Claire Peyratout

Aggregation of thiacyanine derivatives on polyelectrolytes

The electrostatic binding of a series of anionic disulfopropyl-thiacyanine (THIA) derivatives to poly(allylamine hydrochloride) (PAH) in aqueous solution is investigated by optical spectroscopy. From strong variations of the absorption, fluorescence, and excitation spectra of the dyes upon binding to PAH, conclusions are drawn about the binding modes. The unsubstituted THIA forms a ground state complex with PAH which exhibits a blue shift in absorption, like H-aggregates, but also a strongly enhanced fluorescence with respect to the free dye. The methyl derivative behaves similarly. The absorption of the methoxy derivative also exhibits a hypsochromic shift upon addition of PAH but its fluorescence is quenched. In the case of the dichloro-derivative, addition of PAH induces the formation of J-aggregates. The fluorescence quantum yield is strongly enhanced from 0.008 for the monomer to 0.48 for the PAH-J-aggregate. The monochloro-THIA shows a competition between the J-aggregate formation and the dye–PAH complex formation. Similarly to the methoxy-THIA, the fluorescence is quenched upon addition of PAH. The stability of the PAH-J-adduct to replacement by polyanions is larger than for the H-adduct, although the aggregation tendency of the dyes in pure water displays an opposite behaviour. Results show that the optical properties and the aggregate structures of sensitizing cyanine molecules can be tuned widely by interaction with polyelectrolytes.

Comparison of surface properties between kaolin and metakaolin in concentrated lime solutions

The surface adsorption of calcium hydroxide onto kaolin and metakaolin was investigated by monitoring with atomic emission spectroscopy and pH measurements the amounts of ions left in solution after exposing clays to calcium hydroxide solutions of various concentrations. Both clays adsorb calcium and hydroxyl ions but differently. Kaolin adsorbs calcium hydroxide not only at the edges of the clay particles but also onto the basal faces. The adsorbed hydrated calcium ions form a layer on the clay particle surfaces, preventing further dissolution of the clay mineral platelet. Metakaolin shows high pozzolanic activity, which provides the quick formation of hydrated phases at the interfaces between metakaolin and lime solutions. The nature of the hydration products has been investigated using X-ray diffraction (XRD) and differential thermal analysis (DTA). The most important hydrated phases like CSH (hydrated calcium silicate) and C(2)ASH(8) (gehlenite) have been identified.

Surface modifications of illite in concentrated lime solutions investigated by pyridine adsorption

The surface properties and functionality of an illite-containing clay mineral in alkaline solutions containing increasing quantities of calcium hydroxide were investigated using FTIR monitored pyridine adsorption at various temperatures. Results showed that the binding behavior of electron donor pyridine molecules to illite platelets is mostly governed by electron acceptor sites located at the edges of the clay particles. The binding of bulky hydrated calcium ions at the surface of the illite platelets decreases the surface area of illite. Moderate thermal treatments up to 450°C do not affect the structure of the clay mineral but strongly decrease the number of Lewis and Brönsted sites available at the edges of the clay platelets.

Geomaterial foams: role assignment of raw materials in the network formation

The geomaterial foams studied is based on geopolymerization reactions, which is a type of geosynthesis that involves silico-aluminates. Its study during formation has however revealed a different behavior than geopolymer, suggesting the formation of various networks. This work investigates the interaction between initial compounds (metakaolin, silica fume, potassium-based solution) by a kind of mixture decomposition to ultimately understand the formation mechanism of foam. The structural evolution was determined using thermal analysis, FTIR spectroscopy and 27Al and 29Si MAS-NMR measurements. The use of different raw materials in combination with various solutions demonstrates the formation of various species in solution. The reactivity of the solution will then evolve in different ways. The Si/K ratio controlled the type of species created and, particularly, the reactivity in the mixture. From the various reactions of dissolution and polycondensation that were deduced, we could identify the composition of the four networks (K0.5SiAl0.75O6.8H8.6; K2Si2O5; KAlSi2O4,1.5H2O; and amorphous silica) constituting the foam.

Mechanical properties of hemp-lime reinforced mortars: influence of the chemical treatment of fibers

In addition to being an environmental-friendly material, hemp fibers are also inexpensive reinforcements in thermoplastics or concrete composites, due to their intrinsic mechanical, thermal, and acoustic properties. However, chemical treatments of fibers are required to enhance the matrix/fiber interface. In this article, the influence of the addition of hemp fibers chemically treated or not, on the mechanical properties of a lime composite was investigated. Mechanical properties were evaluated by ultrasonic pulse echography and four-point bending test associated with acoustic emission analysis. The physical and chemical surface modifications of the hemp fibers were monitored with gas chromatography and scanning electron microscopy. It appears that chemical modification of the fiber surface degrades the amorphous materials present in the fiber structure, which results in an increase in the surface reactivity and also improves the mechanical properties of the composites.

Nanocomposites Derived from Montmorillonite and Metallosupramolecular Polyelectrolytes: Modular Compounds for Electrorheological Fluids

Nanocomposites made from Na-montmorillonite and metallo-supramolecular polyelectrolytes (MEPE) based on nickel and ditopic bis-terpyridine ligands are prepared by an aqueous synthesis. Intercalation is confirmed by IR-spectroscopy, thermogravimetric analysis, and X-ray powder diffraction. The rheological response in the presence of an electric field of the dispersed nanocomposites in silicone oil is measured with a rheometer. The nanocomposites show a distinct electrorheological effect depending on the concentration and the kind of intercalated species. The effect occurs with a low content of active material while only very small currents are observed.

Dispersion of phyllosilicates in aqueous suspensions: Role of the nature and amount of surfactant

HYPOTHESES The present work aims at investigating the effect of pH values and additives on the dispersion of two 1:1 dioctahedral phyllosilicates in the presence of water. Two model clays are used for this purpose, BIP kaolin and NZCC halloysite, presenting the same surface chemistry but different morphologies. The effect of sodium hexametaphosphate, sodium silicate and sodium carbonate is discussed. EXPERIMENTS Kaolin and halloysite powders were first characterized using X-ray diffraction, thermogravimetric analysis and scanning electron microscopy. Subsequently, suspensions containing 8 mass% of each clay were prepared with or without additives. Experimental measurements regarding the pH values, the zeta potential and the rheological behavior were performed to determine the most suitable additive. FINDINGS Results show that the conformation of halloysite particles changes regarding pH values of suspensions and is strongly related to the surface charges of these particles. At their natural pH values, halloysite and kaolin suspensions exhibit zeta potentials equal to -50 and -20 mV respectively. This trend indicates that halloysite-based suspensions are well dispersed compared to kaolin-based suspensions. Sodium hexametaphosphate is the most suitable dispersant for both clays. The rheological characterization regarding further applications in casting process indicates a shear-thinning behavior for all studied compositions.

Monolayers and membranes from amphiphilic polymers

Nanometer thin, elastomeric membranes with considerable application potential in micro mechanics and materials science can be prepared by transferring monomolecular layers of polymers with ionic head groups from the water surface to solid substrates with holes. If monolayers of liquid polymers are transferred to substrates with openings they initially cover the openings, but finally rupture within a couple of minutes after transfer. However, if the polymer monolayers are stabilised by vitrification, chemical or physical cross-linking, they can be transferred to cover openings in solids substrates as stable membranes. Especially if monolayers of low glass transition polymers are cross-linked, elastomeric membranes are obtained, which might find application in micro mechanical devices like membrane valves and pumps. Incorporation of either a second, incompatible polymer or hydrophobised colloids leads to laterally structured and porous membranes.

Role of Alkaline Cations on Geomaterial Foams

The synthesis of geopolymers based on alkaline polysialate, was achieved at slightly elevated temperature, by alkaline activation of raw minerals and industrial waste. The materials were prepared from a solution containing dehydroxylated kaolinite and alkaline hydroxide pellets dissolved in potassium or sodium silicate. Then the mixture was transferred to a polyethylene mould sealed with a top and placed in an oven at 70°C during 24 hours. The addition of an industrial waste, silica fume, leads to the formation of an in-situ inorganic foam. Whatever the alkaline cation, foam formation occurs. The properties depend on the viscosity of silicate precursors due to the amount of water and to the size of alkaline.

Structural and microstructural studies of montmorillonite-based multilayer nanocomposites

HYPOTHESES Montmorillonite, an abundant raw material, is a good candidate to obtain textured nanocomposites. However, the resulting structure of the composite depends on the dispersant used. This work aims at investigating the effect of organic polysaccharides, namely carboxymethylcellulose (CMC) or chitosan (Ch) differing by their side groups, on the resulting structure of montmorillonite-based nanocomposites. EXPERIMENTS The effect of sodium hexametaphosphate and of two polysaccharide derivatives (carboxymethylcellulose and chitosan) combined with montmorillonite on the structure and microstructure of resulting composite films was investigated using particle size analysis, rheological measurements, thermogravimetric analysis, X-ray diffraction, scanning electron microscopy and flexural properties measurements of the textured films. FINDINGS Results showed that the film structure and microstructure depend on the additive. The high organization (and resulting toughness) of the montmorillonite/sodium hexametaphosphate films results from an exfoliated then layered microstructure, whereas the addition of polysaccharide derivatives leads to the particle agglomeration. In this case, two mechanisms are in competition: surface adsorption and intercalation between exfoliated platelets.