Comparative studies on the surface/interface properties and aggregation behavior of mono-rhamnolipid and di-rhamnolipid

The surface properties of mono-rhamnolipid (Rha-C10-C10, R1) and di-rhamnolipid (Rha-Rha-C10-C10, R2) were investigated after separation and purification. The effects of environmental factors on equilibrium surface tension of these surfactants were studied by changing the temperature, salinity, and pH. Results show that R1 possesses a better surface activity than R2, but both are stable at low pH values and high temperature. Moreover, the diffusion and adsorption processes of R1 and R2 were studied by dynamic surface tension measurements. The initial adsorption processes of R1 and R2 were diffusion-controlled, and the effective diffusion coefficient of R1 was higher than that of R2 at the same concentration. We also monitored the dynamic interfacial tension curves of R1 and R2 with or without aging at high temperature, revealing that both feature high temperature resistance, but R1 exhibited a better interfacial activity than R2. For aggregation behavior in the bulk phase, dynamic light scattering and UV - vis spectrophotometry were used to measure and observe the aggregation of rhamnolipids R1 and R2 at different temperatures and pH values. Results show the vesicle-to-micelle transformation of R1 and R2 aggregates with decreasing pH. This result is attributed to the considerable influence of solution pH to the dissociation degree of rhamnolipids. Thus, pH values significantly influence particle size distribution.