Solution Self-Assembly and Adsorption at the Air-Water Interface of the Monorhamnose and Dirhamnose Rhamnolipids and Their Mixtures

The self-assembly in solution and adsorption at the air water interface, measured by small-angle neutron scattering, SANS, and neutron reflectivity, NR, of the monorhamnose and dirhamnose rhamnolipicls (R1, R2) and their mixtures, are discussed. The production of the deuterium-labeled rhamnolipids (required for the NR studies) from a Pseudomonas aeruginosa culture and their separation into the pure RI and R2 components is described. At the air water interface, RI and R2 exhibit Langmuir-like adsorption isotherms, with saturated area/molecule values of about 60 and 75 angstrom(2), respectively. In R1/R2 mixtures, there is a strong partitioning of RI to the surface and R2 competes less favorably because of the steric or packing constraints of the larger R2 dirhamnose headgroup. In dilute solution ( < 20 mM), RI and R2 form small globular micelles, LI, with aggregation numbers of about 50 and 30, respectively. At higher solution concentrations, RI has a predominantly planar structure, La (unilamellar, ULV, or bilamellar, BLV, vesicles) whereas R2 remains globular, with an aggregation number that increases with increasing surfactant concentration. For R1/122 mixtures, solutions rich in R2 are predominantly micellar whereas solutions rich in RI have a more planar structure. At an intermediate composition (60 to 80 mot % RI), there are mixed La/L, and Li/La regions. However, the higher preferred curvature associated with R2 tends to dominate the mixed Rl/R2 microstructure and its associated phase behavior.