Influence of temperature and oil-to-surfactant ratio on micellar growth in aqueous solutions of C12E5 with decane

The influence of temperature and of added oil on the size and shape of micelles in the isotropic L1 phase of the system pentaethyleneglycol-monododecylether (C12E5)+water+decane has been investigated by static and dynamic light scattering. The micellar growth as a function of the solute concentration was studied for oil-free aqueous solutions of C12E5 and for oil-containing micellar solutions at fixed mass fraction (α=0.10) of the oil in the oil+surfactant mixture at three temperatures (13, 19 and 22°C). In the oil-free system, the mean molar mass of the wormlike micelles, Mw, decreases as temperature is lowered at fixed concentration. This trend is enhanced in the oil-containing system (α=0.10), and a transition from oil-swollen wormlike micelles to microemulsion droplets is observed when the temperature is lowered from 19 to 13°C. However, at 13°C the oil-swollen micelles are apparently not spherical, and droplet microemulsions that behave like a hard-sphere system are observed only when the oil-to-surfactant ratio is increased beyond α=0.10.