Interface-subphase interactions of rhamnolipids in aqueous rhamnose solutions

Effects of the presence of rhamnose (RH) sugar in the bulk phase on the interfacial properties of rhamnolipids (RL) are investigated in this work. Rhamnolipid compositions chosen to be studied were R1, R2, and their mixtures in the ratios of R2/R1 = 1.2 and R2/R1 = 1.07. Rhamnose in the bulk phase had a constant ratio (RH/RL) of 0.5, 1, 2, or 3 to the rhamnolipids. The results obtained from surface tension measurements were supported by surface shear viscosity and conductivity measurements. The interactions between rhamnolipids were confirmed with photographs taken under cross-polarized light. The overall effect of these properties was observed in the foam ability and stability of the solutions. The effect of rhamnose in the bulk phase is dependent on the composition of the monolayer; significant variations are observed with pure R1 and R2. In the rhamnolipid mixtures, rhamnose molecules penetrate into the monolayer at very low concentrations. At higher rhamnolipid concentrations, rhamnose molecules form bridges between the rhamnolipid molecules, lending rigidity to the monolayer.