The effect of water on the microstructure and properties of benzene/[bmim][AOT]/[bmim][BF4] microemulsions

Abstract

In the present contribution, results concerning the role of small amounts of water in the 1-butyl-3-methylimidazolium bis 2-ethylhexyl sulfosuccinate ([bmim][AOT]) based 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF₄])-in-benzene reverse microemulsions are emphasized. The microemulsion aggregates have demonstrated features in common with traditional aqueous systems, such as a normal microemulsion droplet size and decreased stability due to the addition of a polar component. Dynamic light scattering (DLS) showed that the size change of microemulsion with added water depends on the loaded [bmim][BF₄] content in the microemulsion: when the [bmim][BF₄] content is low, the microemulsion diameter first decreases and then increases, while the size remains about the same for microemulsions with a moderate [bmim][BF₄] loading and a successive increase in size was found for high-loaded [bmim][BF₄] microemulsions. ¹H NMR along with two-dimensional rotating frame nuclear Overhauser effect (NOE) experiments (ROESY) revealed that water molecules formed wide interactions with both 1-butyl-3-methylimidazolium ([bmim]) and bis 2-ethylhexyl sulfosuccinate ([AOT]), leading to a decrease in the headgroup area of [bmim][AOT], i.e. α value, which will decrease the microemulsion size. On the other hand, addition of water can simultaneously swell the microemulsions, causing an increase in the diameter. It is also deduced that the Coulomb forces between the [AOT] and [bmim] should be one of the main driving forces for the formation of [bmim][BF₄]-in-benzene microemulsions.