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

Abstract

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...