Freezing-induced self-assembly of amphiphilic molecules.

The self-assembly of amphiphilic molecules usually takes place in a liquid phase, near room temperature. Here, using small angle X-ray scattering (SAXS) experiments performed in real time, we show that freezing of aqueous solutions of copolymer amphiphilic molecules can induce self-assembly below 0 °C.

Mechanism of self-assembly in the synthesis of silica mesoporous materials: in situ studies by X-ray and neutron scattering.

In this review, recent progress in the understanding of the formation of various silica mesoporous materials is reported. Owing to time-resolved experiments using Small Angle X-ray or Neutron Scattering (SAXS or SANS), it is possible to follow in situ the formation of a material during its synthesis via the Cooperative Templating Mechanism (CTM). Such experiments directly provide unique information about the structural properties of the material inside the synthesis solution. One of the main findings is that phase transformations often occur in the material prior to the stabilisation of its final structure. Moreover, during the very early stages of the synthesis, it is also possible to detect the first hybrid silica-surfactant micellar aggregates, prior to the precipitation of the material, as reported in the case of the SBA-15 materials. All these experiments allow a better understanding of the formation mechanisms and of the influence of the many synthesis parameters. These results open the way to a better prediction and control of the structure of mesoporous materials.

Rheo-SAXS investigation of shear-thinning behaviour of very anisometric repulsive disc-like clay suspensions.

Aqueous suspensions of swelling clay minerals exhibit a rich and complex rheological behaviour. In particular, these repulsive systems display strong shear-thinning at very low volume fractions in both the isotropic and gel states. In this paper, we investigate the evolution with shear of the orientational distribution of aqueous clay suspensions by synchrotron-based rheo-SAXS experiments using a Couette device. Measurements in radial and tangential configurations were carried out for two swelling clay minerals of similar morphology and size, Wyoming montmorillonite and Idaho beidellite. The shear evolution of the small angle x-ray scattering (SAXS) patterns displays significantly different features for these two minerals. The detailed analysis of the angular dependence of the SAXS patterns in both directions provides the average Euler angles of the statistical effective particle in the shear plane. We show that for both samples, the average orientation is fully controlled by the local shear stress around the particle. We then apply an effective approach to take into account multiple hydrodynamic interactions in the system. Using such an approach, it is possible to calculate the evolution of viscosity as a function of shear rate from the knowledge of the average orientation of the particles. The viscosity thus recalculated almost perfectly matches the measured values as long as collective effects are not too important in the system.

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.

Exploring the facets of "soft crystals" using an Atomic Force Microscope.

We obtained monocrystalline droplets in a thermotropic cubic phase, of approximate size 100 microm, deposited on a flat surface. The facets of these soft crystals are explored using both an optical microscope and an AFM. The height of individual steps on the principal facets and the lateral distance between steps in vicinal facets are measured using AFM in imaging (tapping) mode. Moreover, the elastic modulus is measured locally, using the AFM tip (in contact mode) as a local rheological probe.

New investigation on the tetragonal liquid-crystalline phase or SmQ.

Chiral liquid crystals, for which twist and smectic order strongly compete, can exhibit complex architectures. This is the case of the SmQ phase which shows a high degree of 3D order (tetragonal or hexagonal) as well as a liquid signature. We have combined X-ray scattering and optical observations on a new compound exhibiting a SmQ phase to discriminate between the two models proposed by A.M. Levelut et al. (J. Phys. II 7, 981 (1997)) at the molecular level for the phase with the I4(1)22 space group.

Phase transition between single crystals of two thermotropic cubic phases from a mixture of 3,5-didodecyloxybenzoic acid and C18-ANBC.

A reversible phase transition between two different thermotropic cubic phases is observed in a mixture of two carboxylic acids. Such a transition was characterised by single-crystal X-ray diffraction and direct optical microscopy; an image of a facetted droplet just at the transition between the two phases is shown in the picture.

Devil's staircase-type faceting of a cubic lyotropic liquid crystal

The faceting of monocrystals of the lyotropic cubic liquid crystals in equilibrium with a humid atmosphere is observed. Experiments reveal the presence of more than 60 different types of facets on the surface of a spherical crystal of radius R = 1 mm. The devil's staircase type of faceting has been predicted theoretically when the interaction between steps on the crystal surface is repulsive.