Mixing of Two Immiscible Liquids within the Polymer Microgel Adsorbed at Their Interface

High-performance displacement by microgel-in-oil suspension in heterogeneous porous media: Microscale visualization and quantification

HYPOTHESIS

Preferential flow in porous media is commonly encountered and decreases the multiphase displacement efficiency. Here, we synthesized microgel-in-oil in suspension and demonstrated that microgel-in-oil as a novel additive could present self-adaptive transport behavior and introduce a novel multiphase displacement mode for improving displacement efficiency in heterogeneous porous media.

EXPERIMENTS

We investigated the microgel-in-oil formation process and characterized their morphology with fluorescence microscopy and Cryo-SEM. The suspension displacement performance in heterogeneous porous media was evaluated using a microfluidic chip containing a preferential flow pathway (PFP) and a parallel matrix region. The displacement results of microgel-in-oil were compared to plain microgel particles and analyzed from pore-scale particle transport behavior to macroscopic multiphase flow patterns.

FINDINGS

The results show that suspension with moderate microgel-in-oil yields the optimal displacement efficiency. Fewer microgel-in-oil cannot alter the flow direction, while too many microgel-in-oil would block the PFP region. The topological analysis identified that suspensions with moderate microgel-in-oil content could achieve the strongest sweeping and carrying abilities that contribute to the highest displacement efficiency. The synergistic transport of microgel-in-oil and plain microgel particles would result in local pressure fluctuations to divert displacing fluid from PFP into the matrix region, which explains the above flow behavior.

A Study on the Mechanisms of Nanoparticle-Stabilized High Internal Phase Emulsions Constructed by Cross-Linking Egg White Protein Isolate with Different Transglutaminase Concentrations

There is an increasing interest in the development of high internal phase emulsions (HIPE) stabilized by food-grade nanoparticles due to their potential applications in the food industry. In this study, cross-linked egg white protein isolates (cEPIs) are prepared by adding 10 u/g, 20 u/g, and 40 u/g of transglutaminase (TG), and the impacts of interface properties of cEPIs and emulsifying of HIPEs are investigated. Relative to the native EPI, the cEPIs have more irregular and agglomerated morphology, and the turbidity and hydrophobicity are significantly increased. The particle size and zeta potential of cEPIs considerably varied with the addition of TG. In HIPE, the formation, physical properties, and microstructure are characterized by visual observations, the Turbiscan stability index, and CLSM. The results indicated that stable and gel-like HIPEs are formed by cEPIs at oil internal phase (φ) values of 0.75–0.90. Especially for the enzyme additions of 20 u/g, the cEPIs had the best storage stability and the lowest TSI value (2.50) and formed a gel network structure at φ values of 0.9 microscopically. Overall, this study can enrich the theoretical frame of interface properties by enzyme treatment. Besides, it would be of great importance for the research of HIPE stabilized by cEPIs appropriate to be applied in food formulations.

Interplay of Local Heating, Nanoconfinement, and Tunable Liquid-Wall Interactions Drive Rapid Imbibition and Pronounced Mixing Between Two Immiscible Liquids

Designing novel and energy-efficient strategies for disturbing stable interfaces between two immiscible liquids hold the key for a myriad of applications. In this Letter, we propose a highly effective strategy where localized heating (costing less energy) of an interface between two immiscible liquids confined in a nanochannel enable rapid imbibition and mixing between these two liquids. The exact dynamics (imbibition or mixing) depend on the relative wettability of these two liquids to the nanochannel wall. For the case where one liquid is philic and the other is phobic to the nanochannel wall, local heating makes a particular liquid imbibe into the zone occupied by the other liquid with the philic liquid occupying near-wall locations and the phobic liquid occupying the bulk (far wall) positions. The extent of imbibition is quantified in terms of the interfacial thickness between the two liquids, which is found to be larger than the case where the entire system is heated (costing greater energy). We further show that this interfacial thickness can be enhanced by changing the position (along the nanochannel) of localized heating. Finally, we demonstrate that for the immiscible two liquid systems having identical wetting interactions with the wall, the lack of preference of occupying the near wall location by any of the liquids lead to their enhanced mixing in the presence of the localized heating (that imparts additional energy to the liquids enforcing them to cross over to the side of the other liquid).

Microgels as globular protein model systems

Understanding globular protein adsorption to fluid interfaces, their interfacial assembly, and structural reorganization is not only important in the food industry, but also in medicine and biology. However, due to their intrinsic structural complexity, a unifying description of these phenomena remains elusive. Herein, we propose N-isopropylacrylamide microgels as a promising model system to isolate different aspects of adsorption, dilatational rheology, and interfacial structure at fluid interfaces with a wide range of interfacial tensions, and compare the results with the ones of globular proteins. In particular, the steady-state spontaneously-adsorbed interfacial pressure of microgels correlates closely to that of globular proteins, following the same power-law behavior as a function of the initial surface tension. However, the dilatational rheology of spontaneously-adsorbed microgel layers is dominated by the presence of a loosely packed polymer corona spread at the interface, and it thus exhibits a similar mechanical response as flexible, unstructured proteins, which are significantly weaker than globular ones. Finally, structurally, microgels reveal a similar spreading and flattening upon adsorption as globular proteins do. In conclusion, microgels offer interesting opportunities to act as powerful model systems to unravel the complex behavior of proteins at fluid interfaces.

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.

Vortex fluidic induced mass transfer across immiscible phases

Mixing immiscible liquids typically requires the use of auxiliary substances including phase transfer catalysts, microgels, surfactants, complex polymers and nano-particles and/or micromixers. Centrifugally separated immiscible liquids of different densities in a 45° tilted rotating tube offer scope for avoiding their use. Micron to submicron size topological flow regimes in the thin films induce high inter-phase mass transfer depending on the nature of the two liquids. A hemispherical base tube creates a Coriolis force as a 'spinning top' (ST) topological fluid flow in the less dense liquid which penetrates the denser layer of liquid, delivering liquid from the upper layer through the lower layer to the surface of the tube with the thickness of the layers determined using neutron imaging. Similarly, double helical (DH) topological flow in the less dense liquid, arising from Faraday wave eddy currents twisted by Coriolis forces, impact through the less dense liquid onto the surface of the tube. The lateral dimensions of these topological flows have been determined using 'molecular drilling' impacting on a thin layer of polysulfone on the surface of the tube and self-assembly of nanoparticles at the interface of the two liquids. At high rotation speeds, DH flow also occurs in the denser layer, with a critical rotational speed reached resulting in rapid phase demixing of preformed emulsions of two immiscible liquids. ST flow is perturbed relative to double helical flow by changing the shape of the base of the tube while maintaining high mass transfer between phases as demonstrated by circumventing the need for phase transfer catalysts. The findings presented here have implications for overcoming mass transfer limitations at interfaces of liquids, and provide new methods for extractions and separation science, and avoiding the formation of emulsions.

Engulfing Behavior of Nanoparticles into Thermoresponsive Microgels: A Mesoscopic Simulation Study

The engulfing of nanoparticles into microgels provides a versatile platform to design nano- and microstructured materials with various shape anisotropies and multifunctional properties. Manipulating the spontaneous engulfment process remains elusive. Herein, we report a mesoscopic simulation study on the engulfing behavior of nanoparticles into thermoresponsive microgels. The effects of the multiple parameters, including binding strength, temperature, and nanoparticle size, are examined systematically. Our simulation results disclose three engulfing states at different temperatures, namely full-engulfing, half-engulfing, and surface contact. The engulfing depth is determined by the complementary balance of interfacial elastocapillarity. Specifically, the van der Waals interaction of hybrid microgel-nanoparticle offers the capillary force while the internally networked structure of microgel reinforces the elasticity repulsion. Our study, validated by relevant experimental results, provides a mechanistic understanding of the interfacial elastocapillarity for nanoparticle-microgels.

Sorption Characteristics of Polymer Brushes in Equilibrium with Solvent Vapors

While polymer brushes in contact with liquids have been researched intensively, the characteristics of brushes in equilibrium with vapors have been largely unexplored, despite their relevance for many applications, including sensors and smart adhesives. Here, we use molecular dynamics simulations to show that solvent and polymer density distributions for brushes exposed to vapors are qualitatively different from those of brushes exposed to liquids. Polymer density profiles for vapor-solvated brushes decay more sharply than for liquid-solvated brushes. Moreover, adsorption layers of enhanced solvent density are formed at the brush–vapor interface. Interestingly and despite all of these effects, we find that solvent sorption in the brush is described rather well with a simple mean-field Flory–Huggins model that incorporates an entropic penalty for stretching of the brush polymers, provided that parameters such as the polymer–solvent interaction parameter, grafting density, and relative vapor pressure are varied individually.

Influence of Charges on the Behavior of Polyelectrolyte Microgels Confined to Oil-Water Interfaces

The role of electrostatics on the interfacial properties of polyelectrolyte microgels has been discussed controversially in the literature. It is not yet clear if, or how, Coulomb interactions affect their behavior under interfacial confinement. In this work, we combine compression isotherms, atomic force microscopy imaging, and computer simulations to further investigate the behavior of pH-responsive microgels at oil-water interfaces. At low compression, charged microgels can be compressed more than uncharged microgels. The in-plane effective area of charged microgels is found to be smaller in comparison to uncharged ones. Thus, the compressibility is governed by in-plane interactions of the microgels with the interface. At high compression, however, charged microgels are less compressible than uncharged microgels. Microgel fractions located in the aqueous phase interact earlier for charged than for uncharged microgels because of their different swelling perpendicular to the interface. Therefore, the compressibility at high compression is controlled by out-of-plane interactions. In addition, the size of the investigated microgels plays a pivotal role. The charge-dependent difference in compressibility at low compression is only observed for small but not for large microgels, while the behavior at high compression does not depend on the size. Our results highlight the complex nature of soft polymer microgels as compared to rigid colloidal particles. We clearly demonstrate that electrostatic interactions affect the interfacial properties of polyelectrolyte microgels.

Computational Design of Nanostructured Soft Interfaces: Focus on Shape Changes and Spreading of Cubic Nanogels

Understanding the dynamics of gels at soft interfaces is vital for a range of applications, from biocatalysis and drug delivery to enhanced oil recovery applications. Herein, we use dissipative particle dynamics simulations to focus on the shape changes of a cubic nanogel as it adsorbs from the aqueous phase onto the oil-water interface, effectively acting as a compatibilizer. Upon adsorption at the interface, the hydrogel spreads over the interface, adopting various shapes depending on its size and cross-link density. We characterize these shapes by the shape anisotropy and an effective extent of spreading. We highlight the differences between these characteristics for cubic and spherical nanogels and show that the choice of the cubic shape over the spherical one results in a wider range of topographies that could be dynamically prescribed onto the soft interface due to the gels' adsorption. We first validate our model parameters with respect to the known experimental values for polyacrylamide (PAAm) gels and focus on spreading and shape changes of PAAm nanogels onto the oil-water interfaces. We then probe the behavior of active gels by changing an affinity of the polymer matrix for the solvent, which can be caused by the application of an external stimulus (light, temperature, or change in the chemical composition of solvent). Furthermore, we focus on the interactions between multiple gels placed at the liquid-liquid interface. We show that controlling the shapes and the clustering of the gels at the interfaces via variations in solvent quality result in tailoring the dynamics and topography of soft nanostructured interfaces. Hence, our findings provide insights into the design of soft active nanostructured interfaces with topographies controlled externally via solvent quality.

Scavenging One of the Liquids versus Emulsion Stabilization by Microgels in a Mixture of Two Immiscible Liquids

It is known that microgels can serve as soft, permeable and stimuli-responsive alternative of solid colloidal particles to stabilize oil-water emulsions. The driving force for the adsorption of the microgels on interface of two immiscible liquids is a shielding of unfavorable oil-water contacts by adsorbed subchains, that is, the decrease of the surface tension between the liquids. Such phenomenon usually proceeds if volume fractions of the two liquids are comparable with each other and the microgel concentration is not high enough. The natural question arises: what is going on with the system in the opposite case of strongly asymmetric mixture (one of the liquids (oil) has a very small fraction) or high microgel concentration (the overall volume of the microgels exceeds the volume of the minor oil component)? Here we demonstrate that the microgels uptake the oil whose concentration within the microgels can be orders of magnitude higher than outside, leading to the additional microgel swelling (in comparison with the swelling in water). Thus, the microgels can serve as scavengers and concentrators of liquids dissolved in water. At first glance, this effect seems counterintuitive. However, it has a clear physical reason related to the incompatibility of oil and water. Absorption of the oil by microgels reduces unfavorable oil-water contacts by microgel segments: the microgels have a higher concentration of the segments at the periphery, forming a shell. The microgels with uptaken oil are stable toward aggregation at very small oil concentration in the mixture. However, an increase in the oil concentration can lead to aggregation of the microgels into dimers, trimers, and so on. The increasing concentration of oil mediates the attraction between the microgels: the oil in the aggregates appears to be localized in-between the microgels instead of their interior, which is accompanied by the release of the elastic stress of the microgels. A further increase in the oil concentration results in a growth of the size of the oil droplets between the microgels and the number of the microgels at the droplet's periphery, that is, the emulsion is formed.

Towards the realistic computer model of precipitation polymerization microgels

In this paper we propose a new method of coarse-grained computer simulations of the microgel formation in course of free radical precipitation polymerization. For the first time, we simulate the precipitation polymerization process from a dilute solution of initial components to a final microgel particle with coarse grained molecular dynamics, and compare it to the experimental data. We expect that our simulation studies of PNIPA-like microgels will be able to elucidate the subject of nucleation and growth kinetics and to describe in detail the network topology and structure. Performed computer simulations help to determine the characteristic phases of the growth process and show the necessity of prolongated synthesis for the formation of stable microgel particles. We demonstrate the important role of dangling ends in microgels, which occupy as much as 50% of its molecular mass and have previously unattended influence on the swelling behavior. The verification of the model is made by the comparison of collapse curves and structure factors between simulated and experimental systems, and high quality matching is achieved. This work could help to open new horizons in studies that require the knowledge of detailed and realistic structures of the microgel networks.

Microgels at fluid-fluid interfaces for food and drinks

Various aspects of microgel adsorption at fluid-fluid interfaces of relevance to emulsion and foam stabilization have been reviewed. The emphasis is on the wider non-food literature, with a view to highlighting how this understanding can be applied to food-based systems. The various different types of microgel, their methods of formation and their fundamental behavioral traits at interfaces are covered. The latter includes aspects of microgel deformation and packing at interfaces, their deformability, size, swelling and de-swelling and how this affects their surface activity and stabilizing properties. Experimental and theoretical methods for measuring and modelling their behaviour are surveyed, including interactions between microgels themselves at interfaces but also other surface active species. It is concluded that challenges still remain in translating all the possibilities synthetic microgels offer to microgels based on food-grade materials only, but Nature's rich tool box of biopolymers and biosurfactants suggests that this field will still open up important new avenues of food microstructure development and control.

Membrane Wrapping Pathway of Injectable Hydrogels: From Vertical Capillary Adhesion to Lateral Compressed Wrapping

Membrane wrapping pathway of injectable hydrogels (IHs) plays a vital role in the nanocarrier effectiveness and biomedical safety. Although considerable progress in understanding this complicated process has been made, the mechanism behind this process has remained elusive. Herein, with the help of large-scale dissipative particle dynamics simulations, we explore the molecular mechanism of membrane wrapping by systematically examining the IH architectures and hydrogel-lipid binding strengths. To the best of our knowledge, this is the first report on the membrane wrapping pathway on which IHs transform from vertical capillary adhesion to lateral compressed wrapping. This transformation results from the elastocapillary deformation of networked gels and nanoscale confinement of the bilayer membrane, and it takes long time for the IHs to be fully wrapped owing to the high energy barriers and wrapping-induced shape deformation. Collapsed morphologies and small compressed angles are identified in the IH capsules with a thick shell or strong binding strength to lipids. In addition, the IHs binding intensively to the membrane exhibit special nanoscale mixing and favorable deformability during the wrapping process. Our study provides a detailed mechanistic understanding of the influence of architecture and binding strength on the IH membrane wrapping efficiency. This work may serve as rational guidance for the design and fabrication of IH-based drug carriers and tissue engineering.

Amphiphilic microgels adsorbed at oil-water interfaces as mixers of two immiscible liquids

Amphiphilic microgels adsorbed at an oil-water interface were studied by means of dissipative particle dynamics (DPD) simulations. The hydrophobic (A) and hydrophilic (B) monomer units in the polymer network are considered to be randomly distributed. Effects of the crosslinking density, interfacial tension between the liquids, their selectivity as solvents towards species A and B, and the degree of incompatibility between the A and B units on the internal microgel structure and distribution of the liquids are considered. The most important predictions are that (i) two immiscible liquids can homogeneously be mixed within the microgels and (ii) the adsorbed microgels contain a high fraction of the liquids (they are swollen at the interface). Simultaneous fulfillment of these two conditions can have a high impact on the design of new and efficient catalytic systems. In particular, such microgels can mix immiscible reactants dissolved in water and oil and trigger chemical reactions in the presence of a catalyst embedded into the microgel.

Nanogels and Microgels: From Model Colloids to Applications, Recent Developments, and Future Trends

Nanogels and microgels are soft, deformable, and penetrable objects with an internal gel-like structure that is swollen by the dispersing solvent. Their softness and the potential to respond to external stimuli like temperature, pressure, pH, ionic strength, and different analytes make them interesting as soft model systems in fundamental research as well as for a broad range of applications, in particular in the field of biological applications. Recent tremendous developments in their synthesis open access to systems with complex architectures and compositions allowing for tailoring microgels with specific properties. At the same time state-of-the-art theoretical and simulation approaches offer deeper understanding of the behavior and structure of nano- and microgels under external influences and confinement at interfaces or at high volume fractions. Developments in the experimental analysis of nano- and microgels have become particularly important for structural investigations covering a broad range of length scales relevant to the internal structure, the overall size and shape, and interparticle interactions in concentrated samples. Here we provide an overview of the state-of-the-art, recent developments as well as emerging trends in the field of nano- and microgels. The following aspects build the focus of our discussion: tailoring (multi)functionality through synthesis; the role in biological and biomedical applications; the structure and properties as a model system, e.g., for densely packed arrangements in bulk and at interfaces; as well as the theory and computer simulation.

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

Soft particles display highly versatile properties with respect to hard colloids and even more so at fluid-fluid interfaces. In particular, microgels, consisting of a cross-linked polymer network, are able to deform and flatten upon adsorption at the interface due to the balance between surface tension and internal elasticity. Despite the existence of experimental results, a detailed theoretical understanding of this phenomenon is still lacking due to the absence of appropriate microscopic models. In this work, we propose an advanced modeling of microgels at a flat water/oil interface. The model builds on a realistic description of the internal polymeric architecture and single-particle properties of the microgel and is able to reproduce its experimentally observed shape at the interface. Complementing molecular dynamics simulations with in situ cryo-electron microscopy experiments and atomic force microscopy imaging after Langmuir-Blodgett deposition, we compare the morphology of the microgels for different values of the cross-linking ratios. Our model allows for a systematic microscopic investigation of soft particles at fluid interfaces, which is essential to develop predictive power for the use of microgels in a broad range of applications, including the stabilization of smart emulsions and the versatile patterning of surfaces.

Temperature-Induced Re-Entrant Morphological Transitions in Block-Copolymer Micelles

Using a combination of a mean-field theoretical method and the numerical Scheutjens-Fleer self-consistent field approach, we predict that it is possible to have re-entrant morphological transitions in nanostructures of diblock copolymers upon variation in temperature-mediated solubility of the associating blocks. This peculiar effect is explained by the different rates in variation of the density of the collapsed core domains and the corresponding interfacial energy as a function of the temperature. The theoretical findings are supported by existing experimental observations of reversed sequences of the morphological transitions occurring upon temperature variation in solutions of amphiphilic block copolymers.

Numerical modelling of non-ionic microgels: an overview

Microgels are complex macromolecules. These colloid-sized polymer networks possess internal degrees of freedom and, depending on the polymer(s) they are made of, can acquire a responsiveness to variations of the environment (temperature, pH, salt concentration, etc.). Besides being valuable for many practical applications, microgels are also extremely important to tackle fundamental physics problems. As a result, these last years have seen a rapid development of protocols for the synthesis of microgels, and more and more research has been devoted to the investigation of their bulk properties. However, from a numerical standpoint the picture is more fragmented, as the inherently multi-scale nature of microgels, whose bulk behaviour crucially depends on the microscopic details, cannot be handled at a single level of coarse-graining. Here we present an overview of the methods and models that have been proposed to describe non-ionic microgels at different length-scales, from the atomistic to the single-particle level. We especially focus on monomer-resolved models, as these have the right level of details to capture the most important properties of microgels, responsiveness and softness. We suggest that these microscopic descriptions, if realistic enough, can be employed as starting points to develop the more coarse-grained representations required to investigate the behaviour of bulk suspensions.

Microgel-stabilized liquid crystal emulsions enable an analyte-induced ordering transition

Microgels enable reversible stabilization of liquid crystal (LC) emulsions in ways that facilitate analysis of LC droplets that undergo an analyte-triggered conformational transition.

Adaptive structure of gels and microgels with sliding cross-links: enhanced softness, stretchability and permeability

We propose an experimentally-inspired model of gels and microgels with sliding cross-links, and use this model to study the mechanical and structural properties with molecular dynamics simulations. In the model, the gels and microgels are made of linear polymer chains with threaded rings, which are capable of sliding along the chains, and bulky end-groups keeping the rings threaded (thus mimicking polyrotaxanes); the chains are covalently linked to each other not through the backbones but through the rings. Both gels and microgels are shown to be much softer in the regime of intermediate and large deformations and also much more stretchable than the topologically equivalent chemical counterparts. The physical reason for that is the mobility of the cross-links which leads to the formation of long, longitudinally oriented "subchains" between cross-linked rings upon uniaxial deformation. The microgels are tested for adsorption on a solid flat surface and for interaction with colloidal particles of different sizes. We demonstrate that the sliding microgel is subjected to stronger flattening on the surface than the chemical one. Enforced penetration of solid particles into the sliding microgel without breaking of covalent bonds is predicted even if the size of the particles is comparable to or larger than the mesh size of the chemical microgel and smaller than the size of polyrotaxane. This penetration is accompanied by the disappearance of the cavity: the microgel is characterized by adaptive porosity tunable to the guest-object.

Shell-corona microgels from double interpenetrating networks

Polymer microgels with a dense outer shell offer outstanding features as universal carriers for different guest molecules. In this paper, microgels formed by an interpenetrating network comprised of collapsed and swollen subnetworks are investigated using dissipative particle dynamics (DPD) computer simulations, and it is found that such systems can form classical core-corona structures, shell-corona structures, and core-shell-corona structures, depending on the subchain length and molecular mass of the system. The core-corona structures consisting of a dense core and soft corona are formed at small microgel sizes when the subnetworks are able to effectively separate in space. The most interesting shell-corona structures consist of a soft cavity in a dense shell surrounded with a loose corona, and are found at intermediate gel sizes; the area of their existence depends on the subchain length and the corresponding mesh size. At larger molecular masses the collapsing network forms additional cores inside the soft cavity, leading to the core-shell-corona structure.

Amphiphilic Arborescent Copolymers and Microgels: From Unimolecular Micelles in a Selective Solvent to the Stable Monolayers of Variable Density and Nanostructure at a Liquid Interface

Amphiphilic arborescent block copolymers of two generations (G2 and G3) and polymer microgels, obtained via cross-linking of diblock copolymers, were studied in a selective solvent and at liquid interface via dissipative particle dynamics (DPD) simulations. Depending on the primary structure, single arborescent macromolecules in selective solvent can have both core-corona and multicore structures. Self-assembly of the G2, G3, and microgels in the selective solvent is compared with equivalent linear diblock copolymers. The latter self-assemble into spherical micelles of large enough aggregation number. On the contrary, stability of unimolecular micelles is a feature of the arborescent copolymers and microgels, whereas their ability to aggregate is very low. Adsorption of the single molecules at liquid (oil-water) interface leads to their flattening and segregation of the amphiphilic blocks: hydrophilic and hydrophobic blocks are exposed toward water and oil, respectively. Depending on the character of interactions between monomer units, which can be controlled by temperature or solvent(s) quality, Janus, patchy, and nanosegregated structures can be formed within the macromolecules. Their self-assembly at the interface can lead to the formation of both loose and dense monolayers, which can be homogeneous and nanostructured. The pretty fast adsorption kinetics of G2 macromolecules make them efficient stabilizers of emulsions.

A polymer microgel at a liquid-liquid interface: theory vs. computer simulations

We propose a mean-field theory and dissipative particle dynamics (DPD) simulations of swelling and collapse of a polymer microgel adsorbed at the interface of two immiscible liquids (A and B). The microgel reveals surface activity and lowers A-B interfacial tension. Attempting to occupy as large an interfacial area as possible, the microgel undergoes anisotropic deformation and adopts a flattened shape. Spreading over the interface is restricted by polymer subchain elasticity. The equilibrium shape of the microgel at the interface depends on its size. Small microgels are shown to be more oblate than the larger microgels. Increasing microgel cross-link density results in stronger reduction of the surface tension and weaker flattening. As the degree of immiscibility of A and B liquids increases, the microgel volume changes in a non-monotonous fashion: the microgel contraction at moderate immiscibility of A and B liquids is followed by its swelling at high incompatibility of the liquids. The segregation regime of the liquids within and outside the microgel is different. Being segregated outside the microgel, the liquids can be fully (homogeneously) mixed or weakly segregated within it. The density profiles of the liquids and the polymer were plotted under different conditions. The theoretical and the DPD simulation results are in good agreement. We hope that our findings will be useful for the design of stimuli responsive emulsions, which are stabilized by the microgel particles, as well as for their practical applications, for instance, in biocatalysis.