering of anisotropic particles at fluid oil-water interfaces. Three different core-shell systems were synthesized such that they have the same elliptical hematite-silica core but differ with respect to thickness and stiffness of the soft microgel shell. Compression isotherms, the shape of individual core-shell microgels, and their 2D order at a decane-water interface are investigated by means of the Langmuir-Blodgett technique combined with ex-situ atomic force microscopy (AFM) imaging as well as dissipative particle dynamics (DPD) simulations. We show how the softness, size, and anisotropy of the microgel shell affect the side-to-side vs tip-to-tip ordering of anisotropic hybrid microgels as well as the alignment with respect to the direction of compression in the Langmuir trough. A large, soft microgel shell leads to an ordered structure with tip-to-tip alignment directed perpendicular to the direction of compression. In contrast, a thin and harder microgel shell leads to side-to-side ordering orientated parallel to the compression direction. In addition, the thin and harder microgel shell induces clustering of the microgels in the dilute state, indicating the presence of strong capillary interactions. Our findings highlight the relevance of softness for the complex ordering of anisotropic hybrid microgels at interfaces.

Soft Colloidal Particles at Fluid Interfaces

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.

Soft Particles at Liquid Interfaces: From Molecular Particle Architecture to Collective Phase Behavior

Soft particles such as microgels can undergo significant and anisotropic deformations when adsorbed to a liquid interface. This, in turn, leads to a complex phase behavior upon compression. To date, experimental efforts have predominantly provided phenomenological links between microgel structure and resulting interfacial behavior, while simulations have not been entirely successful in reproducing experiments or predicting the minimal requirements for the desired phase behavior. Here, we develop a multiscale framework to link the molecular particle architecture to the resulting interfacial morphology and, ultimately, to the collective interfacial phase behavior. To this end, we investigate interfacial morphologies of different poly(N-isopropylacrylamide) particle systems using phase-contrast atomic force microscopy and correlate the distinct interfacial morphology with their bulk molecular architecture. We subsequently introduce a new coarse-grained simulation method that uses augmented potentials to translate this interfacial morphology into the resulting phase behavior upon compression. The main novelty of this method is the possibility to efficiently encode multibody interactions, the effects of which are key to distinguishing between heterostructural (anisotropic collapse) and isostructural (isotropic collapse) phase transitions. Our approach allows us to qualitatively resolve existing discrepancies between experiments and simulations. Notably, we demonstrate the first in silico account of the two-dimensional isostructural transition, which is frequently found in experiments but elusive in simulations. In addition, we provide the first experimental demonstration of a heterostructural transition to a chain phase in a single-component system, which has been theoretically predicted decades ago. Overall, our multiscale framework provides a phenomenological bridge between physicochemical soft-particle characteristics at the molecular scale and nanoscale and the collective self-assembly phenomenology at the macroscale, serving as a stepping stone toward an ultimately more quantitative and predictive design approach.

Microgels at interfaces, from mickering emulsions to flat interfaces and back

In this review, we cover the topic of p(NIPAM) based microgels at interfaces, revisiting classical studies in light of the newest ones. In particular, we focus on their use as emulsifiers in the so-called mickering emulsions, i.e. Pickering emulsion stabilized by soft particles. Given the complexity of the experimental characterization and simulation of these soft particles at interfaces, the review is structured in progressive complexity levels, until we reach the highly interesting and promising responsiveness to stimuli of mickering emulsions. We start from the lowest level of complexity, the current understanding of the behavior of single microgels confined at a flat interface. Then, we discuss their collective behavior upon crowding, their responsiveness at interfaces, and their macroscopic properties as microgel films. Once we have the necessary characterization tools, we proceed to discuss the complex and convoluted picture of responsive mickering emulsions. The way is rough, with current controversial and contradicting studies, but it holds promising results as well. We state open questions worth of being tackled by the Soft Matter community, and we conclude that it is worth the trouble of continuing after the master theory of microgel interfacial activity, as it will pave the way to widely adopt responsive mickering emulsions as the worthy Pickering emulsion successors.

One-Step Formation of Double Emulsions Stabilized by PNIPAM-based Microgels: The Role of Co-monomer

Microgels have been widely used as particulate emulsifiers to stabilize emulsions due to their multiresponsiveness and deformability. Generally, microgels stabilize oil-in-water (o/w) emulsions, whereas occasionally water-in-oil (w/o) emulsions are reported using oils like n-octanol in which microgels can swell. However, the use of microgels to stabilize double emulsions (DEs) remains scarce. In this work, we report a special poly(N-isopropylacrylamide)- (PNIPAM-) based microgel to obtain water-in-oil-in-water (w/o/w) DEs in one step with the introduction of 1-vinylimidazole (VIM) as comonomer and hydroxy silicone oil as the oily phase. By comparison, when methacrylic acid (MAA) is used, an o/w emulsion will be obtained. The same holds true even when we freeze-dry and redisperse the microgels in the oil. Compared with PNIPAM-co-MAA microgel, PNIPAM-co-VIM microgel achieves a lower interfacial tension (IFT) when dispersed in the aqueous phase. This interfacial affinity of PNIPAM-co-VIM is believed to result from acid-base interaction between VIM and hydroxyl groups of the silicone oil, the same interaction used for preparing silica-vinyl polymer composite particles. Increasing the particle concentrations from 0.05% to 0.9% (w/v), we observe the inversion from w/o to o/w/o and w/o/w emulsions. When the oil fraction is changed from 0.1 to 0.9, the emulsion morphology evolves from o/w and w/o/w to w/o emulsions. At last, we examine the emulsifying ability of PNIPAM-co-VIM microgel with other oils and find that w/o/w emulsions are obtained with edible oils as well. Considering the similarity between microgels and biopolymers, the discovery in this work will help in designing food-grade emulsifiers to form edible DEs.

Mechanochemical Cellular Membrane Internalization of Nanohydrogels: A Large-Scale Mesoscopic Simulation

By combining large-scale dissipative particle dynamics and steered molecular dynamics simulations, we investigate the mechanochemical cellular internalization pathways of homogeneous and heterogeneous nanohydrogels and demonstrate that membrane internalization is determined by the crosslink density and encapsulation ability of nanohydrogels. The homogeneous nanohydrogels with a high crosslink density and low encapsulation ability behave as soft nanoparticles partially wrapped by the membrane, while those with a low crosslink density and high encapsulation ability permeate into the membrane. Regardless of the crosslink density, the homogeneous nanohydrogels undergo typical dual morphological deformations. The local lipid nanodomains are identified at the contacting region between the membrane and nanohydrogels because of different diffusion behaviors between lipid and receptor molecules during the internalization process. The yolk@shell heterogeneous nanohydrogels present a different mechanochemical cellular internalization pathway. The yolk with strong affinity is directly in contact with the membrane, resulting in partial membrane wrapping, and the contacting area is much reduced when compared to homogenous nanohydrogels, leading to a smaller lipid nanodomain and thus avoiding related cellular toxicity. Our findings provide a critical mechanism understanding of the biological pathways of nanohydrogels and may guide the molecular design of the hydrogel-based materials for controlled release drug delivery, tissue engineering, and cell culture.

Interactions between a responsive microgel monolayer and a rigid colloid: from soft to hard interfaces

Responsive poly-N-isopropylacrylamide-based microgels are commonly used as model colloids with soft repulsive interactions. It has been shown that the microgel-microgel interaction in solution can be easily adjusted by varying the environmental parameters, e.g., temperature, pH, or salt concentration. Furthermore, microgels readily adsorb to liquid-gas and liquid-liquid interfaces forming responsive foams and emulsions that can be broken on-demand. In this work, we explore the interactions between microgel monolayers at the air-water interface and a hard colloid in the water. Force-distance curves between the monolayer and a silica particle were measured with the Monolayer Particle Interaction Apparatus. The measurements were conducted at different temperatures and lateral compressions, i.e., different surface pressures. The force-distance approach curves display long-range repulsive forces below the volume phase transition temperature of the microgels. Temperature and lateral compression reduce the stiffness of the monolayer. The adhesion increases with temperature and decreases with a lateral compression of the monolayer. When compressed laterally, the interactions between the microgels are hardly affected by temperature, as the directly adsorbed microgel fractions are nearly insensitive to temperature. In contrast, our findings show that the temperature-dependent swelling of the microgel fractions in the aqueous phase strongly influences the interaction with the probe. This is explained by a change in the microgel monolayer from a soft to a hard repulsive interface.

Capillary interactions between soft capsules protruding through thin fluid films

When a suspension dries, the suspending fluid evaporates, leaving behind a dry film composed of the suspended particles. During the final stages of drying, the height of the fluid film on the substrate drops below the particle size, inducing local interface deformations that lead to strong capillary interactions among the particles. Although capillary interactions between rigid particles are well studied, much is still to be understood about the behaviour of soft particles and the role of their softness during the final stages of film drying. Here, we use our recently-introduced numerical method that couples a fluid described using the lattice Boltzmann approach to a finite element description of deformable objects to investigate the drying process of a film with suspended soft particles. Our measured menisci deformations and lateral capillary forces, which agree well with previous theoretical and experimental works in case of rigid particles, show that the deformations become smaller with increasing particle softness, resulting in weaker lateral interaction forces. At large interparticle distances, the force approaches that of rigid particles. Finally, we investigate the time dependent formation of particle clusters at the late stages of the film drying.