Effect of bacterial surfactants on the phase behavior of miscible pollutants in water

Abstract

Rhamnolipids are bacterial amphiphiles. In addition to emulsifying hydrophobic solvents, they affect the phase behaviour of miscible solvents. In mixtures of toluene and water, rhamnolipids mediate the migration of metal ions (e.g., iron and copper) from the water to the toluene phase. Also, rhamnolipids phase separate the miscible solvent tetrahydrofuran (THF) from water, yielding emulsions even in the absence of toluene. This is because they compete with THF for hydrogen (H) bonding with similar water species. Water is an ensemble of species, including single donors (SD) or double (DD) donors, and single acceptors (SA) or double (DA) acceptors. In pure water, SD-SA and DD-DA have similar abundance and are dominant, as shown by attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR). With 50–200 g/L rhamnolipids, a water species intermediate between SD-SA and DD-DA is dominant, indicating that rhamnolipid-water interactions mainly occur through this species. This same species primarily interacts with THF, at 50–80% THF. At lower THF percentages, a similar (albeit not identical) species dominates, namely DD-DA, explaining separation. Emulsions are THF in water, as demonstrated by synchrotron mid-infrared spectro-microscopy and confocal microscopy using a hydrophobic dye. Synchrotron small angle X-ray scattering (SAXS) showed that rhamnolipids self- assemble into micelles, which contain THF. These findings have potential implications for miscible pollutant migration in groundwater, and their toxicity to rhamnolipid-secreting bacteria.