Erwan Paineau

Liquid-crystalline nematic phase in aqueous suspensions of a disk-shaped natural beidellite clay.

After size-selection and osmotic pressure measurements at fixed ionic strength, the behavior of aqueous colloidal suspensions of anisotropic disklike beidellite clay particles has been investigated by combining optical observations under polarized light, rheological, and small angle X-ray scattering (SAXS) experiments. The obtained phase diagrams (volume fraction/ionic strength) reveal, for ionic strength below 10(-3) M/L, a first-order isotropic/nematic (I/N) phase transition before gel formation at low volume fractions, typically around 0.5%. This I/N transition line displays a positive slope for increasing ionic strength and shifts toward lower volume fraction with increasing particle size, confirming that the system is controlled by repulsive interactions. The swelling laws, derived from the interparticle distances obtained by SAXS, display a transition from isotropic swelling at low volume fractions to lamellar swelling at higher volume fractions. The liquid-crystal properties have then been investigated in detail. Highly aligned nematic samples can be obtained in three different ways, by applying a magnetic field, an ac electric field, and by spontaneous homeotropic anchoring on surfaces. The birefringence of the fluid nematic phase is negative with typical values around 5 x 10(-4) at a volume fraction of about 0.6%. High nematic order parameters have been obtained as expected for well-aligned samples. The nematic director is aligned parallel to the magnetic field and perpendicular to the electric field.

Aqueous suspensions of natural swelling clay minerals. 1. Structure and electrostatic interactions.

In this article, we present a general overview of the organization of colloidal charged clay particles in aqueous suspension by studying different natural samples with different structural charges and charge locations. Small-angle X-ray scattering experiments (SAXS) are first used to derive swelling laws that demonstrate the almost perfect exfoliation of clay sheets in suspension. Using a simple approach based on geometrical constraints, we show that these swelling laws can be fully modeled on the basis of morphological parameters only. The validity of this approach was further extended to other clay data from the literature, in particular, synthetic Laponite. For all of the investigated samples, experimental osmotic pressures can be properly described by a Poisson-Boltzmann approach for ionic strength up to 10(-3) M, which reveals that these systems are dominated by repulsive electrostatic interactions. However, a detailed analysis of the Poisson-Boltzmann treatment shows differences in the repulsive potential strength that are not directly linked to the structural charge of the minerals but rather to the charge location in the structure for tetrahedrally charged clays (beidellite and nontronites) undergoing stronger electrostatic repulsions than octahedrally charged samples (montmorillonites, laponite). Only minerals subjected to the strongest electrostatic repulsions present a true isotropic to nematic phase transition in their phase diagrams. The influence of ionic repulsions on the local order of clay platelets was then analyzed through a detailed investigation of the structure factors of the various clay samples. It appears that stronger electrostatic repulsions improve the liquidlike positional local order.

Aqueous suspensions of natural swelling clay minerals. 2. Rheological characterization.

We report in this article a comprehensive investigation of the viscoelastic behavior of different natural colloidal clay minerals in aqueous solution. Rheological experiments were carried out under both dynamic and steady-state conditions, allowing us to derive the elasticity and yield stress. Both parameters can be renormalized for all sizes, ionic strength, and type of clay using in a first approach only the volume of the particles. However, applying such a treatment to various clays of similar shapes and sizes yields differences that can be linked to the repulsion strength and charge location in the swelling clays. The stronger the repulsive interactions, the better the orientation of clay particles in flows. In addition, a master linear relationship between the elasticity and yield stress whose value corresponds to a critical deformation of 0.1 was evidenced. Such a relationship may be general for any colloidal suspension of anisometric particles as revealed by the analysis of various experimental data obtained on either disk-shaped or lath- and rod-shaped particles. The particle size dependence of the sol-gel transition was also investigated in detail. To understand why suspensions of larger particles gel at a higher volume fraction, we propose a very simplified view based on the statistical hydrodynamic trapping of a particle by an another one in its neighborhood upon translation and during a short period of time. We show that the key parameter describing this hydrodynamic trapping varies as the cube of the average diameter and captures most features of the sol-gel transition. Finally, we pointed out that in the high shear limit the suspension viscosity is still closely related to electrostatic interactions and follows the same trends as the viscoelastic properties.

Orientational Order of Colloidal Disk-Shaped Particles under Shear-Flow Conditions: a Rheological−Small-Angle X-ray Scattering Study

The structure of a colloidal dispersion consisting of anisometric natural clay particles (beidellite) was followed under shear-flow conditions by small-angle X-ray scattering (SAXS) measurements in a Couette-type cell. It is shown that in this shear-thinning dispersion an orientational order develops with increasing shear rate. By use of two different geometrical configurations for SAXS measurements, corresponding to incident beam parallel and perpendicular to flow velocity gradient (radial and tangential configurations, respectively), it is observed that SAXS patterns are anisotropic in both geometries, meaning that particles tend to align along a preferred orientation with their normal in velocity gradient direction, and further they partly rotate around flow streamlines. Quantitative interpretation of these results is successfully achieved upon derivation of a probability distribution function accounting for biaxial particle orientation. From this distribution and following geometrical arguments, the viscosity of the suspension was calculated for each shear rate and found to correctly compare with rheological measurements, thereby appropriately relating the anisotropy of the SAXS patterns to macroscopic flow behavior of the suspension.

X-ray scattering determination of the structure of water during carbon nanotube filling.

We present in situ monitoring of water filling of single-walled carbon nanotubes at room temperature, using X-ray scattering. A systematic method is developed to determine the water radial density profile. Water filling is homogeneous below about 5% in mass, whereas it structures into three layers above. These results should motivate further theoretical and simulations studies and allow getting a better understanding of the very peculiar properties of water confined in hydrophobic environment.

Liquid–crystalline properties of aqueous suspensions of natural clay nanosheets

Clay minerals, like beidellite or nontronite, spontaneously exfoliate in water and form colloidal suspensions of nanosheets. In a given range of concentration, these suspensions display a nematic liquid–crystalline phase whose structure and properties can be conveniently studied in detail by polarized-light microscopy and small-angle X-ray scattering (SAXS). Moreover, in situ SAXS investigations of sheared clay suspensions provide information about their flow properties, both in the isotropic and nematic phases. The colloidal nematic phase shows the classical properties of usual nematics, such as surface anchoring and electric-field and magnetic-field alignment. Thus, nematic single domains can be produced. The isotropic phase also displays strong electro-optic effects in moderate electric fields. Finally, we describe a few examples of applications of such systems and we show how these studies could be extended to suspensions of other types of nanosheets.

Tailoring highly oriented and micropatterned clay/polymer nanocomposites by applying an a.c. electric field.

Clay/polymer nanocomposites have recently raised much interest because of their widespread industrial applications. Nevertheless, controlling both clay platelet exfoliation and orientation during polymerization still remains challenging. Herein, we report the elaboration of clay/polymer nanocomposite hydrogels from aqueous suspensions of natural swelling clays submitted to high-frequency a.c. electric fields. X-ray scattering experiments have confirmed the complete exfoliation of the clay sheets in the polymer matrix, even after polymerization. Moreover, polarized light microscopy shows that the clay platelets were perfectly oriented by the electric field and that this field-induced alignment was frozen in by in situ photopolymerization. This procedure allowed us to not only produce uniformly aligned samples but also pattern platelet orientation, at length scales down to 20 μm. This straightforward and cheap nanocomposite patterning technique can be easily extended to a wide range of natural or synthetic inorganic anisotropic particles.

Effects of added silica nanoparticles on the nematic liquid crystal phase formation in beidellite suspensions

In this article, we present a study of the liquid crystal phase behavior of mixed suspensions of the natural smectite clay mineral beidellite and nonadsorbing colloidal silica particles. While virtually all smectite clays dispersed in water form gels at very low concentrations, beidellite displays a first order isotropic-nematic phase transition before gel formation (J. Phys. Chem. B, 2009, 113, 15858-15869). The addition of silica nanospheres shifts the concentrations of the coexisting isotropic and nematic phases to slightly higher values while at the same time markedly accelerating the phase separation process. Furthermore, beidellite suspensions at volume fractions above the isotropic-nematic phase separation, trapped in a kinetically arrested gel state, liquefy on the addition of silica nanospheres and proceed to isotropic-nematic phase separation. Using small-angle X-ray scattering (SAXS), we probe the structural changes caused by the addition of the silica nanospheres, and we relate the modification of the phase transition kinetics to the change of the rheological properties.

Intracellular fate of carbon nanotubes inside murine macrophages: pH-dependent detachment of iron catalyst nanoparticles.

BackgroundCarbon nanotubes (CNT) are a family of materials featuring a large range of length, diameter, numbers of walls and, quite often metallic impurities coming from the catalyst used for their synthesis. They exhibit unique physical properties, which have already led to an extensive development of CNT for numerous applications. Because of this development and the resulting potential increase of human exposure, an important body of literature has been published with the aim to evaluate the health impact of CNT. However, despite evidences of uptake and long-term persistence of CNT within macrophages and the central role of those cells in the CNT-induced pulmonary inflammatory response, a limited amount of data is available so far on the CNT fate inside macrophages. Therefore, the overall aim of our study was to investigate the fate of pristine single walled CNT (SWCNT) after their internalization by macrophages.MethodsTo achieve our aim, we used a broad range of techniques that aimed at getting a comprehensive characterization of the SWCNT and their catalyst residues before and after exposure of murine macrophages: X-ray diffraction (XRD), High Resolution (HR) Transmission Electron Microscopy (TEM), High Angle Annular Dark Field-Scanning TEM (HAADF-STEM) coupled to Electron Energy Loss Spectroscopy (EELS), as well as micro-X-ray fluorescence mapping (μXRF), using synchrotron radiation.ResultsWe showed 1) the rapid detachment of part of the iron nanoparticles initially attached to SWCNT which appeared as free iron nanoparticles in the cytoplasm and nucleus of CNT-exposed murine macrophages, and 2) that blockade of intracellular lysosomal acidification prevented iron nanoparticles detachment from CNT bundles and protected cells from CNT downstream toxicity.ConclusionsThe present results, while obtained with pristine SWCNT, could likely be extended to other catalyst-containing nanomaterials and surely open new ways in the interpretation and understanding of CNT toxicity.

Pulmonary exposure to metallic nanomaterials during pregnancy irreversibly impairs lung development of the offspring

Abstract Due to the growing commercial applications of manufactured nanoparticles (NPs), toxicological studies on NPs, especially during the critical window of development, are of major importance. The aim of the study was to assess the impact of respiratory exposure to metallic and metal oxide NPs during pregnancy on lung development of the offspring and to determine the key parameters involved in lung alterations. Pregnant mice were exposed to weekly doses of 100 μg (total dose 300 μg) of titanium dioxide (TiO2), cerium oxide (CeO2), silver (Ag) NPs or saline solution by nonsurgical intratracheal instillation. The offspring lungs were analyzed at different stages of lung development: fetal stage (gestational day 17.5), pulmonary alveolarization (post-delivery day 14.5) and lung maturity (post-delivery day 49.5). Regardless of the type of NP, maternal exposure during gestation induced long-lasting impairment of lung development of the offspring. This effect was accompanied by: i) decreased placental efficiency together with the presence of NPs in placenta, ii) no increase of inflammatory mediators present in amniotic fluid, placenta or offspring lungs and iii) decreased pulmonary expression of vascular endothelial growth factor-α (VEGF-α) and matrix metalloproteinase 9 (MMP-9) at the fetal stage, and fibroblast growth factor-18 (FGF-18) at the alveolarization stage. Respiratory exposure to metallic NPs during pregnancy induces stereotyped impairment of lung development with a lasting effect in adult mice, independently of the chemical nature of the NP.