The effect of attractive interactions on the nematic order of gels

¹ Laboratoire de Physique des Solides, Bât. 510, URA 002 CNRS, Université Paris Sud 91405 Orsay cedex, France
² Laboratoire pour l'Utilisation du Rayonnement Electromagnétique, Bât. 209D UMR 130 CNRS, Université Paris Sud - 91405 Orsay cedex, France
³ Laboratoire de Chimie de la Matière Condensée, URA 1466 CNRS Université Pierre et Marie Curie - 4 Place Jussieu, 75252 Paris cedex, France

Received: 3 May 1999
Accepted: 26 July 1999

Abstract

Aqueous suspensions of 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.