Evidence of Water-in-Ionic Liquid Microemulsion Formation by Nonionic Surfactant Brij-35

Abstract

Brij-35, a common and popular nonionic surfactant, is shown to form water-in-ionic liquid (w/IL) microemulsions with IL 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim]­[PF₆]) as the bulk phase. The presence of w/[bmim]­[PF₆] microemulsions is hinted by the significantly increased solubility of water in Brij-35 solution of [bmim]­[PF₆]. The formation of w/[bmim]­[PF₆] microemulsions by Brij-35 is confirmed using dynamic light scattering (DLS) and small-angle X-ray scattering (SAXS) measurements. Brij-35 forms reverse micelle-type aggregates within [bmim]­[PF₆] in the absence of added-water. These reverse micelles become w/[bmim]­[PF₆] microemulsions as the water is added to the system. As the water loading (<i>w</i>₀) is increased, the average diameter of the aggregates increases. Fourier transform infrared (FTIR) absorbance data reveal the presence of both “bound” and “free” water within the system. The “bound” water is associated with the water pools of the w/[bmim]­[PF₆] microemulsions. Excited-state proton transfer (ESPT) involving probe pyranine shows deprotonation of pyranine within the water pools of the w/[bmim]­[PF₆] microemulsions