Abstract
Neutron spin-echo spectroscopy and small-angle scattering measurementswere performed to determine how the isomeric structure and concentration ofC4-cosurfactants (i.e. butyl alcohols) influence structure and dynamics in four-componentwater-in-oil microemulsions. The system investigated was AOT/butanol/water/n-octane at roomtemperature (AOT denotes sodium di-2-ethyl hexylsulfosuccinate), deuterated to achieve contrastof 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 comparedwith samples prepared with tert-butyl alcohol and, in a few cases, sec-butyl alcohol. Datawere analysed using a core–shell model for polydisperse spherical droplets, allowing for thepresence of shape fluctuations. It was found that all structural isomers of the cosurfactantled to a similar decrease in droplet size with increasing alcohol content. In all cases, dropletsize and shape fluctuations were observed to increase with alcohol content; however,the effect was most pronounced for size fluctuations (i.e. polydispersity) in thepresence of tert-butanol. The data indicates that tert-butanol has a higher degree ofpenetration into the water core, leading to a reduced influence on the effective area persurfactant head group on the droplet surface. There is also evidence that an increaseddroplet–droplet attraction upon adding tert-butanol drives phase separation in thesystem.
Published Version
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