Ethanol Foams Stabilized by Isobutyl-Based POSS-Organosilica Dual-Particle Assemblies.

Nonaqueous foams in low-surface tension solvents (<25 mN·m-1) are highly desired for applications in fire extinguishers and detoxification gels. However, their formation is a Holy Grail of the chemical industry due to the need for stabilizers with low surface energy and high recyclability. Herein, we disclose a new strategy to generate abundant foams in ethanol and a variety of low-surface tension solvents relying on the interfacial coadsorption of two different particles. The particles consist of surface-active fluorinated silica particles, used as a stabilizer, and a novel amphiphilic polyhedral oligomeric silsesquioxane (POSS) decorated with isobutyl cage substituents, used as a frother. The interaction between POSS and fluorinated particles at the ethanol-air interface was thoroughly investigated by combining physicochemical methods (contact angle, dynamic surface tension, and dynamic light scattering methods) and catalytic tests using the model aerobic oxidation reaction of benzyl alcohol. Both particles could be conveniently recycled for at least 5 consecutive runs with high foamability and catalytic activity.

Oil foams stabilized by POSS/organosilica particle assemblies: application for aerobic oxidation of aromatic alcohols.

A novel amphiphilic polyhedral oligomeric silsesquioxane (POSS) with surfactant-like behavior was synthesized. By combining this new POSS, used as a frother, with surface-active catalytic organosilica particles, used as a stabilizer, we designed a dual particle system able to generate foams in pure organic solvents. Tunable foamability and foam stability were achieved in a variety of organic solvents by simply adjusting the POSS concentration. As a result, the catalytic activity was drastically boosted in the aerobic oxidation of pure aromatic alcohols under 1 bar O2 pressure. Particles were conveniently recycled with high foamability and the catalytic efficiency was maintained for at least 7 consecutive runs.

Organic foams stabilized by Biphenyl-bridged organosilica particles.

HYPOTHESIS: Can surface-active particles be designed à la carte just by incorporating functional groups mimicking the structure of the solvent and gas? This is based on the idea that, to achieve good foamability, the particle wettability needs to be finely tuned to adjust the liquid-particle and gas-particle surface tensions. In practice, could particles containing phenyl rings and alkyl chains assemble at the air-liquid interface and stabilize foams based on aromatic solvents? EXPERIMENTS: A library of organosilica particles was prepared by sol-gel synthesis using aromatic organosilane precursors. The particles were characterized by TGA, FTIR and 13C/29Si MAS NMR. The foaming properties were studied after hand shaking and high-speed homogenization. The influence of particle wettability and solvent properties on foam formation was systematically investigated. A comparison was carried out between biphenyl-bridged particles and various stabilizers on foamability in benzyl alcohol. FINDINGS: Biphenyl-bridged particles could stabilize foams in aromatic solvents with a high foam volume fraction up to 96% using Ultra-Turrax. The presence of biphenyl rings and short alkyl chains was crucial for foamability. Organic foams were prepared for aromatic solvents with intermediate surface tension (35-44 mN m-1) and contact angle in the range 32-53°. Biphenyl-bridged particles outperformed polytetrafluoroethylene and fluorinated surfactants in benzyl alcohol.

Pickering Interfacial Catalysis for Aerobic Alcohol Oxidation in Oil Foams.

Oil foams stabilized by surface-active catalytic particles bearing fluorinated chains and Pd nanoparticles allowed fast and efficient aerobic oxidation of a variety of aromatic and aliphatic alcohols compared to bulk catalytic systems at ambient O2 pressure. High foam stability was achieved at low particle concentration (<1 wt %) provided that the contact angle locates in the range 41°-73°. The catalytic performance was strongly affected by the foaming properties, with 7-10 times activity increase in pure O2 compared to nonfoam systems. Intermediate foam stability was required to achieve good catalytic activity, combining large interfacial area and high gas exchange rate. Particles were conveniently recycled with high foamability and catalytic efficiency maintained for at least seven consecutive runs.

Multiphase Microreactors Based on Liquid-Liquid and Gas-Liquid Dispersions Stabilized by Colloidal Catalytic Particles.

Pickering emulsions, foams, bubbles, and marbles are dispersions of two immiscible liquids or of a liquid and a gas stabilized by surface-active colloidal particles. These systems can be used for engineering liquid-liquid-solid and gas-liquid-solid microreactors for multiphase reactions. They constitute original platforms for reengineering multiphase reactors towards a higher degree of sustainability. This Review provides a systematic overview on the recent progress of liquid-liquid and gas-liquid dispersions stabilized by solid particles as microreactors for engineering eco-efficient reactions, with emphasis on biobased reagents. Physicochemical driving parameters, challenges, and strategies to (de)stabilize dispersions for product recovery/catalyst recycling are discussed. Advanced concepts such as cascade and continuous flow reactions, compartmentalization of incompatible reagents, and multiscale computational methods for accelerating particle discovery are also addressed.

Unveiling Cells' Local Environment during Cryopreservation by Correlative In Situ Spatial and Thermal Analyses.

Cryopreservation is the only fully established procedure to extend the lifespan of living cells and tissues, a key to activities spanning from fundamental biology to clinical practice. Despite its prevalence and impact, the central aspects of cryopreservation, such as the cell's physicochemical environment during freezing, remain elusive. Here we address that question by coupling in situ microscopic directional freezing to visualize cells and their surroundings during freezing with the freezing-medium phase diagram. We extract the freezing-medium spatial distribution in cryopreservation, providing a tool to describe the cell vicinity at any point during freezing. We show that two major events define the cells' local environment over time: the interaction with the moving ice front and the interaction with the vitreous moving front, a term we introduce here. Our correlative strategy may be applied to cells relevant to clinical research and practice and may help in the design of new cryoprotective media based on local physicochemical cues.

Hierarchical chirality expression of gemini surfactant aggregates via equilibrium between chiral nucleotide and nonchiral mono-anions.

The assembling behaviors of nonchiral dicationic amphiphilic molecules (gemini) in the presence of the mixture of chiral anionic nucleotides and nonchiral anions are investigated. We demonstrate that subtle balance of various physico-chemical parameters and the competition between chiral and nonchiral anions at the interface of gemini assemblies influences the expression of molecular chirality at the micrometer scale through the hierarchical molecular assembly.

A temperature-controlled stage for laser scanning confocal microscopy and case studies in materials science.

If confocal microscopy is an ubiquitous tool in life science, its applications in chemistry and materials science are still, in comparison, very limited. Of particular interest in these domains is the use of confocal microscopy to investigate temperature-dependent phenomena such as self-assembly, diffusio- or thermophoresis, or crystal growth. Several hurdles must be solved to develop a temperature-controlled stage for laser scanning confocal microscopy, in particular regarding the influence of an elevated temperature gradient close to the microscope objective, which most people try very hard to avoid. Here we report the design of a temperature-controlled stage able to generate stable temperature gradients in both positive and negative temperature range and does not require use of liquid nitrogen. Our setup provides an excellent control of the temperature gradient, which can be coupled with a controlled displacement of the sample, making it useful in particular for a variety of solidification, chemistry, and interfacial problems. We illustrate the benefits of our setup with several case studies of interest in chemistry and materials science: the 3D real-time imaging of ice growth, the segregation of hard particles by growing crystals, the freezing behaviour of single emulsions, the self-shaping of oil droplets upon cooling, and the self-assembly of amphiphile molecules into helical structures. These results show how confocal microscopy coupled with a temperature-controlled stage that provides a controlled temperature gradient is a welcome addition to the toolkit of chemists and materials scientists.

Five-dimensional imaging of freezing emulsions with solute effects.

The interaction of objects with a moving solidification front is a common feature of many industrial and natural processes such as metal processing, the growth of single crystals, the cryopreservation of cells, or the formation of sea ice. Interaction of solidification fronts with objects leads to different outcomes, from total rejection of the objects to their complete engulfment. We imaged the freezing of emulsions in five dimensions (space, time, and solute concentration) with confocal microscopy. We showed that the solute induces long-range interactions that determine the solidification microstructure. The local increase of solute concentration enhances premelting, which controls the engulfment of droplets by the front and the evolution of grain boundaries. Freezing emulsions may be a good analog of many solidification systems where objects interact with a solidification interface.

Synthesis of poly(diallyldimethylammonium) capped copper hexacyanoferrate (CuHCF) nanoparticles: An efficient stabiliser for Pickering emulsions.

Poly(diallyldimethylammonium chloride) (PDDA) capped copper hexacyanoferrate nanoparticles (CuHCF NPs) with the controlled size are prepared following a precipitation method. While PDDA and CuHCF are not active at the cyclohexane/water interface, PDDA capped CuHCF NPs present synergistic property permitting them to stabilise oil-in-water emulsions for weeks. These latter can be considered as promising precursors for the development of new porous materials considering the fact that the NPs present some interests in the field of nuclear decontamination.

Water/ice phase transition: The role of zirconium acetate, a compound with ice-shaping properties.

Few compounds feature ice-shaping properties. Zirconium acetate is one of the very few inorganic compounds reported so far to have ice-shaping properties similar to that of ice-shaping proteins, encountered in many organisms living at low temperature. When a zirconium acetate solution is frozen, oriented and perfectly hexagonal ice crystals can be formed and their growth follows the temperature gradient. To shed light on the water/ice phase transition while freezing zirconium acetate solution, we carried out differential scanning calorimetry measurements. From our results, we estimate how many water molecules do not freeze because of their interaction with Zr cations. We estimate the colligative properties of the Zr acetate on the apparent critical temperature. We further show that the phase transition is unaffected by the nature of the base which is used to adjust the pH. Our results provide thus new hints on the ice-shaping mechanism of zirconium acetate.

Time-Lapse, in Situ Imaging of Ice Crystal Growth Using Confocal Microscopy.

Ice crystals nucleate and grow when a water solution is cooled below its freezing point. The growth velocities and morphologies of the ice crystals depend on many parameters, such as the temperature of ice growth, the melting temperature, and the interactions of solutes with the growing crystals. Three types of morphologies may appear: dendritic, cellular (or fingerlike), or the faceted equilibrium form. Understanding and controlling which type of morphology is formed is essential in several domains, from biology to geophysics and materials science. Obtaining, in situ, three dimensional observations without introducing artifacts due to the experimental technique is nevertheless challenging. Here we show how we can use laser scanning confocal microscopy to follow in real-time the growth of smoothed and faceted ice crystals in zirconium acetate solutions. Both qualitative and quantitative observations can be made. In particular, we can precisely measure the lateral growth velocity of the crystals, a measure otherwise difficult to obtain. Such observations should help us understand the influence of the parameters that control the growth of ice crystals in various systems.

Chiral colloids: homogeneous suspension of individualized SiO2 helical and twisted nanoribbons.

Finely tuned chiral nanometric silica fibers were synthesized based on sol-gel chemistry using organic self-assembly as a template. The optimization of the sol-gel process in acidic conditions allowed us to reduce the transcription time by a factor of 10. These nanohelices were successfully fragmented while preserving the fine internal structures from several micrometers to several hundreds of nanometers in length by a sonication method previously reported for carbon nanotubes. By carefully choosing the nature of the solvent, the sonication power, pH in the case of water, and densification of the silica walls by freeze-drying, the homogeneous and stable colloidal suspensions of individualized chiral nanometric silica ribbons with controlled length were obtained.

Effect of Hofmeister and alkylcarboxylate anionic counterions on the Krafft temperature and melting temperature of cationic gemini surfactants.

The effect of counterions was investigated to probe the principal ionic effects on the solubility in water and melting behavior of cationic gemini surfactants. We focused on two types of counterions: (1) small inorganic counterions that are typically taken from the Hofmeister series were studied to focus on the effect of ion type and (2) n-alkylcarboxylate counterions were studied to focus on the effect of the hydrophobicity of counterions. The Krafft temperature (Tk) and melting temperature (Tm) were obtained by conductivity measurements, calorimetric measurements, and optical microscopy observation. The results clearly indicate that Tk, which represents the solubility of surfactants, is not determined by a single parameter of ions such as the hydration free energy, as is too often assumed, but rather by the combined effects between the hydrophobicity of anions associated with other effects such as the polarizability, dehydrated ion size, and ionic morphology. In parallel, our observation demonstrated that all of the surfactants showed a transition from a crystalline phase to a thermotropic liquid-crystalline phase at around ca. 70 °C, which transformed to an isotropic liquid phase at around ca. 150 °C, and that the transition temperatures depended strongly on the counterion type. The counterion effects on the solubilization and melting behaviors were then compared with micellization properties that have been reported previously. These results provide new insight into understanding the effect of ions on the delicate balance of forces controlling the solution properties and aggregate morphology of charged amphiphilic molecules. Specifically, the solubilization properties of these cationic surfactants with various counterions were determined mainly by the subtle interplay between the hydration of counterions and the dissociation energies (stability of crystallinity) of the ion pair.