In-situ SAXS study of aqueous clay suspensions submitted to alternating current electric fields.

Aqueous colloidal suspensions of clay platelets display a sol/gel transition that is not yet understood. Depending on the nature of the clay, liquid-crystalline behavior may also be observed. For example, the suspensions of beidellite display a nematic phase whereas those of montmorillonite do not.

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.

Elaboration and characterization of nanoliposome made of soya; rapeseed and salmon lecithins: application to cell culture.

Health benefits of unsaturated fatty acids have been demonstrated over the last decades. Nanotechnology provided new process to produce particles such as liposomes and nanoliposomes made of pure phospholipids. These techniques are already used in pharmaceutics to augment the bioavailability and the bioefficiency of drugs.

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.

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.

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.

Highly concentrated emulsions: 1. Average drop size determination by analysis of incoherent polarized steady light transport.

The analysis of incoherent polarized steady light transport is reported as a convenient technique for the drop size determination in highly concentrated oil-in-water emulsions. The studied system consists in heptane-in-water emulsions stabilized with a copolymeric surfactant (Synperonic PE®/L64). Hundred grams of parent emulsions, at two volume fractions of dispersed phase (φ=0.

Sol-gel and isotropic/nematic transitions in aqueous suspensions of natural nontronite clay. Influence of particle anisotropy. 2. Gel structure and mechanical properties.

After size-selection, the phase behavior of aqueous suspensions of nontronite clay was analyzed by osmotic pressure measurements, rheological experiments, and small-angle X-ray scattering. All the measurements confirm that for ionic strength < or =10(-3) M/L, the system is purely repulsive. By combining results from osmotic pressure measurements and X-ray scattering, it appears that the pressure of the system can be well-described using a simple Poisson-Boltzmann treatment based on the interaction between charged infinite parallel planes.

Sol/gel and isotropic/nematic transitions in aqueous suspensions of natural nontronite clay. Influence of particle anisotropy. 1. Features of the i/n transition.

The phase behavior of a natural nontronite clay was studied for size-selected particles by combining osmotic pressure measurements, visual observations under polarized light, and rheological experiments. In parallel, the positional and orientational correlations of the particles were analyzed by small-angle X-ray scattering. Aqueous suspensions of nontronite exhibit a true isotropic/nematic (I/N) transition that occurs before the sol/gel transition, for ionic strengths below 10(-3) M/L.

Birefringence determination in turbid media.

We study the influence of birefringence on incoherent polarized light transport in turbid media. In particular, Mueller matrices backscattered by a diffusing medium are modified by the birefringence of the suspending phase. We study this effect both theoretically, through Monte Carlo simulations, and experimentally with a highly birefringent xanthane solution in which particles are added at various concentrations to modify its turbidity.

Liquid-crystalline aqueous clay suspensions.

This article demonstrates the occurrence of a true isotropic/nematic transition in colloidal Brownian aqueous suspensions of natural nontronite clay. The liquid-crystalline character is further evidenced by polarized light microscopy and small-angle x-ray scattering experiments in the presence and absence of modest external magnetic fields. The complete phase diagram ionic strength/volume fraction then exhibits a clear biphasic domain in the sol region just before the gel transition in contrast with the situation observed for other swelling clays in which the sol/gel transition hinders the isotropic/nematic transition.

Size determination by use of two-dimensional Mueller matrices backscattered by optically thick random media.

Here we are concerned with the systematic study of polarized light transport in thick, isotropic, homogeneous random media and of the associated inverse problem. An original spatial and intensity rescaling of the polarization transport allows one to account implicitly for the volume fraction. This parameter elimination permits a complete exploration, by means of Monte Carlo simulations of the dependence of polarized light transport on microscopic parameters.

Phase diagrams of Wyoming Na-montmorillonite clay. Influence of particle anisotropy.

Natural Na-Wyoming montmorillonite was size fractionated by successive centrifugation. Polydisperse particles with average sizes of 400, 290, and 75 nm were then obtained. As the structural charge of the particles belonging to three fractions (determined by cationic exchange capacity measurements) is the same, such a procedure allows studying the effect of particle anisotropy on the colloidal phase behavior of swelling clay particles.