Étude de différents modes d’organisation cristal-liquide de suspensions colloïdales à base de nanofeuillets minéraux

Colloidal" liquid-crystals, suspensions of anisotropic nanoparticles, are currently raising renewed interest as they provide convenient model systems to test various statistical physics theories and to better understand some behaviors that appear during various industrial processes. In this thesis, we are interested in different modes of liquid-crystal organization of colloidal suspensions based on mineral nanosheets. We started with the study of the influence of the doping of two different types of liquid-crystal suspensions of nanosheets (natural and synthetic) by different kinds of water-soluble polymers. Then, another subject led us to synthesize gold-clay hybrid nanoparticles (based on beidellite clay) and to explore the behavior of their colloidal suspensions. Furthermore, we have examined a very original mode of organization of nanosheets discovered during the study of synthetic hectorite suspensions. Finally, we studied the influence of a pH change of acidic suspensions of H3Sb3P2O14 nanosheets, in order to widen the field of their potential applications. We neutralized these suspensions by addition of a strong base. All these studies were performed using a wide range of complementary techniques such as observations of samples in natural and polarized light, optical microscopy, application of magnetic and electric fields, X-ray scattering (DRX, SAXS, WAXS), transmission electron microscopy (TEM), X-ray photoemission electron microscopy (XPEEM), adsorption isotherms, zetametry, thermogravimetric analysis (TGA) and UV-visible spectroscopy. All these techniques, complemented by theoretical approaches, have allowed us to reach a rather complete description of different original samples at the micro and macroscopic scales, to study the spontaneous self-assembly of nanoparticles, and to manipulate these colloidal organizations by external fields. In conclusion, this thesis presents a wide panorama of different original phenomena illustrating the great variety and subtlety of spontaneous organization modes that affect colloidal suspensions of nanosheets.Les cristaux-liquides « colloïdaux », qui sont des suspensions de nanoparticules anisotropes, présentent actuellement un regain d’intérêt car ils fournissent des systèmes-modèles commodes pour tester diverses théories de physique statistique et pour mieux appréhender certains comportements qui apparaissent lors de divers processus industriels. Dans cette thèse, nous nous sommes intéressés à différents modes d’organisation cristal-liquide de suspensions colloïdales à base de nanofeuillets minéraux. Nous avons commencé par l’étude de l’influence du dopage de deux différents types de suspensions cristal-liquides de nanofeuillets (naturels et synthétiques) par différentes sortes de polymères hydrosolubles. Ensuite, un autre sujet nous a conduits à synthétiser des nanoparticules hybrides or-argile (à base d’argile beidellite) et à explorer le comportement de leurs suspensions colloïdales. Puis, nous avons examiné un mode très original d’organisation de nanofeuillets qui a été mis en évidence lors de l’étude de suspensions d’hectorites synthétiques. Finalement, nous avons étudié l’influence d’un changement de pH de suspensions acides de nanofeuillets synthétiques de H3Sb3P2O14, afin élargir le champ de leurs applications potentielles. Nous les avons ainsi neutralisées par addition d'une base forte.Toutes ces études ont été réalisées en utilisant une vaste panoplie de techniques complémentaires telles que les observations d’échantillons en lumière naturelle et polarisée, la microscopie optique, l’application de champs magnétique et électrique, la diffusion des rayons X (DRX, SAXS, WAXS), la microscopie électronique en transmission (MET), la microscopie électronique à photoémission de rayons X (XPEEM), les isothermes d’adsorption, la zetametrie, l’analyse thermogravimétrique (ATG) et la spectroscopie UV-visible. Toutes ces techniques, complétées par les approches théoriques nous ont permis d’atteindre une description assez complète de différents échantillons originaux aux échelles micro et macroscopique, d’y étudier l’auto-assemblage spontané de nanoparticules, et de manipuler ces organisations colloïdales par des champs extérieurs. En conclusion, ce mémoire de thèse présente donc un panorama de différents phénomènes originaux illustrant la très grande variété et subtilité des modes d’organisation spontanée qui affectent les suspensions colloïdales de nanofeuillets

Study of different crystal-liquid organization modes of colloidal suspensions based on mineral nanosheets

Les cristaux-liquides « colloïdaux », qui sont des suspensions de nanoparticules anisotropes, présentent actuellement un regain d’intérêt car ils fournissent des systèmes-modèles commodes pour tester diverses théories de physique statistique et pour mieux appréhender certains comportements qui apparaissent lors de divers processus industriels. Dans cette thèse, nous nous sommes intéressés à différents modes d’organisation cristal-liquide de suspensions colloïdales à base de nanofeuillets minéraux. Nous avons commencé par l’étude de l’influence du dopage de deux différents types de suspensions cristal-liquides de nanofeuillets (naturels et synthétiques) par différentes sortes de polymères hydrosolubles. Ensuite, un autre sujet nous a conduits à synthétiser des nanoparticules hybrides or-argile (à base d’argile beidellite) et à explorer le comportement de leurs suspensions colloïdales. Puis, nous avons examiné un mode très original d’organisation de nanofeuillets qui a été mis en évidence lors de l’étude de suspensions d’hectorites synthétiques. Finalement, nous avons étudié l’influence d’un changement de pH de suspensions acides de nanofeuillets synthétiques de H3Sb3P2O14, afin élargir le champ de leurs applications potentielles. Nous les avons ainsi neutralisées par addition d'une base forte.Toutes ces études ont été réalisées en utilisant une vaste panoplie de techniques complémentaires telles que les observations d’échantillons en lumière naturelle et polarisée, la microscopie optique, l’application de champs magnétique et électrique, la diffusion des rayons X (DRX, SAXS, WAXS), la microscopie électronique en transmission (MET), la microscopie électronique à photoémission de rayons X (XPEEM), les isothermes d’adsorption, la zetametrie, l’analyse thermogravimétrique (ATG) et la spectroscopie UV-visible. Toutes ces techniques, complétées par les approches théoriques nous ont permis d’atteindre une description assez complète de différents échantillons originaux aux échelles micro et macroscopique, d’y étudier l’auto-assemblage spontané de nanoparticules, et de manipuler ces organisations colloïdales par des champs extérieurs. En conclusion, ce mémoire de thèse présente donc un panorama de différents phénomènes originaux illustrant la très grande variété et subtilité des modes d’organisation spontanée qui affectent les suspensions colloïdales de nanofeuillets.Colloidal" liquid-crystals, suspensions of anisotropic nanoparticles, are currently raising renewed interest as they provide convenient model systems to test various statistical physics theories and to better understand some behaviors that appear during various industrial processes. In this thesis, we are interested in different modes of liquid-crystal organization of colloidal suspensions based on mineral nanosheets. We started with the study of the influence of the doping of two different types of liquid-crystal suspensions of nanosheets (natural and synthetic) by different kinds of water-soluble polymers. Then, another subject led us to synthesize gold-clay hybrid nanoparticles (based on beidellite clay) and to explore the behavior of their colloidal suspensions. Furthermore, we have examined a very original mode of organization of nanosheets discovered during the study of synthetic hectorite suspensions. Finally, we studied the influence of a pH change of acidic suspensions of H3Sb3P2O14 nanosheets, in order to widen the field of their potential applications. We neutralized these suspensions by addition of a strong base. All these studies were performed using a wide range of complementary techniques such as observations of samples in natural and polarized light, optical microscopy, application of magnetic and electric fields, X-ray scattering (DRX, SAXS, WAXS), transmission electron microscopy (TEM), X-ray photoemission electron microscopy (XPEEM), adsorption isotherms, zetametry, thermogravimetric analysis (TGA) and UV-visible spectroscopy. All these techniques, complemented by theoretical approaches, have allowed us to reach a rather complete description of different original samples at the micro and macroscopic scales, to study the spontaneous self-assembly of nanoparticles, and to manipulate these colloidal organizations by external fields. In conclusion, this thesis presents a wide panorama of different original phenomena illustrating the great variety and subtlety of spontaneous organization modes that affect colloidal suspensions of nanosheets

Destabilization of the Nematic Phase of Clay Nanosheet Suspensions by Polymer Adsorption

International audienceComplex aqueous mixtures comprised of swelling clays and hydrosoluble polymers naturally occur in soils and play a major role in pedogenesis. They are also very often used for formulating oil-well drilling fluids, paints, and personal-care products. The suspensions of some natural clays, thanks to their large nanoparticle aspect ratio, spontaneously form nematic liquid-crystalline phases where the particles align parallel to each other, which affects their flow properties. We observed that adding small amounts of hydrosoluble polymers to these clay suspensions destabilizes the nematic phase with respect to the isotropic (disordered) phase. The polymers that we used (poly(ethylene oxide) and dextran) were too small to adopt particle-bridging conformations and small-angle X-ray scattering experiments showed that the structure of the nematic phase is not altered by polymer doping. However, the adsorption isotherm shows that the macromolecules adsorb onto the clay nanosheets, effectively coating them with a polymer layer. Our extension of Onsager’s theory for polymer-coated platelets properly captures the experimental phase diagram and shows how the nematic phase destabilization can be due to the polymer adsorbing more on the platelet faces than at the rim. Because the flow properties of the nematic phase are very different from those of the isotropic phase, the presence or absence of the former phase is an important factor to be determined and considered to explain the rheological behavior of these complex systems

Fine tuning the structural colours of photonic nanosheet suspensions by polymer doping

International audienceAqueous suspensions of nanosheets are readily obtained by exfoliating low-dimensional mineral compounds like H3Sb3P2O14. The nanosheets self-organize, at low concentration, into a periodic stack of membranes, i.e. a lamellar liquid-crystalline phase. Due to the dilution, this stack has a large period of a few hundred nanometres, it behaves as a 1-dimensional photonic material and displays structural colours. We experimentally investigated the dependence of the period on the nanosheet concentration. We theoretically showed that it cannot be explained by the usual DLVO interaction between uniform lamellae but that the particulate nature of nanosheet-laden membranes must be considered. Moreover, we observed that adding small amounts of 100 kDa poly(ethylene oxide) (PEO) decreases the period and allows tuning the colour throughout the visible range. PEO adsorbs on the nanosheets, inducing a strong reduction of the nanosheet charge. This is probably due to the Lewis-base character of the EO units of PEO that become protonated at the low pH of the system, an interpretation supported by theoretical modeling. Oddly enough, adding small amounts of 1 MDa PEO has the opposite effect of increasing the period, suggesting the presence of an additional intermembrane repulsion not yet identified. From an applied perspective, our work shows how the colours of these 1-dimensional photonic materials can easily be tuned not only by varying the nanosheet concentration (which might entail a phase transition) but also by adding PEO. From a theoretical perspective, our approach represents a necessary step towards establishing the phase diagram of aqueous suspensions of charged nanosheets

Do aqueous suspensions of smectite clays form a smectic liquid-crystalline phase?

International audienceBottom-up strategies for the production of well-defined nanostructures often rely on the self-assembly of anisotropic colloidal particles (nanowires and nanosheets). These building blocks can be obtained by delamination in a solvent of low-dimensionality crystallites. To optimize particle availability, determination of the delamination mechanism and the different organization stages of anisotropic particles in dispersion is essential. We address this fundamental issue by exploiting a recently developed system of fluorohectorite smectite clay mineral that delaminates in water, leading to colloidal dispersions of single-layer, very large (≈20 μm) clay sheets at high dilution. We show that when the clay crystallites are dispersed in water, they swell to form periodic one-dimensional stacks of fluorohectorite sheets with very low volume fraction (<1%) and therefore huge (≈100 nm) periods. Using optical microscopy and synchrotron X-ray scattering, we establish that these colloidal stacks bear strong similarities, yet subtle differences, with a smectic liquid-crystalline phase. Despite the high dilution, the colloidal stacks of sheets, called colloidal accordions, are extremely robust mechanically and can persist for years. Moreover, when subjected to AC electric fields, they rotate as solid bodies, which demonstrates their outstanding internal cohesion. Furthermore, our theoretical model captures the dependence of the stacking period on the dispersion concentration and ionic strength and explains, invoking the Donnan effect, why the colloidal accordions are kinetically stable over years and impervious to shear and Brownian motion. Because our model is not system specific, we expect that similar colloidal accordions frequently appear as an intermediate state during the delamination process of two-dimensional crystals in polar solvents

Crystalline restacking of 2D-materials from their nanosheets suspensions

International audienceWe report here the highly ordered restacking of the layered phosphatoantimonic dielectric materials H$_{3(1−x)}$M$_{3x}$Sb$_3$P$_2$O$_{14}$, (where M = Li, Na, K, Rb, Cs and 0 ≤ x ≤ 1), from their nanosheets dispersed in colloidal suspension, induced by a simple pH change using alkaline bases. H$_3$Sb$_3$P$_2$O$_{14}$ aqueous suspensions are some of the rare examples of colloidal suspensions based on 2D materials exhibiting a lamellar liquid crystalline phase. Because the lamellar period can reach several hundred nanometers, the suspensions show vivid structural colors and because these colors are sensitive to various chemicals, the suspensions can be used as sensors. The structures of the lamellar liquid crystalline phase and the restacked phase have been studied by X-ray scattering (small and wide angle), which has followed the dependence of the lamellar/restacked phase equilibrium on the cation exchange rate, x. The X-ray diffraction pattern of the restacked phase is almost identical to that of the M$_3$Sb$_3$P$_2$O$_{14}$ crystalline phase, showing that the restacking is highly accurate and avoids the turbostratic disorder of the nanosheets classically observed in nanosheet stacking of other 2D materials. Strikingly, the restacking process exhibits features highly reminiscent of a first-order phase transition, with the existence of a phase coexistence region where both ∼1 nm (interlayer spacing of the restacked phase) and ∼120 nm lamellar periods can be observed simultaneously. Furthermore, this first-order phase transition is well described theoretically by incorporating a Lennard-Jones-type lamellar interaction potential into an entropy-based statistical physics model of the lamellar phase of nanosheets. Our work shows that the precise cation exchange produced at room temperature by a classical neutralization reaction using alkaline bases leads to a crystal-like restacking of the exfoliated free Sb$_3$P$_2$O$_{14}$$^{3−}$ nanosheets from suspension, avoiding the turbostratic disorder typical of van der Waals 2D materials, which is detrimental to the controlled deposition of nanosheets into complex integrated electronic, spintronic, photonic or quantum structures