Pickering emulsions stabilized by inside/out Janus nanotubes: Oil triggers an evolving solid interfacial layer.

HYPOTHESIS: In the field of Pickering emulsion, original inside/ouside Janus clays nanoparticles are investigated for their emulsification properties. Imogolite is a tubular nanomineral of the clay family having both inner and outer hydrophilic surfaces. A Janus version of this nanomineral with an inner surface fully covered by methyl groups can be obtained directly by synthesis (Imo-CH3, hybrid imogolite). The hydrophilic/hydrophobic duality of the Janus Imo-CH3 allows the nanotubes to be dispersed in an aqueous suspension and enables emulsification of non-polar compounds due to the hydrophobic inner cavity of the nanotube. EXPERIMENTS: Through the combination of Small Angle X-ray Scattering (SAXS), interfacial observations and rheology, the stabilization mechanism of imo-CH3 in oil-water emulsions has been investigated. FINDINGS: Here, we show that interfacial stabilization of an oil-in-water emulsion is rapidly obtained at a critical Imo-CH3 concentration as low as 0.6 wt%. Below this concentration threshold, no arrested coalescence is observed, and excess oil is expelled from the emulsion through a cascading coalescence mechanism. The stability of the emulsion above the concentration threshold is reinforced by an evolving interfacial solid layer resulting from the aggregation of Imo-CH3 nanotubes that is triggered by the penetration of confined oil front into the continuous phase.

Correction: Interpenetration of fractal clusters drives elasticity in colloidal gels formed upon flow cessation.

Correction for 'Interpenetration of fractal clusters drives elasticity in colloidal gels formed upon flow cessation' by Noémie Dagès et al., Soft Matter, 2022, 18, 6645-6659, https://doi.org/10.1039/D2SM00481J.

Interpenetration of fractal clusters drives elasticity in colloidal gels formed upon flow cessation.

Colloidal gels are out-of-equilibrium soft solids composed of attractive Brownian particles that form a space-spanning network at low volume fractions. The elastic properties of these systems result from the network microstructure, which is very sensitive to shear history. Here, we take advantage of such sensitivity to tune the viscoelastic properties of a colloidal gel made of carbon black nanoparticles. Starting from a fluidized state at an applied shear rate 0, we use an abrupt flow cessation to trigger a liquid-to-solid transition. We observe that the resulting gel is all the more elastic when the shear rate 0 is low and that the viscoelastic spectra can be mapped on a master curve. Moreover, coupling rheometry to small angle X-ray scattering allows us to show that the gel microstructure is different from gels solely formed by thermal agitation where only two length scales are observed: the dimension of the colloidal and the dimension of the fractal aggregates. Competition between shear and thermal energy leads to gels with three characteristic length scales. Such gels structure in a percolated network of fractal clusters that interpenetrate each other. Experiments on gels prepared with various shear histories reveal that cluster interpenetration increases with decreasing values of the shear rate 0 applied before flow cessation. These observations strongly suggest that cluster interpenetration drives the gel elasticity, which we confirm using a structural model. Our results, which are in stark contrast to previous literature, where gel elasticity was either linked to cluster connectivity or to bending modes, highlight a novel local parameter controlling the macroscopic viscoelastic properties of colloidal gels.

Condensation and growth of amorphous aluminosilicate nanoparticles via an aggregation process.

The precipitation of zeolite nanoparticles involves the initial formation of metastable precursors, such as amorphous entities, that crystallize through non-classical pathways. Here, using reactive force field-based simulations, we reveal how aluminosilicate oligomers grow concomitantly to the decondensation of silicate entities during the initial step of the reaction. Aluminate clusters first form in the solution, thus violating the Loewenstein rule in the first instant of the reaction, which is then followed by their connection with silicate oligomers at the terminal silanol groups before reorganization to finally diffuse within the silicate oligomers to form stable amorphous aluminosilicate nanoparticles that do obey the Loewenstein rule. Our results clearly indicate that aluminate does not serve as the nucleation center for the growth of aluminosilicates in a nucleation-like process but rather proceeds via an aggregation process. The coexistence of aluminosilicate oligomers and small silicate entities induces a phase separation that promotes the precipitation of zeolites with aging.

Time-connectivity superposition and the gel/glass duality of weak colloidal gels.

Colloidal gels result from the aggregation of Brownian particles suspended in a solvent. Gelation is induced by attractive interactions between individual particles that drive the formation of clusters, which in turn aggregate to form a space-spanning structure. We study this process in aluminosilicate colloidal gels through time-resolved structural and mechanical spectroscopy. Using the time-connectivity superposition principle a series of rapidly acquired linear viscoelastic spectra, measured throughout the gelation process by applying an exponential chirp protocol, are rescaled onto a universal master curve that spans over eight orders of magnitude in reduced frequency. This analysis reveals that the underlying relaxation time spectrum of the colloidal gel is symmetric in time with power-law tails characterized by a single exponent that is set at the gel point. The microstructural mechanical network has a dual character; at short length scales and fast times it appears glassy, whereas at longer times and larger scales it is gel-like. These results can be captured by a simple three-parameter constitutive model and demonstrate that the microstructure of a mature colloidal gel bears the residual skeleton of the original sample-spanning network that is created at the gel point. Our conclusions are confirmed by applying the same technique to another well-known colloidal gel system composed of attractive silica nanoparticles. The results illustrate the power of the time-connectivity superposition principle for this class of soft glassy materials and provide a compact description for the dichotomous viscoelastic nature of weak colloidal gels.

Characterization of a Geopolymer Foam by X-ray Tomography.

Metakaolin based geopolymer foams were synthesized at room temperature by direct foaming using hydrogen peroxide (H2O2) as a blowing agent and two types of surfactants such as AER5 and CTAB allowing to tune the connection between two adjacent cells. In the field of decontamination process of liquid wastes, the knowledge of the topology of the generated macroporous network is a primary of interest. Due to the complex structure of porous material, 2D conventional techniques as optical or scanning electron microscopy are often not able to provide all the necessary informations. The 3D networks were therefore characterized by X-ray tomography to determine the morphological structure parameters that is useful to manufacture geopolymer material for filtration applications. The porosity, the pore size distribution and constriction between adjacent cells, as well as the connection rates between pores were analyzed by the iMorph program. The results show that the total porosity increases from 26 to 74% when the initial concentration of H2O2 increases, which is in complete agreement with the tomography results. Materials synthetized from CTAB surfactant are poorly connected whereas those generated from AER5 surfactant have a higher mean cell size (at equivalent initial H2O2 concentration) and are fully connected, which will facilitate the transport of fluid through the material. These features have a strong impact on the value of permeability coefficients of the geopolymer foams. Indeed, permeabilities calculated from a Pore Network Modeling (PNM) approach or Kozeny-Carman equation, are ranged in between 10-14 to 10-10 m2 depending on the cell connectivity, the throat size and the total porosity.

Investigation of the Structure of Concentrated NaOH Aqueous Solutions by Combining Molecular Dynamics and Wide-Angle X-ray Scattering.

Classical molecular dynamics has been performed with explicit polarization on NaOH aqueous solutions from 0.5 mol L-1 up to 9.7 mol L-1. We adapted a force field of OH- for polarizable simulation in order to reproduce the NaOH structural and thermodynamics properties in aqueous solutions. A good agreement between theoretical and experimental results has been found. Wide-angle X-ray scattering (WAXS) intensities issued from molecular dynamics are compared to experimental ones measured on Synchrotron facilities. The structure of the first coordination shell of Na+ has been studied to determine the variation of the oxygen number and hydroxide oxygen around the cation. In addition, Na+-OH- McMillan-Mayer potential issued from molecular dynamics simulations has been calculated and allows for calculating Na+-OH- pair association constant of 0.1 L mol-1, which is in good agreement with the experiments.

Structural Evolution during geopolymerization from an early age to consolidated material.

Time-resolved rheology, small angle X-ray scattering (SAXS), and electron paramagnetic resonance (EPR) techniques were used to study the polymerization of geopolymers. These polymers are inorganically synthesized by the alkaline activation of an aluminosilicate source (metakaolin) in aqueous solution. The influence of the alkali activator (Na(+), K(+), and Cs(+)) was investigated at room temperature. As observed through the variation of the viscoelastic moduli (G', G"), curing proceeds in steps that are well pronounced when NaOH is used. These steps correspond to a specific dissolution/polycondensation mechanism and are smoothed when the size of the alkali cations increases. This size effect also has an impact on the gelation time (maximum of tan δ). Structural analysis through SAXS experiments allows us to characterize these mechanisms on the nanoscale and to show that the growth of the geopolymer is due to the aggregation of oligomers with a size that is even smaller than the cation is chaotropic. Finally, water behavior during geopolymerization was assessed by using a spin probe. The results show that the spin-probe signal progressively disappears during the first moment of the reaction and reappears when the solid polymeric gel is formed, highlighting the role of water molecules in the different chemical reactions during the process. The EPR signal is in fact increasingly masked as the ion size decreases (because of the strength of the hydration shell). At the end of the reaction, some water molecules were released within the pores, restoring the visibility of the isotropic spin-probe signal.

NMR 1D-imaging of water infiltration into mesoporous matrices.

It is shown that coupling nuclear magnetic resonance (NMR) 1D-imaging with the measure of NMR relaxation times and self-diffusion coefficients can be a very powerful approach to investigate fluid infiltration into porous media. Such an experimental design was used to study the very slow seeping of pure water into hydrophobic materials. We consider here three model samples of nuclear waste conditioning matrices which consist in a dispersion of NaNO(3) (highly soluble) and/or BaSO(4) (poorly soluble) salt grains embedded in a bitumen matrix. Beyond studying the moisture progression according to the sample depth, we analyze the water NMR relaxation times and self-diffusion coefficients along its 1D-concentration profile to obtain spatially resolved information on the solution properties and on the porous structure at different scales. It is also shown that, when the relaxation or self-diffusion properties are multimodal, the 1D-profile of each water population is recovered. Three main levels of information were disclosed along the depth-profiles. They concern (i) the water uptake kinetics, (ii) the salinity and the molecular dynamics of the infiltrated solutions and (iii) the microstructure of the water-filled porosities: open networks coexisting with closed pores. All these findings were fully validated and enriched by NMR cryoporometry experiments and by performing environmental scanning electronic microscopy observations. Surprisingly, results clearly show that insoluble salts enhance the water progression and thereby increase the capability of the material to uptake water.