A PM-IRRAS Investigation of Monorhamnolipid Orientation at the Air–Water Interface

The rhamnolipid biosurfactants have been considered as possible “green” alternatives to synthetic surfactants due to their greater compatibility with the environment and excellent surface active properties. In order to understand the molecular orientation of rhamnolipids at the air–water interface, a new monorhamnolipid with two octadecyl chains, Rha–C18–C18, has been studied at the air–water interface with polarization modulated-infrared reflection absorption spectroscopy (PM-IRRAS). Since rhamnolipids possess a carboxylic acid, and hence exhibit pH-dependent properties, their water surface orientation is studied in solutions of pH 2, 5, and 8. Rhamnolipids have also been reported to form strong complexes with Pb²⁺; thus, the effect of the presence of Pb²⁺ on molecular orientation at the interface is also investigated. PM-IRRA spectra indicate an increase in alkyl chain order and a decrease in alkyl chain tilt angle as the surface pressure of the monolayer increases, with pH-independent tilt angles ranging from 63° to 45°. Molecular modeling using Spartan provides insight into the cause of this large tilt angle as being due to the nature of the monorhamnolipid packing at the air–water interface as dictated by its large hydrophilic headgroup