A detailed study of the synthesis of aqueous vanadium pentoxide nematic gels

Vanadium pentoxide (V2O5) gels are suspensions of V2O5 ribbonlike particles dispersed in water. At volume fractions larger than 0.7%, they form a lyot ropic nematic liquid-crystalline phase. We investigate here three different methods of synthesis (by ion-exchange, from peroxovanadic species, and by dissolution of V2O5 powders) and suggest a common chemical mechanism giving rise to these V2O5 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 s ubsequent dispersion of V2O5 condensed moieties to their electric surface c harge. The fractal structure of the transient flocculate is demonstrated. W e then give a complete physical description of the formation of the ribbons and their organization in a nematic phase. Finally, we try to describe thi s polymerization process by borrowing concepts devised to understand the se lf-assembly of amphiphilic molecules.