Quaternary Ammonium Bromide Surfactant Oligomers in Aqueous Solution:  Self-Association and Microstructure

Surfactant oligomers are made up of x (≥2) amphiphilic moieties connected at the level of, or close to, the headgroups by spacer group(s). This paper examines the effect of the degree of oligomerization x on the self-assembling of cationic surfactant oligomers at interfaces and in the bulk. The quaternary ammonium bromide surfactant oligomers investigated are made up of dodecyldimethyl- and dodecylmethylammonium bromide moieties connected by short polymethylene spacers −(CH2)s− (of carbon number s). The properties investigated are the surface occupied by the surfactant at the air/solution and silica/solution interfaces, the critical micellization concentration (cmc), the micelle ionization degree at the cmc, the micelle micropolarity and microviscosity, and the microstructure and rheology of the solution. As x is increased, the surfactant layers at interfaces become more dense, the cmc decreases, the micelle microviscosity increases, the micelle shape for oligomers with s = 3 changes from spherical (x = 1) to wormlike (x = 2), branched wormlike (x = 3), and ring like (x = 4). Last the zero-shear viscosity of the oligomer aqueous solutions starts to increase very rapidly and by orders of magnitude at surfactant concentrations above a value C* that decreases as x is increased. On the contrary the micelle ionization degree at the cmc and micelle micropolarity are nearly independent of x. These results are discussed in terms of surfactant packing parameter. They emphasize once more the possibilities offered by surfactant oligomers in obtaining surfactant organized assemblies with new architectures and solutions with improved and adjustable properties.