Solubilizing amino acids and polypeptides in supercritical CO2 via reverse micelle formation

Water-in-CO2 reverse micelles stabilized by ionic perfluoroalkyl and non-ionic perfluoropolyether surfactants were used to host amino acids and polypeptides in supercritical (SC) CO2. The minimum pressure at which micellization occurs (Ptrans) was found to be affected by the surfactant concentration and H2O/CO2 ratio. Due to the differences in hydrophilic/CO2 balance, the two surfactants exhibited different phase behaviors at 19.3 MPa and 40 °C. At certain H2O/CO2/surfactant compositions, while coagulation was more evident when using perfluoroalkyl surfactant, flocculation was found to dominate the system containing perfluoropolyether surfactant. The presence of amino acid and polypeptide in reverse micelles was found to increase the Ptrans of the system due to enthalpic and entropic changes. In perfluoroalkyl reverse micelles, the Ptrans was found to increase with increasing hydrophilicity of the amino acid added. The presence of hydrophobic moieties in amino acids promotes interfacial solubilization that eventually resulted in lower Ptrans. On the other hand, the nature of the amino acid was not found to affect Ptrans in perfluoropolyether reverse micelle. Overall, this work demonstrates that polypeptides with varying molecular weight could be solubilized in water-in-CO2 reverse micelles using perfluoropolyether while perfluoroakyl surfactants were ineffective.