Microstructural Study of Aqueous Solutions of Octadecylamide Oligo(oxyethylene)ether

Abstract

The nonionic surfactant octadecylamide oligo(oxyethylene)ether is soluble with water, giving rise to an isotropic solution phase over the entire concentration range at 298 K. Solution properties of surfactants have been investigated by applying several experimental techniques. The existence of nonspherical micelles is indicated by a large broadening of the 1H NMR linewidths. The surfactant self-diffusion coefficient determined by the pulse gradient spin echo NMR method identifies the presence of two types of micellar aggregates—small spherical micelles in coexistence with large nonspherical micelles. The radii of spherical micelles (20–30 Å) are almost independent of surfactant concentration. The nonspherical micelles formed at a very high dilute solution grow dramatically in size with surfactant concentration until a maximum (≈20% surfactant) is reached, after which there is a slow decrease in micellar size. Direct imaging by the cryo-TEM technique also identifies a mixture of spheroidal- and thread-like micelles in dilute solutions. The solution exhibits high viscosity which increases rapidly with concentration, (maximum at ≈20%) and then decreases again. Stress vs strain flow curves show the characteristics non-Newtonian flow of the pseudoplastic type. High viscosity measured at low strain decreases to a concentration-independent constant low value at high shear strain.