Influence of the interfacial tension on the microstructural and mechanical properties of microgels at fluid interfaces

Modulating the conformation of microgels by complexation with inorganic nanoparticles

HYPOTHESIS

The complexation of microgels with rigid nanoparticles is an effective way to impart novel properties and functions to the resulting hybrid particles for applications such as in optics, catalysis, or for the stabilization of foams/emulsions. The nanoparticles affect the conformation of the polymer network, both in bulk aqueous environments and when the microgels are adsorbed at a fluid interface, in a non-trivial manner by modulating the microgel size, stiffness and apparent contact angle.

EXPERIMENTS

Here, we provide a detailed investigation, using light scattering, in-situ atomic force microscopy and nano-indentation experiments, of the interaction between poly(N-isopropylacrylamide) microgels and hydrophobized silica nanoparticles after mixing in aqueous suspension to shed light on the network reorganization upon nanoparticle incorporation.

FINDINGS

The addition of nanoparticles decreases the microgels' bulk swelling and thermal response. When adsorbed at an oil-water interface, a higher ratio of nanoparticles influences the microgel's stiffness as well as their hydrophobic/hydrophilic character by increasing their effective contact angle, consequently modulating the monolayer response upon interfacial compression. Overall, these results provide fundamental understanding on the complex conformation of hybrid microgels in different environments and give inspiration to design new materials where the combination of a soft polymer network and nanoparticles might result in additional functionalities.

The compression of deformed microgels at an air/water interface

The compression of deformed hydrogel microspheres (microgels) at air/water interfaces was investigated using a Langmuir-Blodgett trough with simultaneous in situ visualization of the process using a fluorescent microscope. The relationship between the structure of the microgel arrays and the compression behavior was clarified using microgels with different degrees of crosslinking.

Synthetic and biopolymeric microgels: Review of similarities and difference in behaviour in bulk phases and at interfaces

This review discusses the current knowledge of interfacial and bulk interactions of biopolymeric microgels in relation to the well-established properties of synthetic microgels for applications as viscosity modifiers and Pickering stabilisers. We present a timeline showing the key milestones in designing microgels and their bulk/ interfacial performance. Poly(N-isopropylacrylamide) (pNIPAM) microgels have remained as the protagonist in the synthetic microgel domain whilst proteins or polysaccharides have been primarily used to fabricate biopolymeric microgels. Bulk properties of microgel dispersions are dominated by the volume fraction (ϕ) of the microgel particles, but ϕ is difficult to pinpoint, as addressed by many theoretical models. By evaluating recent experimental studies over the last five years, we find an increasing focus on the analysis of microgel elasticity as a key parameter in modulating their packing at the interfaces, within the provinces of both synthetic and biopolymeric systems. Production methods and physiochemical factors shown to influence microgel swelling in the aqueous phase can have a significant impact on their bulk as well as interfacial performance. Compared to synthetic microgels, biopolymer microgels show a greater tendency for polydispersity and aggregation and do not appear to have a core-corona structure. Comprehensive studies of biopolymeric microgels are still lacking, for example, to accurately determine their inter- and intra- particle interactions, whilst a wider variety of techniques need to be applied in order to allow comparisons to real systems of practical usage.

Exploring the 3D Conformation of Hard-Core Soft-Shell Particles Adsorbed at a Fluid Interface

The encapsulation of a rigid core within a soft polymeric shell allows obtaining composite colloidal particles that retain functional properties, e.g., optical or mechanical. At the same time, it favors their adsorption at fluid interfaces with a tunable interaction potential to realize tailored two-dimensional (2D) materials. Although they have already been employed for 2D assembly, the conformation of single particles, which is essential to define the monolayer properties, has been largely inferred via indirect or ex situ techniques. Here, by means of in situ atomic force microscopy experiments, the authors uncover the interfacial morphology of hard-core soft-shell microgels, integrating the data with numerical simulations to elucidate the role of the core properties, of the shell thicknesses, and that of the grafting density. They identify that the hard core can influence the conformation of the polymer shells. In particular, for the case of small shell thickness, low grafting density, or poor core affinity for water, the core protrudes more into the organic phase, and the authors observe a decrease in-plane stretching of the network at the interface. By rationalizing their general wetting behavior, such composite particles can be designed to exhibit specific inter-particle interactions of importance both for the stabilization of interfaces and for the fabrication of 2D materials with tailored functional properties.

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.

Impact of polyelectrolyte adsorption on the rheology of concentrated poly(N-isopropylacrylamide) microgel suspensions

We explore the impact of three water-soluble polyelectrolytes (PEs) on the flow of concentrated suspensions of poly(N-isopropylacrylamide) (PNIPAm) microgels with thermoresponsive anionic charge density. By progressively adding the PEs to a jammed suspension of swollen microgels, we show that the rheology of the mixtures is remarkably influenced by the sign of the PE charge, PE concentration and hydrophobicity only when the temperature is increased above the microgel volume phase transition temperature T_c, namely when microgels collapse, they are partially hydrophobic and form a volume-spanning colloidal gel. We find that the original gel is strengthened close to the isoelectric point, attained when microgels are mixed with cationic PEs, while PE hydrophobicity rules the gel strengthening at very high PE concentrations. Surprisingly, we find that polyelectrolyte adsorption or partial embedding of PE chains inside the microgel periphery occurs also when anionic polymers of polystyrene sulfonate with a high degree of sulfonation are added. This gives rise to colloidal stabilization and to the melting of the original gel network above T_c. Contrastingly, the presence of polyelectrolytes in suspensions of swollen, jammed microgels results in a weak softening of the original repulsive glass, even when an apparent isoelectric condition is met. Our study puts forward the crucial role of electrostatics in thermosensitive microgels, unveiling an exciting new way to tailor the flow of these soft colloids and highlighting a largely unexplored path to engineer soft colloidal mixtures.

Mechanism of Reduced Glutathione Induced Lysozyme Defolding and Molecular Self-Assembly

The distinctive assembly behaviors of lysozyme (Lys) feature prominently in food, materials, biomedicine, and other fields and have intrigued many scholars. Although our previous work suggested that reduced glutathione (GSH) could induce lysozyme to form interfacial films at the air/water interface, the underlying mechanism is still obscure. In the present study, the effects of GSH on the disulfide bond and protein conformation of lysozyme were investigated by fluorescence spectroscopy, circular dichroism spectroscopy, and infrared spectroscopy. The findings demonstrated that GSH was able to break the disulfide bond in lysozyme molecules through the sulfhydryl/disulfide bond exchange reaction, thereby unraveling the lysozyme. The β-sheet structure of lysozyme expanded significantly, while the contents of α-helix and β-turn decreased. Furthermore, the interfacial tension and morphology analysis supported that the unfolded lysozyme tended to arrange macroscopic interfacial films at the air/water interface. It was found that pH and GSH concentrations had an impact on the aforementioned processes, with higher pH or GSH levels having a positive effect. This paper on the exploration of the mechanism of GSH-induced lysozyme interface assembly and the development of lysozyme-based green coatings has better instructive significance.

Buckling and Interfacial Deformation of Fluorescent Poly(N-isopropylacrylamide) Microgel Capsules

Hollow microgels are fascinating model systems at the crossover between polymer vesicles, emulsions, and colloids as they deform, interpenetrate, and eventually shrink at higher volume fraction or when subjected to an external stress. Here, we introduce a system consisting of microgels with a micrometer-sized cavity enabling a straightforward characterization in situ using fluorescence microscopy techniques. Similarly to elastic capsules, these systems are found to reversibly buckle above a critical osmotic pressure, conversely to smaller hollow microgels, which were previously reported to deswell at high volume fraction. Simulations performed on monomer-resolved in silico hollow microgels confirm the buckling transition and show that the presented microgels can be described with a thin shell model theory. When brought to an interface, these microgels, that we define as microgel capsules, strongly deform and we thus propose to utilize them to locally probe interfacial properties within a theoretical framework adapted from the Johnson-Kendall-Roberts (JKR) theory. Besides their capability to sense their environment and to address fundamental questions on the elasticity and permeability of microgel systems, microgel capsules can be further envisioned as model systems mimicking anisotropic responsive biological systems such as red blood and epithelial cells thanks to the possibility offered by microgels to be synthesized with custom-designed properties.

Probing the Interfacial Properties of Oil-Water Interfaces Decorated with Ionizable, pH Responsive Silica Nanoparticles

Particle-stabilized emulsions (Pickering emulsions) have recently attracted significant attention in scientific studies and for technological applications. The interest stems from the ease of directly assembling the particles at interfaces and modulating the interfacial properties. In this paper, we demonstrate the formation of stable, practical emulsions leveraging the assembly of ionizable, pH responsive silica nanoparticles, surface-functionalized by a mixture of silanes containing amine/ammonium groups, which renders them positively charged. Using pH as the trigger, the assembly and the behavior of the emulsion are controlled by modulating the charges of the functional groups of the nanoparticle and the oil (crude oil). In addition to their tunable charge, the particular combination of silane coupling agents leads to stable particle dispersions, which is critical for practical applications. Atomic force microscopy and interfacial tension (IFT) measurements are used to monitor the assembly, which is controlled by both the electrostatic interactions between the particles and oil and the interparticle interactions, both of which are modulated by pH. Under acidic conditions, when the surfaces of the oil and the nanoparticles (NPs) are positively charged, the NPs are not attracted at the interface and there is no significant reduction in the IFT. In contrast, under basic conditions in which the oil carries a high negative charge and the amine groups on the silica are deprotonated while still positively charged because of the ammonium groups, the NPs assemble at the interface in a closely packed configuration yielding a jammed state with a high dilatational modulus. As a result, two oil droplets do not coalesce even when pushed against each other and the emulsion stability improves significantly. The study provides new insights into the directed assembly of nanoparticles at fluid interfaces relevant to several applications, including environmental remediation, catalysis, drug delivery, food technology, and oil recovery.

Compression of colloidal monolayers at liquid interfaces: in situ vs. ex situ investigation

The assembly of colloidal particles at liquid/liquid or air/liquid interfaces is a versatile procedure to create microstructured monolayers and study their behavior under compression. When combined with soft and deformable particles such as microgels, compression is used to tune not only the interparticle distance but also the underlying microstructure of the monolayer. So far, the great majority of studies on microgel-laden interfaces are conducted ex situ after transfer to solid substrates, for example, via Langmuir-Blodgett deposition. This type of analysis relies on the stringent assumption that the microstructure is conserved during transfer and subsequent drying. In this work, we couple a Langmuir trough to a custom-built small-angle light scattering setup to monitor colloidal monolayers in situ during compression. By comparing the results with ex situ and in situ microscopy measurements, we conclude that Langmuir-Blodgett deposition can alter the structural properties of the colloidal monolayers significantly.

The interfacial and assembly properties of in situ producing silica nanoparticle at oil-water interface

In multiphase materials, structured fluid-fluid interfaces can provide mechanical resistance against destabilization, applicable for conformance control, Pickering emulsion, liquid 3D printing and molding, etc. Currently all research prepare the particle-ladened fluid-fluid interfaces by dispersing ex situ acquired particles to the immiscible interface, which limits their application in the harsh environment, such as oil reservoir which can impair particle stability and transport ability. Here, we investigated the interfacial and assembly properties of the interface where SiO2 nanoparticles (NPs) were in situ produced. The experimental results show that ammonia as catalyst could accelerate the processes of silica NPs formation as well as the interfacial tension (IFT) evolution. High temperature could not accelerate the reaction processes to achieve the lowest equilibrium IFT, but it induced the sine-wave IFT evolution curves regardless of the presence of ammonia. The equilibrium IFTs corresponded to the saturation states of interfaces trapping with SiO2 NPs, while the sine-wave fluctuating patterns of IFT were attributed to the alternating transition between interfacial jammed and unjammed states changing along with the reaction process. Silica NPs diffusing into aqueous phase with high salinity also showed good stability, due to the abundant surface decoration with in situ anchored organic species.

In situ imaging of the three-dimensional shape of soft responsive particles at fluid interfaces by atomic force microscopy

The reconfiguration of individual soft and deformable particles upon adsorption at a fluid interface underpins many aspects of their dynamics and interactions, ultimately regulating the properties of monolayers of relevance for applications. In this work, we demonstrate that atomic force microscopy can be used for the in situ reconstruction of the three-dimensional conformation of model poly(N-isopropylacrylamide) microgels adsorbed at an oil-water interface. We image the particle topography from both sides of the interface to characterize its in-plane deformation and to visualize the occurrence of asymmetric swelling in the two fluids. In addition, the technique enables investigating different fluid phases and particle architectures, as well as studying the effect of temperature variations on particle conformation in situ. We envisage that these results open up an exciting range of possibilities to provide microscopic insights into the single-particle behavior of soft objects at fluid interfaces and into the resulting macroscopic material properties.

Raspberry-Shaped Microgels Assembled at the Oil-Water Interface by Heterocoagulation of Complementary Microgels

Raspberry-shaped particles have attracted increasing interest due to their tunable surface morphologies and physicochemical properties. A variety of covalent and noncovalent strategies have been developed for the fabrication of raspberry-shaped particles. However, most of these strategies are complex or require precise control of solution conditions. In this work, we develop a direct approach for the fabrication of noncovalent raspberry-shaped microgels. Our strategy works through the electrostatically driven heterocoagulation of binary microgels with complementary functional groups at the oil-water interface. By introducing hexanoic acid (HA) into the oil phase, stable inverse water-in-oil (w/o) Pickering emulsions could be stabilized solely by HA-swollen microgels or self-assembled raspberry-shaped microgels. Furthermore, the formation mechanism and the interfacial properties of interfaces laden with raspberry-shaped microgels were investigated. The results indicate that HA can effectively improve the hydrophobicity and interfacial activity of microgels. In addition, raspberry-shaped microgels achieve high coverage on the droplet surface, resulting in the elastic interface and excellent stability of emulsions. We envision that these results will not only fill a knowledge gap in the field of soft matter interfacial self-assembly, but also will shed light on the rational design of raspberry-shaped soft colloids and the on-demand control of interfacial rheology. In addition, we expect that our results will contribute to wider applications of microgel-stabilized emulsions, including cascade catalysis, microreactor, and in vivo drug delivery.

In-situ study of the impact of temperature and architecture on the interfacial structure of microgels

The structural characterization of microgels at interfaces is fundamental to understand both their 2D phase behavior and their role as stabilizers that enable emulsions to be broken on demand. However, this characterization is usually limited by available experimental techniques, which do not allow a direct investigation at interfaces. To overcome this difficulty, here we employ neutron reflectometry, which allows us to probe the structure and responsiveness of the microgels in-situ at the air-water interface. We investigate two types of microgels with different cross-link density, thus having different softness and deformability, both below and above their volume phase transition temperature, by combining experiments with computer simulations of in silico synthesized microgels. We find that temperature only affects the portion of microgels in water, while the strongest effect of the microgels softness is observed in their ability to protrude into the air. In particular, standard microgels have an apparent contact angle of few degrees, while ultra-low cross-linked microgels form a flat polymeric layer with zero contact angle. Altogether, this study provides an in-depth microscopic description of how different microgel architectures affect their arrangements at interfaces, and will be the foundation for a better understanding of their phase behavior and assembly.

How Softness Matters in Soft Nanogels and Nanogel Assemblies

Softness plays a key role in determining the macroscopic properties of colloidal systems, from synthetic nanogels to biological macromolecules, from viruses to star polymers. However, we are missing a way to quantify what the term "softness" means in nanoscience. Having quantitative parameters is fundamental to compare different systems and understand what the consequences of softness on the macroscopic properties are. Here, we propose different quantities that can be measured using scattering methods and microscopy experiments. On the basis of these quantities, we review the recent literature on micro- and nanogels, i.e. cross-linked polymer networks swollen in water, a widely used model system for soft colloids. Applying our criteria, we address the question what makes a nanomaterial soft? We discuss and introduce general criteria to quantify the different definitions of softness for an individual compressible colloid. This is done in terms of the energetic cost associated with the deformation and the capability of the colloid to isotropically deswell. Then, concentrated solutions of soft colloids are considered. New definitions of softness and new parameters, which depend on the particle-to-particle interactions, are introduced in terms of faceting and interpenetration. The influence of the different synthetic routes on the softness of nanogels is discussed. Concentrated solutions of nanogels are considered and we review the recent results in the literature concerning the phase behavior and flow properties of nanogels both in three and two dimensions, in the light of the different parameters we defined. The aim of this review is to look at the results on micro- and nanogels in a more quantitative way that allow us to explain the reported properties in terms of differences in colloidal softness. Furthermore, this review can give researchers dealing with soft colloids quantitative methods to define unambiguously which softness matters in their compound.

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.

A broad perspective to particle-laden fluid interfaces systems: from chemically homogeneous particles to active colloids

Particles adsorbed to fluid interfaces are ubiquitous in industry, nature or life. The wide range of properties arising from the assembly of particles at fluid interface has stimulated an intense research activity on shed light to the most fundamental physico-chemical aspects of these systems. These include the mechanisms driving the equilibration of the interfacial layers, trapping energy, specific inter-particle interactions and the response of the particle-laden interface to mechanical perturbations and flows. The understanding of the physico-chemistry of particle-laden interfaces becomes essential for taking advantage of the particle capacity to stabilize interfaces for the preparation of different dispersed systems (emulsions, foams or colloidosomes) and the fabrication of new reconfigurable interface-dominated devices. This review presents a detailed overview of the physico-chemical aspects that determine the behavior of particles trapped at fluid interfaces. This has been combined with some examples of real and potential applications of these systems in technological and industrial fields. It is expected that this information can provide a general perspective of the topic that can be exploited for researchers and technologist non-specialized in the study of particle-laden interfaces, or for experienced researcher seeking new questions to solve.

Microgels react to force: mechanical properties, syntheses, and force-activated functions

Microgels are colloidal polymer networks with high molar mass and properties between rigid particles, flexible macromolecules, and micellar aggregates. Their unique stimuli-responsiveness in conjunction with their colloidal phase behavior render them useful for many applications ranging from engineering to biomedicine. In many scenarios either the microgel's mechanical properties or its interactions with mechanical force play an important role. Here, we firstly explain microgel mechanical properties and how these are measured by atomic force microscopy (AFM), then we equip the reader with the synthetic background to understand how specific architectures and chemical functionalities enable these mechanical properties, and eventually we elucidate how the interaction of force with microgels can lead to the activation of latent functionality. Since the interaction of microgels with force is a multiscale and multidisciplinary subject, we introduce and interconnect the different research areas that contribute to the understanding of this emerging field in this Tutorial Review.

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.