Formation of Water-in-CO2 Microemulsions with Non-fluorous Surfactant Ls-54 and Solubilization of Biomacromolecules

Abstract

A non-fluorous and non-siloxane nonionic surfactant Ls-54 can form water-in-CO2 microemulsions, which can solubilize lysozyme. The figure shows the UV absorption band of lysozyme (276 nm) solubilized in 0.02 M Ls-54 based water-in-CO2 microemulsions with W0=8.0 at 308.15 K and 22.00 MPa. The solubility of Ls-54 surfactant in supercritical CO2 was determined. It was found that the surfactant was highly soluble in SC CO2 and the water-in-CO2 microemulsions could be formed, despite it being a non-fluorous and non-siloxane nonionic surfactant. The main reasons for the high solubility and formation of the microemulsions may be that the surfactant has four CO2-philic groups (propylene oxide) and five hydrophilic groups (ethylene oxide) and its molecular weight are relatively low. The results of this work provide useful information for designing CO2-soluble non-fluorous and non-siloxane surfactants. The phase behavior of the CO2/Ls-54/H2O system, solvatochromic probe study, and the UV spectrum of lysozyme proved the existence of water domains in the SC CO2 microemulsions. The method of synchrotron radiation small-angle X-ray scattering was used to obtain the structural information on the Ls-54 based water-in-CO2 reverse micelles. By using the Guinier plot (ln I(q) versus q 2) on the data sets in a defined small q range (0.022–0.040 Å−1), the radii of the reverse micelles were obtained at different pressures and molar ratio of water to surfactant, W0, which were in the range of 20.4–25.2 Å.