EVIDENCE FOR LOCAL ORIENTATIONAL ORDER IN SALT-FREE WORM-LIKE MICELLES - A TRANSIENT ELECTRIC BIREFRINGENCE STUDY

Transient electric birefringence experiments were performed on a aqueo us dispersions of salt-free charged giant micelles of a recently intro duced gemini surfactant (referred as 12-2-12). In the dilute regime, t he system consists of small rod-like micelles, while at higher concent rations (semi-dilute region) the micelles are semi-flexible and entang led. For concentrations far from c, either dilute or semi-dilute regi me, the birefringence has the same sign suggesting that the micelles a lign with the electric field in the same direction in both dilute and entangled regime. For concentrations close to c, an anomalous signal with an opposite sign is observed. For the entire surfactant concentra tion range studied here, we show that the static signals of the electr ic birefringence can be split into three different components with ver y different characteristic times, each component being associated to s pecific orientational mechanism: 1) a parallel alignment to the applie d electric field after free rotation in the dilute regime, or confined rotation of small segments in the semi-dilute regime, 2) an alignment perpendicular to the field that appears near c, and interpreted as t he result of pre-smectic ordering as recently proposed by Cates, 3) a parallel alignment after disentanglement occurring in the semi-dilute regime.