Effect of the alkyl chain length of a hydrophobic ionic liquid (IL) as an oil phase on the phase behavior and the microstructure of H2O/IL/nonionic polyoxyethylene surfactant ternary systems

Abstract

The use of hydrophobic ionic liquids (ILs) as oil phases to formulate a H₂O-in-IL (W/IL) microemulsion is of great significance to enzyme catalysis. In this report, we present the essential influence of the alkyl chain length of ILs on the phase behavior and the microstructure of H₂O/IL/C_nE_m (polyoxyethylene alkyl ether) ternary systems. The ILs are [C₄mim][PF₆] (1-butyl-3-methylimidazolium hexafluorophosphate) and [C₈mim][PF₆] (1-octyl-3-methylimidazolium hexafluorophosphate). For a given nonionic surfactant, the alkyl chain length of ILs has an influence on the water solubilization capacity. The difference in the capacity, however, narrows and even disappears upon significant increase of the hydrophilic chain length of C_nE_m. FTIR spectroscopy and UV-Vis spectroscopy confirm the existence of bulk water in both [C₄mim][PF₆]- and [C₈mim][PF₆]-based C₁₂E₂₃-stabilized W/IL microemulsions. The catalytic activity of laccase hosted in the W/IL microemulsion and the dynamic light scattering measurement reveal that under the same conditions, the water droplet size dispersed in [C₄mim][PF₆] is larger than that in [C₈mim][PF₆]. To establish the newly developed W/IL microemulsion as an effective medium for laccase, a control study is presented, demonstrating that the W/IL microemulsion is superior to the pure ILs for the expression of the catalytic activity of laccase.