The effect of attractive interactions on the nematic order of V2O5 gels

Aqueous suspensions of V2O5 ribbons form a lyotropic nematic phase at volume fractions larger than 0.7%. We investigate the effect of adding a monovalent salt (NaCl) to this charge-stabilized system. Without any added salt, the nematic ordering can be described by the Onsager model based on a hard-core repulsive potential. Adding salt induces the gelation of these suspensions. Very unexpectedly, it also induces the stabilization of the nematic phase with respect to the isotropic one in a way opposite to the predictions of the Onsager model and to the usual behavior of typical hard-rod fluids. Larger salt concentrations eventually lead to flocculation. These observations indicate the increasing influence of attractive interactions which we argue are most probably van der Waals attractions.