Isomeric and concentration effects of C4-cosurfactants on four-component microemulsions investigated by neutron spin-echo and small-angle scattering

Abstract

Neutron spin-echo spectroscopy and small-angle scattering measurements were performed to determine how the isomeric structure and concentration of C4-cosurfactants (i.e. butyl alcohols) influence structure and dynamics in four-component water-in-oil microemulsions. The system investigated was AOT/butanol/water/n-octane at room temperature (AOT denotes sodium di-2-ethyl hexylsulfosuccinate), deuterated to achieve contrast of the surfactant/cosurfactant film. At a fixed volume fraction of 0.06 and a fixed molar ratio of [water]/[AOT] = 20, we studied the effects of increasing the molar ratio of [butanol]/[AOT] from 0 to 30. Data from samples containing the cosurfactant n-butyl alcohol were compared with samples prepared with tert-butyl alcohol and, in a few cases, sec-butyl alcohol. Data were analysed using a core–shell model for polydisperse spherical droplets, allowing for the presence of shape fluctuations. It was found that all structural isomers of the cosurfactant led to a similar decrease in droplet size with increasing alcohol content. In all cases, droplet size and shape fluctuations were observed to increase with alcohol content; however, the effect was most pronounced for size fluctuations (i.e. polydispersity) in the presence of tert-butanol. The data indicates that tert-butanol has a higher degree of penetration into the water core, leading to a reduced influence on the effective area per surfactant head group on the droplet surface. There is also evidence that an increased droplet–droplet attraction upon adding tert-butanol drives phase separation in the system.