Effect of pH on the surface and interfacial behavior of rhamnolipids R1 and R2

Abstract Surface and interfacial tensions of pure R1 and R2 solutions at the pH values of 6.8 and 5 were comparatively studied in this work, with decane and hexadecane as the oil phase. Conductivities were measured as a function of concentration to supplement the surface tension results. Photographs of these solutions under cross-polarized light were taken to show the existence of liquid crystals. Foaming ability and stability of pure rhamnolipid solutions were determined using a modified Bikerman device. Results of the measurements showed that R1 and R2 molecules form compact phases at the surface beginning from very low concentrations. R1 molecules are more surface active at concentrations below CMC independent of the bulk phase pH. Nevertheless, neither the value of CMC nor the minimum surface tension at CMC is affected by the type of rhamnolipid significantly. These depend only on the pH of the solution, since greater interactive forces exist between the undissociated rhamnolipid molecules at pH=5.0 resulting in a greater compaction at the surface monolayer. The interfacial behavior of R1 and R2 molecules at concentrations below CMC differ considerably with the structure of the molecules with which they interact at the interface: R1 molecules are more capable of reducing the interfacial tension than R2 when using decane as the oil phase instead of hexadecane. Strong intermolecular interactions between rhamnolipid molecules allow foam formation to occur only in a narrow range of air flow rates. Stability of the foam depends on the rhamnolipid type with R1 forming more stable foams. Maximum foam volume depends on the pH of the solution and not on the type of rhamnolipid.