Assembly of micelles into higher-order structures with increasing salt concentration

Abstract

Concentrated aqueous surfactant solutions of ∼50% by weight frequently form liquid crystalline mesophases1,2 having repeat distances of ∼50 A. At lower surfactant concentrations, however, the ordering is lost and micelles3 are formed. We present neutron scattering and nuclear magnetic resonance observations of dilute (∼ 1–10% by weight) aqueous ionic surfactant/co-surfactant solutions which show that upon the addition of salt, disperse micellar aggregates assemble into mesophases of higher order which have repeat distances of up to 1,000 A. This raises the question as to the structure of these phases and the origin of the forces responsible for ordering over such large distances. Studies of this assembly type of behaviour may provide insight into the manner in which mixtures of simple surfactant-type molecules might form interconnected network structures such as are found in biological systems.