Aqueous suspensions of vanadium pentoxide (V2O5) ribbons can be considered as a model system for a lyotropic nematic liquid crystal. Electron microscopy on dried samples shows that these mineral particles are roughly 1 nm thick, 10 nm wide and a few hundred nm long. Such sizes can be probed directly on aqueous suspensions using small-angle X-ray scattering (SAXS) techniques, which gives additional information about the nature of the interactions stabilising the system.

The polymorphism of these suspensions is essentially determined by their volume fraction [1]. Dilute suspensions are isotropic whereas more concentrated ones are nematic. This first order transition (at ~ 0.5%) is well described by the Onsager theory of uniaxial nematic ordering which is based on excluded volume interactions.

In the dilute isotropic phase, the analysis of the scattered intensity at very small scattering vectors (form factor analysis) allowed us to determine the width (25 nm), the persistence length (100-200 nm), and the chemical stability of the ribbons depending on pH and concentration [2]. In the nematic phase, investigations on single domains were carried out at scattering angles lower than that at which a correlation peak is observed. A small and very well collimated synchrotron beam is necessary to probe very small single domains and to observe their small angle scattering (Figure 98). In principle, for a nematic phase, this "central" scattering pattern is expected to be anisotropic. However, this has rarely been observed so far because the particles or molecules are usually small (as in thermotropic liquid crystals) so that the scattering anisotropy is too small. In contrast, Figure 98 clearly shows the SAXS anisotropy of these V2O5 suspensions. Besides, a careful analysis of the scattering along the nematic director direction provided an estimate of the contour (or overall) length of the ribbons (800 - 1000 nm) [2].

At larger volume fraction ( > 1.5%), the system exhibits a transition from a sol to a weak physical gel which is still nematic. At low volume fraction ( < 4%), the swelling of this gel is 2-dimensional (expected for a common uniaxial nematic phase) whereas, strangely enough, it is 1-dimensional at higher volume fraction ( > 4%). One way to prove the biaxiality suggested by such a behaviour is to study a single domain of the nematic phase, i.e. to macroscopically orient the sample. Unfortunately, the gel properties of the phase prevent its alignment by an electric or a magnetic field. Nevertheless, shearing these suspensions in a Couette cell was found to be an elegant and efficient means of achieving biaxial orientation. SAXS experiments under shear were performed on ID2 on two nematic gel samples of volume fractions = 2% and 5%. Two orthogonal configurations were defined by sending the X-ray beam either radially through the shear cell or tangentially (Figure 99). Combining the information obtained in these two configurations allowed us to analyse the symmetry of the scattering (Figure 100) and to prove the biaxial character of the more concentrated nematic phase [3]. Moreover, the thermodynamic and flow properties of this biaxial nematic phase are well described by recent theoretical models.

[1] a) P. Davidson, C. Bourgaux, L. Schoutteten, P. Sergot, C. Williams, J. Livage, J. de Physique II, 5, 1577 (1995). b) X. Commeinhes, P. Davidson, C. Bourgaux, J. Livage, Adv. Mat., 9, 900 (1997).
[2] O. Pelletier, C. Bourgaux, O. Diat, P. Davidson, J. Livage, submitted to Eur. Phys. J. B.
[3] O. Pelletier, C. Bourgaux, O. Diat, P. Davidson, J. Livage, Eur. Phys. J. B, Dec 1999, in press.

O. Pelletier (a), C. Bourgaux (b), O. Diat (c), P. Davidson (a), J. Livage (d).

(a) Lab. de physique des Solides, Université de Paris Sud, Orsay (France)
(b) LURE, Université de Paris Sud, Orsay (France)
(c) ESRF, and presently DRFMC/SI3M, CEA Grenoble (France)
(d) Lab. de Chimie de la Matiére Condensée, Univ. Pierre et Marie Curie, Paris (France)