Water solubilization, conductivity and structural characteristics of single and mixed surfactant water-in-oil microemulsions in absence and presence of ionic liquids

• Addition of ionic liquids enhances solubilization of water in mixed microemulsions.
• Different solubilization sites of ionic liquids in microemulsions have been proposed.
• Ionic liquid acts as an effective interfacial curvature-control parameter than NaCl.
• Inclusion of ionic liquid shrinks the droplet size of mixed microemulsions.
• FTIR study reveals that ionic liquids behave as water-structure maker salt.

The present study is focused on the determination of the solubilization capacity and conductance behavior of mixed microemulsions [AOT/Brij-56 or Brij-58 or Brij-76 or TX-100 or Tween-80/Hp or Dc or IPM/water] in presence of ionic liquids (ILs) of different chemical structures and physicochemical properties [viz. bmimCl, hmimCl, bmimBF4 and BzmimCl] at different compositions (Xnonionic = 0, 0.1) at fixed surfactant concentration and temperature. Synergism in solubilization capacity has been evidenced in presence of ILs. All these systems exhibit volume-induced percolation of conductance. The maximum solubilization capacity (ω0,max), [IL]max (concentration of ILs at which synergism occurs) and volume-induced percolation threshold (ωp) have been found to influence by alkyl side-chain length, anion and polarity of ILs, and polar head group, hydrophobic moiety and content of nonionic surfactant and type of oils. Different solubilization sites of bmimCl and hmimCl have been proposed. ILs have been found to be more efficient additives than NaCl in respect of enhancing ω0,max with less [IL]max and reducing ωp. The microstructure of AOT and AOT/Brij-56 systems at different Xnonionic (= 0, 0.1) has been determined by DLS and FTIR measurements in absence and presence of ILs. Droplet diameter (dh) decreases and relative population of bound water increases with increase in [IL]. An attempt has been made to correlate the solubilization capacity in presence of ILs with percolation of conductance vis-à-vis droplet dimensions and the states of confined water to underline mechanism of solubilization phenomenon in these systems.

Schematic diagram of different solubilization sites of ionic liquids in mixed surfactant microemulsion.Figure optionsDownload as PowerPoint slide