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

Aqueous suspensions of V2O5 ribbons form a lyotropic nematic phase at volum e fractions larger than 0.7%. We investigate the effect of adding a monoval ent salt (NaCl) to this charge-stabilized system. Without any added salt, t he nematic ordering can be described by the Onsager model based on a hard-c ore repulsive potential. Adding salt induces the gelation of these suspensi ons. Very unexpectedly, it also induces the stabilization of the nematic ph ase 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 observati ons indicate the increasing influence of attractive interactions which we a rgue are most probably van der Waals attractions.