Surfactant Free Microemulsions as Fluid Scaffolds for Thermal Stabilization of Lysozyme

Abstract

The electrostatic forces supported by hydrogen-bonding (H-bonding) interactions in the presence of surfactants stabilizes the microemulsions in general. It would be quite surprising to have surfactant free microemulsions (SFMEs) predominantly stabilized by weak but large number of H-bonding interactions contrary to common wisdom. Herein, the formulation and characterization of SFMEs comprising a hydrophobic ionic liquid (IL) and a deep eutectic solvent (DES) exhibiting high thermal stability is reported. The constituents of DES namely ethylene glycol (EG) and choline chloride (ChCl) act as polar and amphiphile components, respectively and an IL, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide works as a hydrophobic entity to form SFME. The formation mechanism as well as high temperature thermal stability of SFMEs has been discussed in terms of relative changes in the thickness of interfacial film stabilizing the polar and non-polar pseudo-domains predominantly via alteration in H-bonding interactions, which is supported by computational studies. The sufficiently low interfacial energy in SFMEs has been exploited to thermally stabilize Lysozyme (LYZ) in SFMEs, which showed remarkable thermal stability (up to 150 oC) as revealed by comparative enzyme activity at room temperature after heating, which is quite higher than that observed in buffer. The present study not only adds to the existing knowledge about the formation and stability of SFMEs but is also expected to prompt other researchers for designing relatively greener IL or deep eutectic solvent (DES) based SFMEs for utilization in various biological and other applications.