Quaternary ammonium bromide surfactant oligomers in aqueous solution: Self-association and microstructure

Surfactant oligomers are made up of x (greater than or equal to 2) amphiphi lic moieties connected at the level of, or close to, the headgroups by spac er group(s). This paper examines the effect of the degree bf oligomerizatio n s on the self-assembling of cationic surfactant oligomers at interfaces a nd in the bulk. The quaternary ammonium bromide surfactant oligomers invest igated are made up of dodecyldimethyl- and dodecylmethylammonium bromide mo ieties connected by short polymethylene spacers -(CH2)(s)- (of carbon numbe r s). The properties investigated are the surface occupied by the surfactan t at the air/solution and silica/solution interfaces, the critical micelliz ation concentration (cmc), the micelle ionization degree at the cmc, the mi celle micropolarity and microviscosity, and the microstructure and rheology of the solution. As x is increased, the surfactant layers at interfaces be come more dense, the cmc decreases, the micelle microviscosity increases, t he micelle shape for oligomers with s = 3 changes from spherical (s = 1) to wormlike (x = 2), branched wormlike (x = 3), and ring like (x = 4). Last t he zero-shear viscosity of the oligomer aqueous solutions starts to increas e very rapidly and by orders of magnitude at surfactant concentrations abov e a value C* that decreases as x is increased. On the contrary the micelle ionization degree at the cmc and micelle micropolarity are nearly independe nt of x. These results are discussed in terms of surfactant packing paramet er. They emphasize once more the possibilities offered by surfactant oligom ers in obtaining surfactant organized assemblies with new architectures and solutions with improved and adjustable properties.