A Detailed Study of the Synthesis of Aqueous Vanadium Pentoxide Nematic Gels

Vanadium pentoxide (V 2 O 5 ) gels are suspensions of V 2 O 5 ribbonlike particles dispersed in water. At volume fractions larger than 0.7%, they form a lyotropic nematic liquid-crystalline phase. We investigate here three different methods of synthesis (by ion-exchange, from peroxovanadic species, and by dissolution of V 2 O 5 powders) and suggest a common chemical mechanism giving rise to these V 2 O 5 ribbons. In addition, we investigate in detail the ion-exchange synthesis process by optical microscopy, time-resolved synchrotron small-angle X-ray scattering, rheology, and conductivity measurements. In particular, we relate the flocculation occurring during synthesis and the subsequent dispersion of V 2 O 5 condensed moieties to their electric surface charge. The fractal structure of the transient flocculate is demonstrated. We then give a complete physical description of the formation of the ribbons and their organization in a nematic phase. Finally, we try to describe this polymerization process by borrowing concepts devised to understand the self-assembly of amphiphilic molecules.