Influence of chemical architecture of oils and ionic liquid on the physicochemical and thermodynamic properties and microenvironment of anionic surfactant based microemulsion

• Formation of microemulsions reveals exothermic with negative entropy change.
• Isobutyl benzene penetrates more easily in the surfactant palisade layer than R-(+)-limonene.
• Ionic liquid behaves as water structure maker salt due to increase in proportion of bound water.
• Polarity of oil affects microstructures of microemulsions.
• Rotation of fluoroprobe restricts in presence of ionic liquid due to rigidity of interfacial layer.

HypothesisAlthough several studies on water-in-hydrocarbon oil microemulsions stabilized by anionic surfactant are available in literature, such study on oils possess comparable molar volumes with different chemical architectures and physicochemical properties, herein R-(+)-limonene (LIM) and isobutyl benzene (IBB) derived microemulsions in absence or presence of ionic liquid (IL) warrants uniqueness of this report.ExperimentsThe present study delineates interfacial composition and thermodynamics properties of sodium dodecylsulfate, SDS/1-pentanol/LIM or IBB/water microemulsions in absence or presence of IL, 1-butyl-3-methylimidazolium chloride (bmimCl) as additive by the dilution method. Further, precise characterization of these systems is accomplished by conductivity and DLS studies as function of ω, [bmimCl] and temperature, which provide an insight into unique features of the oil/water interface. Finally, the influence of distinctive features of oils and the states of water organization in nanopool of these microemulsions have been explored from steady state fluorescence emission, fluorescence anisotropy and FTIR measurements.FindingsbmimCl containing microemulsions are more spontaneous than aqueous microemulsions. Addition of bmimCl yields smaller droplets compared to aqueous system. Fluorescence measurements reveal penetration of oil molecules in the surfactant palisade layer vis-a-vis interfacial micropolarity and their consequential effects in microenvironment of microemulsions in absence or presence of bmimCl, using suitable molecular probes. Apart from reciprocal variation in free and bound water with increasing water content, results exhibit mild variation in interfacial water (which is basically trapped water in the hydrophobic tail region of surfactant) by changing oil continuum. Finally, correlation of the results in terms of evaluated physicochemical and thermodynamic parameters has been presented.

Pictorial representation of different extent of penetration of R-(+)-limonene (LIM) or isobutyl benzene (IBB) in SDS derived microemulsions.Figure optionsDownload as PowerPoint slide