Evidence for Local Orientational Order in Salt-Free Worm-Like Micelles: A Transient Electric Birefringence Study

(Received 1 April 1996, received in final form 14 June 1996, accepted 2 July 1996)

Abstract
Transient electric birefringence experiments were performed on a aqueous dispersions of salt-free charged giant micelles of a recently introduced gemini surfactant (referred as 12-2-12). In the dilute regime, the system consists of small rod-like micelles, while at higher concentrations (semi-dilute region) the micelles are semi-flexible and entangled. For concentrations far from c^*, either dilute or semi-dilute regime, the birefringence has the same sign suggesting that the micelles align 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 concentration range studied here, we show that the static signals of the electric birefringence can be split into three different components with very different characteristic times, each component being associated to specific orientational mechanism: 1) a parallel alignment to the applied 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 the result of pre-smectic ordering as recently proposed by Cates, 3) a parallel alignment after disentanglement occurring in the semi-dilute regime.