From Oil-Swollen Wormlike Micelles to Microemulsion Droplets: A Static Light Scattering Study of the L1 Phase of the System Water + C12E5 + Decane

The microstructure of the ternary system pentaethyleneglycol monododecyl ether (C 12 E 5 ), decane and water was investigated by static light scattering in the water-rich region of the phase diagram at constant temperature (22 °C). Micellar size and shape were determined along several pseudobinary sections of constant oil-to-surfactant ratio. The main focus was on dilute solutions ( c m ranging from 0.0004 g cm -3 up to 0.3 g cm -3, where c m denotes the mass concentration of oil plus surfactant in the aqueous phase) and small oil content (α up to 0.35, where α denotes the mass fraction of oil in the oil + surfactant mixture). Our data analysis supports the presumed existence of wormlike micelles at low α. The concentration dependence of the apparent molar mass M app of the micelles can be represented over a wide concentration range (up to the semidilute regime) by a model that combines a power law M w ∝ c m a with the structure factor S ( q = 0) for flexible polymer chains in good solvents. For α ≤ 0.10 the fitted value of the growth exponent a agrees with the mean-field prediction ( a ≈ 0.5). With increasing oil content at given overall solute concentration c m the molar mass M w of the micelles increases up to a maximum near α = 0.07 but decreases rapidly as the oil content is further increased. This decrease of M w is due to a transition from elongated (wormlike) micelles to microemulsion droplets. At high oil content (α = 0.35) the data can be represented by a model of spherical droplets interacting like hard spheres.