The generation of foams using particle-like objects has gained momentum in the past years as such Pickering foams are relevant in many industrial applications. In this context, we investigate the foamability of microgels, which are soft polymeric particles which architecture can be finely tailored during synthesis. Herein, foams were produced by continuously bubbling air into dispersions of poly(N-isopropylacrylamide) (PNIPAM) microgels. The foaming ability of the microgel dispersions has been assessed by combining observations at both the macroscopic and the local scales. Increased microgel concentration and more pronounced core-shell structure lead to a higher foamability, smaller bubbles, and wetter foams. In contrast, microgel size variation has a small effect on the foam properties. These results are correlated to the adsorption kinetics assessed by pendant drop tensiometry. Indeed, faster adsorption kinetics are expected to promote larger surface coverage during bubble formation, which increases its stability against coalescence and the subsequent ability of the microgel dispersion to generate large volumes of foam. Such a hypothesis is confirmed by salt addition, which accelerates microgel adsorption and thus enhances foamability.

Dispersed systems are advantageous to perform heterogeneous reactions, because they present large interfaces favoring reactants transfer. We develop foams stabilized by poly(N-isopropylacrylamide)-based microgels and apply them to leach metal copper, abiding by the rules of green chemistry. Indeed, the microgels are readily recovered by centrifugation and are used multiple times, while oxygen in air bubbles replaces hazardous oxidizing agents. A forced drainage set-up is implemented, in which reinjection of the continuous phase contributes to reducing water consumption. This configuration also enables the continuous leaching of a copper plate in foams with controlled structure as well as straightforward monitoring of the reaction through regular liquid sampling. Thus, copper leaching kinetics are assessed for different foam liquid fractions and bubble sizes, which are demonstrated to be key parameters. Foams with lower liquid fractions enhance kinetics; yet achievable liquid fractions are constrained by foam stability requirements. Finally, microgels are functionalized with carboxylate groups, enabling on-demand capture and release of copper ions depending on the pH of the medium. The development of such a versatile and environmentally-friendly reactive system paves the way toward future applications of reactive Pickering foams that address both economic and ecological challenges in greener industrial processes.

We report the use of polyion complex micelles (PICM) obtained by complexation of poly(sodium acrylate) and poly(N-methyl-2-vinylpyridinium iodide-block-ethylene oxide) as novel stabilizers for water-in-water emulsions consisting in poly(ethylene oxide) (PEO) and dextran mixtures in water. PICM concentrations as low as 1 g/L lead to a strong increase of stability for both PEO-in-dextran and dextran-in-PEO emulsions, as they did not display any macroscopic phase separation after several months. Such long term stability is to our knowledge unprecedented in the literature dedicated to water-in-water emulsions. Moreover, increasing the concentration of PICM further induces a decrease in the droplet size, resulting in a strong reduction of the cream-ing or sedimentation rates, and suppression of the coalescence after a few days. The dissociation of PICM at high salt concentration was taken as an opportunity to trigger the destabilization of the water-in-water emulsions by addition of a concentrated salt solution. PEO-in-dextran emulsions displayed more sensitivity to PICM and salt concentrations than dextran-in-PEO emulsions, in terms of both stability and droplet size, indicating that the underlying stabilization mechanisms are different for the two systems.

We propose that cationic polyelectrolyte molecules deposited on an aqueous solution containing AuCl4 ions can serve as effective templates for the controlled synthesis of gold nanoparticles by the technique of X-ray surface radiolysis. This approach offers the advantage of enabling the in-situ synthesis of metallic nanostructures whose morphologies are determined by the density of the polyelectrolyte layer, and limit the number of chemical byproducts. A diblock copolymer, polystyrene-b-poly(2-(dimethylamino)ethyl methacrylate) (PS-b-PDMAEMA), is spread at the air/aqueous subphase interface using the Langmuir method. The effect of AuCl4-ions on the PS-b-PDMAEMA layer thickness is determined and compared to one obtained on the pure water subphase. Neutron reflectometry revealed that in the presence of AuCl4-ions, a collapse of the PS-b-PDMAEMA chains occurs. The layer thickness is strongly reduced by a factor 4-5 from that of the PS-b-PDMAEMA brush on pure water. The reduction of gold ions is then induced in the vicinity of the interface by X-ray surface radiolysis method. The formation of metallic nanoparticles is monitored in-situ and characterized using grazing incidence X-ray diffraction (GIXD) and total reflection X-ray fluorescence (TRXF). The controlled reduction process by X-rays yielded to the formation of two distinct gold nanostructure morphologies simultaneously: (1) surface-oriented 2D hexagonal nanoplatelets grown directly from the polymer layer, exhibiting crystalline organization confirmed by distinct GIXD patterns with micrometers in plane size and tens of nanometers of thickness as observed by AFM, and (2) spherical nano-colloids of few hundred of nanometers diameters formed in the vicinity of the interface, as demonstrated by GIXD patterns. The work indicates that the approach has potential to form hierarchical material assemblies and has potential for applications in surface-enhanced spectroscopy or heterogeneous catalysis.

Although understanding structure-property relationships in microgels is crucial, it remains limited due to the challenges associated with both controlling and quantitatively characterizing the distribution of crosslinkers within the network. Here, multi-responsive supramolecular poly(N-isopropylacrylamide) (PNIPAM) microgels have been synthesized with either "ultra" core-shell (via batch synthesis) or quasi-homogeneous (via continuous feeding) distributions of a metallosupramolecular charged crosslinker (SC), while maintaining a constant size. The SC presents two main advantages. First, its color and its high electronic contrast allow an easy and precise quantification of both its final content and its spatial distribution in the microgel network respectively. It is found that microgels with a quasi-homogeneous SC distribution exhibit a lower swelling ratios upon decreasing the temperature and/or the salt concentration. Second, the triggered SC cleavage through chemical oxidation allows the microgel disassembly. Both the SC cleavage kinetics and the resulting microgel disassembly are faster when the initial SC distribution is quasi-homogeneous. Furthermore, the molar mass of the disassembled polymer chains can be closely correlated to the initial microgel structure. Overall, this work highlights that the microgel structure is a key parameter in understanding their behavior and optimizing their applications.

Oil-in-water Pickering emulsions can be stabilized by poly-N-Isopropylacrylamide based microgels that adsorb, deform and entangle at the droplets interface. The surface coverage Γemulsion, defined as the mass of microgels per unit interfacial area, likely plays a key role in the emulsion properties as stability and responsiveness. The objective of the present study is to link Γemulsion to the concentration of microgels used during the emulsification process. Γemulsion was monitored by combining droplet size analysis, UV-visible quantification of non-adsorbed microgels and Cryo-SEM visualization of the droplets interface, for a microgel concentration range over almost two decades. We demonstrate the existence of three regimes. At low microgel concentration, in the particle-poor regime, the well-known "Limited Coalescence" process takes place. All the microgels adsorb, the droplet size distribution is narrow and the mean droplet size is inversely proportional to the microgel concentration: a constant minimum value of Γemulsion characterizes this domain. For higher microgel concentrations, microgels partition between the interface and the bulk continuous phase. In this intermediate "Excess" regime, both adsorbed and non-adsorbed microgel increase, proving that microgels compress at the interface to maximize their adsorption. At higher microgel concentrations, a third regime named "Saturation" regime is observed for the first time. Γemulsion then reaches a constant high plateau value while the drop size distribution becomes polydisperse, showing that the fragmentation process dominates over the coalescence. These findings should open new perspectives to better tailor emulsion properties using deformable particles as stabilizers.

Self-assembly of polymers at liquid interfaces using noncovalent interactions has emerged as a promising technique to reversibly produce self-healing membranes. Besides the assembly process, it is also crucial to control the mechanical properties of these membranes. Here, we measure the interfacial rheological properties of PMAA-PPO (poly(methacrylic acid)-poly(propylene oxide)) polymer membranes assembled using hydrogen bonds at the interface between water and a polar oil, Miglyol. Varying the pH enables us to modify the degree of ionization of the PMAA chains and hence their ability to establish hydrogen interactions with PPO. Frequency sweeps of the interfacial layers show a crossover between a viscous regime at low frequencies and an elastic regime at high frequencies. The crossover elastic modulus, measured one hour after the two phases were put into contact, decreases by a half over the pH range investigated, which can be accounted for by a decrease of the layer thickness as pH increases. Furthermore, we find that the crossover frequency varies exponentially with the degree of ionization of PMAA. To account for these observations, we propose a simple picture where the short PPO chains behave as noncovalent cross-linkers that bridge several PMAA chains. The dissociation rate and hence the crossover frequency are controlled by the number of PO units per PPO chain involved in the hydrogen bonds.

Sodium alginate and calcium carbonate are natural materials with a high compositional variability. The molar mass and structural composition of sodium alginate and the crystalline structure of calcium carbonate can affect their interactions in pure water. Herein, we studied the adsorption of sodium alginate onto calcium carbonate microparticles in pure water. The adsorption appears to be driven by electrostatic interactions stabilized by calcium ions at the surface of the particles and in solution due to the dissolution of calcium carbonate in pure water. We found that the adsorption of sodium alginate is favored onto microparticles with more calcite. We also found that the adsorption of sodium alginate polymers with low guluronate content (∼ 30%) onto calcium carbonate particles tends to increase for polymers with lower average molar mass and more flexible polymer chains. Polymers with high guluronate content (∼ 65%), exhibiting a more rigid conformation, tend to adsorb more onto calcite when they present a lower average molar mass polymers as well as a higher proportion of guluronate monomers.

Stimuli-responsive microgels are of great interest not only in fundamental research but also in a broad range of applications. In this study, we synthesize multiresponsive supramolecular poly(N-isopropylacrylamide) (PNIPAM) microgels using a homemade metallo-supramolecular cross-linker (SC) instead of the commonly used N,N'-methylenebis(acrylamide) (BIS). Originally, the supramolecular cross-linker is based on a coordination complex, which endows the microgels with salt responsiveness and degradability, in addition to their thermoresponsive properties as expected from PNIPAM-based materials. Afterward, the use of sodium dodecyl sulfate (SDS) in the conventional dispersion polymerization synthesis not only allows a microgel size tuning but also originally permits the successful incorporation of hydrophilic positively charged cross-linkers. By combining scanning electron microscopy (SEM) observations and light scattering measurements, we can evidence the core-shell-like structure of these new supramolecular microgels and its evolution with the cross-linker content. Finally, such well-controlled multiresponsive microgels could be relevant to stabilize smart emulsions or to perform drug delivery.

The size of acrylamide-based microgels can be decreased by addition of ionic surfactants during the classical dispersion polymerization. Nevertheless, the mechanism of such syntheses is not well understood yet. Here, a "Limited Aggregation Model" is proposed by analogy with the limited coalescence mechanism occurring for Pickering emulsion stabilization. In such a model, nuclei aggregate until a constant, high enough surfactant surface coverage is reached, which ensures colloidal stability. Consequently, the total surface of the growing particles, linked to the inverse of their size, is linearly dependent on the surfactant concentration. This law is verified if the surfactant/polymer particle interaction is high enough to guarantee a "total adsorption" of the surfactants onto the particles. This simple model fits very well with all the data extracted from the literature, including very different synthesis conditions. Finally, it not only permits to predict the microgels size, but it is also an interesting tool to investigate the role of each synthesis parameter like initiator, solvent or polymer. For instance, it shows that the surfactant role is not linked to its charge, proving that a phenomenon complementary to the electrostatic repulsion, related to the surfactant tail, ensures the colloidal stability of the growing collapsed microgels. Inspired by the phenomenon of limited coalescence observed in emulsion stabilization, this work proposes a "Limited Aggregation Model" to predict the size of acrylamide-based microgels synthesized in the presence of ionic surfactants.image

Polymer association at liquid-liquid interfaces is a promising way to spontaneously obtain soft self-healing membranes. In the case of reversible bonding between two polymers, the macromolecules are mobile everywhere within the membrane and they can be absorbed into it at both boundaries due to binding to macromolecules of the other type. In this work, we develop the theoretical model of membrane growth based on these assumptions. The asymptotic dependence of membrane thickness on time as h ∼ t1/2, as typically observed experimentally in a stationary regime, reveals an interdiffusion-controlled process, where the polymer fluxes sustain the polymer absorption. The membrane growth rate is mainly determined by the difference in equilibrium compositions at the boundaries, the association constant, the polymer lengths and mobilities. This model is further used to describe the growth of hydrogel membranes formed via H-bonding of polymers at the interface between a solution of poly(propylene oxide) (PPO) in isopropyl myristate and an aqueous solution of poly(methacrylic acid) (PMAA). The film thickness is measured by reflectometric methods. The growth rate slows down about 25 times for 500-nm-thick films at pH = 5.1 compared to the case of pH = 3. The ionization degree of PMAA solutions is studied by potentiometric methods. Even a small change in ionization is found to influence noticeably the growth rate of the film. In the diffusion-controlled regime, the slowdown can be explained by a drop in the composition gradient in the membrane, whereas the process becomes non-stationary if the absorption of PMAA is hindered by an interfacial electrostatic barrier.

Hypothesis: Aqueous foams are expected to constitute exquisite particularly suitable reactive medium for the oxidation of metals, since the reactant H' can be supplied through the continuous liquid phase, while the reactant O2 can be transported through the gas bubbles.Experiments: To test this hypothesis, we investigated the oxidation of a metallic copper cylinder immersed in an aqueous foam. To study the relation between the transport of these reactants and the kinetics of the chemical reaction we use a forced drainage setup which enables us to control both the advection velocity of the H' ions through the foam and the foam liquid fraction.Findings: We find experimentally that the mass of dissolved copper presents a maximum with the drai-nage flow rate, and thus with the foam liquid fraction. Modeling analytically the transfer of H' and O2 through the foams enables us to show that this non-monotonic behavior results from a competition between the advective flux of H+ ions and the unsteady diffusion of O2 through the thin liquid films which tends to be slower as the area of the thin liquid films decreases with the drainage flow rate and the liquid fraction. This study shows for the first time how to optimize the foam structure and drainage flow in reac-tive foams in which the reactants are present both in the liquid and gaseous phases.(c) 2023 Elsevier Inc. All rights reserved.

The specific design of a water-soluble supramolecular cross-linker based on a terpyridine-iron(II) bis-complex is reported. Copolymerization of this cross-linker with acrylamide monomers in water allows a novel one-step synthesis of metallo-supramolecular hydrogels. The synthesized hydrogels were characterized by rheology, dynamic light scattering, and H-1 double-quantum nuclear magnetic resonance experiments. They reveal great similarities with the rheological behavior of a chemically crosslinked acrylamide network but differences in the structure at low length scales. Characterization also shows that the supramolecular cross-linker behaves similarly to a permanent bond at the observed time scales (from 10(-6) to almost 1000 s), thanks to its relatively high binding energy. However, unlike their chemical counterparts, supramolecular gels show polyelectrolyte swelling behavior and stimulus responsiveness when put in contact with an oxidant. A controlled tuning of the physical-chemical properties of the final gel, ranging from the initial supramolecular gel properties to those of a polymer solution, is then achievable.

Frozen complex coacervate core micelles (C3Ms) were developed as a class of particle stabilizers for Pickering emulsions. The C3Ms are composed of a core of electrostatically interacting weak polyelectrolytes, poly(acrylic acid) (pAA) and poly(dimethylaminopropylacrylamide) (pDMAPAA), surrounded by a corona of water-soluble and surface active poly(N-isopropylacrylamide) (pNiPAM). Mixing parameters of the two polymer solutions, including pH, mixing method, charge ratio, and salinity of the medium, were carefully controlled, leading to monodisperse, colloidally stable C3Ms. A combination of dynamic light scattering and proton nuclear magnetic resonance experiments showed that the C3Ms gradually disassembled from a dynamically frozen core state in pure water into free polyelectrolyte chains above 0.8 M NaCl. Upon formulation of dodecane-in-water emulsions, the frozen C3Ms adsorb as particles at the droplet interfaces in striking contrast with most of the conventional micelles made of amphiphilic block copolymers which fall apart at the interface. Eventually, increasing the salt concentration of the system triggered disassembly of the C3Ms, which led to emulsion destabilization.

The design of biocompatible multiple emulsions is an important challenge in the field of controlled delivery systems for protecting and delivering compounds encapsulated and protected in the innermost phase. In this paper, we use biocompatible water – Miglyol®812 water-in-oil-in-water (W/O/W) emulsions stabilized by a stimuli-responsive diblock copolymer consisting of poly(dimethylsiloxane) (PDMS) and poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) to design an easy-to-process new delivery W/O/W system. Such emulsions are formed in a single emulsification step. They present a high encapsulation yield and are shown to be stable over months. As such, the encapsulation of a hydrophilic dye (Alexa fluor) in the innermost water phase is successfully demonstrated over months. These emulsions are stimulable either by a shift in pH level or in ionic strength. The former destabilizes the multiple emulsion and leads to a simple one while the latter partly maintains the multiple character. Eventually both stimulations are effective in the dye release and molecular mechanisms are proposed for explaining the observed two-stage kinetics of release.

3D printing of hydrogels usually relies on a combination of fine‐tuned material chemistry and polymer chain architecture to obtain an ink with adequate yield‐stress flow, shear‐thinning, and self‐healing behavior. Recent approaches in hydrogel ink design include introduction of reversible covalent or supramolecular bonds or jamming of microparticles into a granular hydrogel. However, the dimensional stability of such systems is typically afforded by a post‐printing covalent cross‐linking step that impedes further on‐demand degradation of the scaffolds. Here, a jammed micro‐gels ink made of thermosensitive poly( N ‐isopropylacrylamide) micro‐gels incorporating terpyridine ligand linkers are proposed as a 3D printable ink that is cured post‐printing by iron (II) cations for long‐term stabilization. The uncross‐linked micro‐gels ink exhibits the rheological characteristics of granular materials, meaning yield‐stress, shear‐thinning and fast recovery. Upon curing, iron (II)‐bis‐terpyridine coordination complexes between neighboring micro‐gels are formed. The supramolecular bonds are sufficiently strong and long‐lived to maintain scaffold integrity during manual handling or immersion in liquid medium for over two months. The thermosensitivity of the micro‐gels endows the printed construct with reversible and cyclable temperature‐induced resolution enhancement, while the supramolecular cross‐linking provides an asset of disintegration on‐demand. The proposed micro‐gel scaffolds are biocompatible, revealing the potential for biomedical applications and 4D bioprinting.

Multiple w/o/w emulsions (MEs) are promising systems for protecting fragile hydrophilic drugs and controlling their release. We explore the capacity of a single pH-sensitive copolymer, PDMS60-b-PDMAEMA(50), and salts, to form and stabilize MEs loaded with sucrose or catechin by a one-step mechanical process or a microfluidic method. ME cytotoxicity was evaluated in various conditions of pH. Using the mechanical process, the most stable emulsions were obtained with Miglyol (R) 812 N and isopropyl myristate in a final pH range of 8-12 and [0.3 M-1 M] NaCl concentrations. Conversely, with the microfluidic method, isopropyl myristate at pH 3 without salt was more efficient. Catechin strongly affected the formation of droplets by the mechanical process but did not modify the conditions of stability of MEs obtained by the microfluidic method. The antioxidant power of catechin was preserved in the inner droplets, even in emulsions prepared by the mechanical method at pH 8. An incomplete release of sucrose and catechin from the emulsions was observed and attributed to the interaction of molecules with the copolymer through hydrogen bonding. This study highlights some of the barriers to break to formulate multiple emulsions stabilized by a PDMS-b-PDMAEMA copolymer or other polymers which can form hydrogen bonds interaction with encapsulated drugs.

The recovery of metals from WEEE, Waste from Electrical and Electronic Equipment, is a major challenge to preserve natural resources. Hydrometallurgy, which consists in leaching metals is a promising method but generates large amounts of effluents which need to be collected and concentrated. In this study we design aqueous leaching foams, composed of 90% v/v of gas and 10% v/v of HCl solution to oxidize and dissolve copper. We take advantage of the oxidizing power of the dioxygen (O2) present in the air bubbles whose fast transfer through the 2 foams enables an efficient oxidation of copper. We then extend the concept of leaching foams to another gas, ozone, to oxidize silver (Ag). We finally show that using an anionic surfactant to complex cupric ions helps improving the dissolution of the metal. These promising results open new recycling routes for metals contained in WEEE, with a lower environmental footprint.

Complexation of polymers at liquid interfaces is an emerging technique to produce all-liquid printable and self-healing devices and membranes. It is crucial to control the assembly process, but the mechanisms at play remain unclear. Using two different reflectometric methods, we investigate the spontaneous growth of H-bonded PPO-PMAA (polypropylene oxide-polymetacrylic acid) membranes at a flat liquid-liquid interface. We find that the membrane thickness h grows with time t as h ∼ t1/2, which is reminiscent of a diffusion-limited process. However, counterintuitively, we observe that this process is faster as the PPO molar mass increases. We are able to rationalize these results with a model which considers the diffusion of the PPO chains within the growing membrane. The architecture of the latter is described as a gel-like porous network, with a pore size much smaller than the radius of the diffusing PPO chains, thus inducing entropic barriers that hinder the diffusion process. From the comparison between the experimental data and the result of the model, we extract some key piece of information about the microscopic structure of the membrane. This study opens the route toward the rational design of self-assembled membranes and capsules with optimal properties.

The recovery of metals from WEEE, Waste from Electrical and Electronic Equipment, is a major challenge to preserve natural resources.Hydrometallurgy, which consists in leaching metals is a promising method but generates large amounts of polluting effluents.In this study we design aqueous leaching foams, composed of 90% v/v of gas and 10% v/v of HCl solution to oxidize and dissolve copper.We take advantage of the oxidizing power of the dioxygen (O2) present in the air bubbles whose fast transfer through the foams enables an efficient oxidation of copper.We then extend the concept of leaching foams to another gas, ozone, to oxidize silver (Ag).We finally show that using an anionic surfactant to complex cupric ions helps improving the dissolution of the metal.These promising results open new recycling routes for metals contained in WEEE, with a lower environmental footprint.