In simulations and experiments, we study the drying of films containing mixtures of large and small colloidal particles in water. During drying, the mixture stratifies into a layer of the larger particles at the bottom with a layer of the smaller particles on top. We developed a model to show that a gradient in osmotic pressure, which develops dynamically during drying, is responsible for the segregation mechanism behind stratification.

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This corrects the article DOI: 10.1103/PhysRevLett.116.118301.

Porous ionic liquids prepared from phosphonium‐based ionic liquids and metal‐organic frameworks (MOFs) are fluid in large ranges of temperature including ambient. It is shown that the ion pairs are too voluminous to enter the pores of the MOF, so the porous liquids remain several months as suspensions with permanent free volume, capable of absorbing large quantities of gases. The increase in gas absorption, when compared with the pure ionic liquids, is proportional to the amount of porous solid in suspension. Structural features of the MOFs and of the ionic liquids are maintained in the suspensions. Thermodynamic analysis and molecular simulations show that the driving force for gas absorption by the porous ionic liquids is energetic as well as structural being controlled by gas‐solid affinity or by the porous liquid free volume. The enthalpy of gas absorption allows easy regeneration of the porous liquid in all cases. The dissolved gases fluidify the porous ionic liquids, different gases having distinct effects on mass transport. The molecular mechanisms that explain the stability of the suspensions and their capacity for gas absorption are identified and point toward easy design rules that will enable numerous applications of these innovative materials as reaction or separation media.

The stabilization of o/w Pickering emulsions in cases of weak adsorption of solid particles at the surface of oil droplets is addressed. Though the adsorption is usually very strong and irreversible when partial wetting conditions are fulfilled, electrostatic repulsions between charged solid particles act against the adsorption. The regime of weak adsorption was reached using charged silica nanoparticles at high pH and low ionic strength. O/w Pickering emulsions of the diisopropyl adipate oil were stabilized by colloidal nanoparticles of Ludox® AS40 consisting of non-aggregated particles of bare silica (hydrophilic). The combination of stability assessment, droplet size and electrokinetic potential measurements at various pH values, adsorption isotherms and cryo-SEM observations of the adsorbed layers disclosed the specificities of the stabilization of Pickering emulsions by adsorption of solid nanoparticles against strong electrostatic repulsions. Not only the long-term stability of emulsions was poor under strong electrostatic repulsions at high pH, but emulsification failed since full dispersion of oil could not be achieved. Emulsion stability was ensured by decreasing electrostatic repulsions by lowering the pH from 9 to 3. Stable emulsions were stabilized by a monolayer of silica particles at 54% coverage of the oil droplet surface at low silica content and an adsorption regime as multilayers was reached at higher concentrations of silica although there was no aggregation of silica in the bulk aqueous phase.

We report the first nitroxide-mediated synthesis of multipod-like silica/polymer latexes by polymerizationinduced self-assembly (PISA) of amphiphilic block copolymers in aqueous emulsion. A water-soluble brush-type PEO-based macroalkoxyamine initiator composed of poly(ethylene oxide) methacrylate and a small amount of styrene (P-[(PEOMA 950 ) 12 -co-S 1 ]-SG1, M n = 11 700 g mol −1 and M w / M n = 1.11) was synthesized and physically adsorbed on the surface of silica particles through hydrogen-bonding interactions. The adsorbed macroalkoxyamine initiator was subsequently employed to initiate the emulsion polymerization of n-butyl methacrylate with a small amount of styrene under mild conditions (85°C). Kinetic analysis indicates that the polymerizations exhibit the same behavior (i.e., the same reaction rates and the same level of control) as those reported in our previous work in the absence of silica under otherwise similar experimental conditions [Qiao et al. Macromolecules 2013, 46, 4285−4295]. This observation is fully consistent with a PISA process taking place at the silica surface. The resulting self-assembled block copolymers formed polymer nodules randomly distributed around the central silica spheres. Varying the macroinitiator concentration or the silica particle size enabled the successful formation of hybrid particles with dumbbell-, daisy-, or raspberry-like morphologies using this new surface-PISA process. ■ INTRODUCTIONColloidal particles with complex shapes such as triangles, pyramids, rods, cubes, nanodisks, star-like, peanuts and other sorts of exotic geometries have attracted considerable attention in the past few years. 1−4 Such complex particles with welldefined compositions and morphologies can find applications in many areas of colloid science and are very promising building blocks for the elaboration of functional advanced materials. 5−7 Among them, multipod-like particles with a controlled number of pods (e.g., dumbbells, dipods, tripods, and beyond) have been the subject of intensive research. 8−15 For instance, colloidal polymer−polymer dumbbells have been produced by controlled phase separation in seeded emulsion polymerization. 9−11 Colloidal polymeric clusters with a precisely defined geometry have been generated by confining latex particles to water-in-oil emulsion droplets and subsequent oil removala process pioneered by Velev et al. 12 in 1996 and further extended to a variety of colloidal systems 13−16 including inorganic particles 15 and binary mixtures of organic and inorganic colloids. 16 Organic/inorganic particles have received increasing interest in the recent literature. 17 Our group has been particularly active in this area, with special attention being focused on the synthesis of polymer/silica biphasic particles. 18−22 Such particles can be obtained by seeded-growth emulsion polymerization using methacryloxymethyltriethoxysilane (MMS) 18,19 or poly(ethylene oxide) methyl ether methacrylate (PEOMA) 20 functionalized silica particles as seeds. Depending on the reacti...

The reversible addition−fragmentation chain transfer (RAFT) polymerization technique was used to synthesize random copolymers of poly(ethylene glycol) methyl ether acrylate) (PEGA) and n-butyl acrylate (BA) and terpolymers of acrylic acid (AA), PEGA and BA with a trithiocarbonate reactive end-group. These macromolecular RAFT agents (macro-RAFTs) were subsequently adsorbed at the surface of size-monodisperse colloidal silica particles with diameters varying between 40 and 450 nm. Adsorption isotherms for both macro-RAFTs could be well fitted to the Langmuir adsorption model, the AA-based macro-RAFT agent showing however a lower maximum adsorption. The adsorbed macro-RAFT agents were subsequently chain extended with a mixture of methyl methacrylate (MMA) and BA by starved feed emulsion polymerization. Cryo-TEM analysis of the resulting hybrid latexes synthesized in the presence of the P(AA-co-PEGAco-BA) terpolymers resulted in multipod-like particles while the P(PEGA-co-BA) copolymers showed the formation of individually and multiencapsulated silica particles depending on the silica particle size. Decreasing the total silica surface area available by decreasing the silica concentration or by increasing the silica particle size resulted in limited coagulation of the latex particles due to a less efficient use of the free nonadsorbing macro-RAFT agent. The feeding process also had a strong impact on particle morphology, and snowman-like particles could be successfully achieved under batch conditions. The use of commercial silica particles instead of homemade silica led to armored latexes illustrating the determinant role of the surface properties of the macro-RAFT-coated inorganic particles in controlling hybrid particle morphology. At last, core−shell particles with a rigid silica core and a soft copolymer shell were obtained for the first time by polymerizing a film-forming monomer mixture showing the high potential of the P(PEGA-co-BA) macro-RAFT agent for the elaboration of polymer-encapsulated silica particles for coating applications.

Use of LAPONITE® clay platelets as Pickering stabilizers of vinylidene chloride-based latexes.