Incompatible Liquids in Confined Conditions

In applications involving organic vapour, the performance of high surface area carbons is often challenged by water vapour in the atmosphere. Small angle neutron scattering (SANS), through its ability to distinguish between different components by means of contrast variation, is ideally suited to investigating the behaviour of adsorbed layers in simultaneous contact with a mixed vapour phase. Even at high relative humidity (RH), water alone forms a discontinuous film composed of clusters on the surface of the oxidized microporous carbon used for these studies. When toluene is also present, all the available carbon surface is wetted. Toluene and water adsorb as a single phase already at RH 11.5%, and the concentration of water present in the adsorbed phase is as high as 2.9 wt.%, far above its solubility in bulk toluene (0.033 wt.% at 25 deg C). At RH 87% the concentration of water in the adsorbed phase is four times higher, approximately 12 wt.%. The recently proposed mechanism of anchoring of the water by the aromatic molecules may provide an explanation for this phenomenon

TiO2-doped resorcinol–formaldehyde (RF) polymer and carbon gels with photocatalytic activity

Resorcinol-formaldehyde (RF) polymer gels offer a relatively easy and versatile route for incorporating metals into a carbon aerogel matrix. The hybrid materials thus obtained are ideal candidates for applications involving enhanced adsorption or catalysis. This paper presents a detailed study of Ti-doped RF and carbon aerogels. The metal was introduced into the system at three different stages of the preparation process: during polymerization, by impregnation of the RF gel, or by impregnation of the carbon gel. The structure and morphology of the samples are compared using low temperature N2 adsorption, SEM, and small and wide angle X-Ray scattering (SAXS/WAXS) methods. The TiO2-doped carbon aerogels display photocatalytic activity in breaking down aromatic compounds

Graphene derivatives in responsive hydrogels: Effect of concentration and surface chemistry

Reduced graphene oxide (RGO) containing composite hydrogels, based on poly(N-isopropylacrylamide) (PNIPA) were prepared by two different methods: (i) by incorporating RGO directly into the polymer matrix; (ii) applying a post-synthesis reduction of the graphene-oxide (GO) already incorporated into the polymer. The samples were compared by various microscopic (small angle neutron scattering, differential scanning calorimetry, 1H NMR spectroscopy, thermogravimetry) and macroscopic (kinetic and equilibrium swelling properties and mechanical testing) techniques. Results from microscopic and macroscopic measurements show that the dispersity of the nanoparticles as well as their interaction with the polymer chains are influenced by their surface chemistry. Incorporation of nanoparticles limits the shrinkage and slows down the kinetics of the thermal response. Both thermogravimetric and solid-state NMR measurements confirmed strong polymer – nanoparticle interaction when hydrophilic GO was used in the synthesis. In these cases, the slow thermal response may be explained by the decrease of the free volume inside the nanocomposite matrix caused by a hypernodal structure. Our results imply that both the chemistry and the concentration of incorporated graphene derivatives are promising in tuning the thermal responsivity of PNIPA

Topological dynamics of micelles formed by geometrically varied surfactants

The molecular architecture of sugar-based surfactants strongly affects their self-assembled structure, i.e., the type of micelles they form, which in turn controls both the dynamics and rheological properties of the system. Here, we report the segmental and mesoscopic structure and dynamics of a series of C16 maltosides with differences in the anomeric configuration and degree of tail unsaturation. Neutron spin-echo measurements showed that the segmental dynamics can be modeled as a one-dimensional array of segments where the dynamics increase with inefficient monomer packing. The network dynamics as characterized by dynamic light scattering show different relaxation modes that can be associated with the micelle structure. Hindered dynamics are observed for arrested networks of worm-like micelles, connected to their shear-thinning rheology, while nonentangled diffusing rods relate to Newtonian rheological behavior. While the design of novel surfactants with controlled properties poses a challenge for synthetic chemistry, we demonstrate how simple variations in the monomer structure can significantly influence the behavior of surfactantsThe authors thank the Swedish Research Council Formas (Grant 2015-666) for funding J.L. The research was performed with financial support from the Vinnova─Swedish Governmental Agency for Innovation Systems within the NextBioForm Competence Centre. The authors also thank the Institut Laue-Langevin, France, for the awarded beamtime (Proposal No. 9-10-1652). NSE data is openly available at doi: 10.5291/ILL-DATA.9-10-1652S

Neutron spin echo monitoring of segmental-like diffusion of water confined in the cores of carbon nanotubes

An unexpected formal similarity to the segmental dynamics of entangled polymers is found for the subdiffusional behavior of water axially confined inside single-wall carbon nanotubes of average diameter d = 1.4 nm

Role of water molecules in the decomposition of HKUST-1: Evidence from adsorption, thermoanalytical, X-ray and neutron scattering measurements

HKUST-1 is a strictly microporous crystalline metal organic framework with pore sizes of 5, 11, and 13.5 Å. Detailed gas adsorption measurements show that its adsorption capacity for water at 20 °C is higher than that for nitrogen at-196 °C, and far exceeds that for methane at 0 °C. Extended exposure to water vapour at high relative humidity, or consecutive adsorption-desorption cycling of water vapour, destroys both the MOF crystal structure and its adsorption capacity, after a reduced number (< 5) of cycles. Destruction proceeds through mesoporous defects that open within the crystal structure, as attested both by the development of hysteresis in the adsorption isotherms and by changes in the small angle X-ray scattering pattern. In the pristine crystal, the structure of the water in the micropores closely resembles that of bulk liquid water. Small angle neutron scattering demonstrates that water is adsorbed preferentially over methane, and that the size of the spherical cavities occupied by the adsorbed water molecules in the intact crystal is consistent with the known pore size structure in this system

Wetting and non-wetting fluids in surface-functionalised activated carbons

International audiencePore filling by non-polar and polar molecules is investigated by small-angle X-ray scattering in activated carbons that had been surface-functionalised to different degrees by oxidation. A pseudo-binary model for the scattering response is used to trace the filling characteristics of water and n-hexane in the pore structure. While the pores are uniformly filled by n-hexane, pore filling by water is only partial. In the latter case, a significant contribution from liquid-vapour interfaces appears and the system becomes fully ternary. This feature is direct evidence of the development of water droplets, which form even in the most oxidised carbon. The appreciable differences between the carbons illustrate the influence on the small-angle X-ray scattering response of surface chemistry and of the polarity of the adsorbed molecules

Effect of molybdenum on the structure formation of resorcinol - formaldehyde hydrogel studied by coherent X-Ray scattering

International audienceA detailed study on the effect of Mo on the gelation process of resorcinol-formaldehyde systems is presented. The evolution of the system was followed by x-ray photon correlation spectroscopy, which allows in situ investigation of the dynamics as well as of the structural evolution in nonequilibrium processes. The Mo was introduced into the system after a pre-polymerization period (PP), the effect of which was also examined. Our results show that the presence of Mo substantially modifies the gelation process by favoring the growth of large compact clusters with weak bonds between them. However, this effect can be reduced by increasing PP

In situ SAXS investigation of structural changes in soft resorcinol-formaldehyde polymer gels during CO2-drying

International audienceDrying is one of the key steps in the preparation of resorcinol-formaldehyde (RF) gels. Supercritical drying has long been known to be a method by which the structure of RF aerogels has the greatest chance of being perfectly preserved. Although several investigations have been conducted into the effects of the applied drying parameters on the final gels, the structural evolution of the gels during the drying process is nevertheless poorly known. In this work we present an in situ small angle X-ray scattering study of (1) the changes that occur in the structure of RF networks during supercritical drying under various conditions, and (2) the kinetics of the drying process. During the pressurization stage the overall structure of the network remains unaffected by the rate of increase of the CO2 pressure. By contrast, depressurization with supercritical CO2 at rates in excess of 4 bar/min, or with liquid CO2 alone, causes bubbles to form and yields a shrunken state of the final network. This densification preferentially shrinks the larger pores, the smaller pores being much less affected. We find that liquid CO2 is also an efficient drying medium that can preserve the structure, but the depressurization stage must always start from the supercritical state. The incomparable advantage of supercritical CO2, however, is that its higher rate of molecular diffusion makes solvent exchange substantially faster than with liquid CO2

In situ SAXS investigation of structural changes in soft resorcinol–formaldehyde polymer gels during CO2-drying

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