A STRUCTURAL STUDY OF THE LYOTROPIC NEMATIC PHASE OF VANADIUM PENTOXIDE GELS

The aqueous suspensions of vanadium pentoxide ribbons are known to pre sent a lyotropic nematic phase. The optical microscopy photographs of these suspensions in the gel and the sol states shown in this paper ar e indeed typical nematic textures. The biphasic (I/N) region shows tac toids which can be aligned by a magnetic field. Small angle X-ray scat tering (SAXS) experiments were performed either on unoriented samples or on samples aligned by shear flow. The scattering is comparable to t hat of the other classic lyotropic nematic phases displayed by stiff o rganic rod-like particles. Scattering patterns give information on the transverse dimensions of the ribbons and on the average distance betw een them as a function of concentration. Estimates of the nematic orde r parameter were also obtained. Finally, the relevance of Onsager's mo del is discussed; the importance of the electrostatic interactions bet ween ribbons in the onset of the nematic ordering of this system is st ressed.