Simulation Study on Molecular Behavior of Rhamnolipids and Biobased Zwitterionic Surfactants at the Oil/Water Interface: Effect of Rhamnose Moiety Structures.

Molecular behavior of rhamnolipid mixed with a biobased zwitterionic surfactant at an n-hexadecane/water interface has been studied, and the effects of a rhamnose moiety and composition are evaluated. Results showed that rhamnolipid abundantly interacts with biobased surfactant EAB by means of hydrophobic interactions between aliphatic tails and electrostatic interactions between headgroups, including the attractive interaction between COO- of rhamnolipids and N+ of biobased surfactants and the repulsive interaction between COO- of both surfactants. Dirhamnolipid has a larger number of bound Na+ and a more stable bound structure of COO- ∼ Na+, which screens the repulsive interaction between two kinds of surfactants and shows a more homogeneous distribution with biobased surfactants. The interfacial tension between n-hexadecane and water has been synergistically reduced by dirhamnolipids mixed with biobased surfactants at a higher molar ratio of biobased surfactants. Monorhamnolipids show a strengthened interaction with N+ of biobased surfactants and a more stable hydrogen bond with water relative to that of dirhamnolipids, and there is no synergistic effect in lowering the interfacial tension for the mixture of monorhamnolipids and biobased surfactants. The present work provides details of the molecular behavior of biosurfactant rhamnolipids mixed with biobased surfactants and obtains the key factor in affecting the interfacial properties of the binary system.

Study on the thermal washing of oily sludge used by rhamnolipid/sophorolipid binary mixed bio-surfactant systems.

Demulsification and crude oil desorption are usually a necessary step for the treatment of oily sludge in the petroleum industry. In this study a binary mixed bio-surfactant (rhamnolipid / sophorolipid, RL/SL) was used to strengthen the removing oil efficiency for oily sludge by thermal washing method. Surface tension values of the single and the mixed surfactants were carried out to investigate the effect of mixing systems on reducing critical micelle concentrations (CMC) value. The models proposed by Clint, Rubingh and Gibbs et al. had been employed to interpret the formation of mixed micelles and synergism and found out in case of the mass ratios of 4:6 the synergism was the strongest in RL and SL mixed surfactant systems, which was selected as the washing agents to treat the oily sludge produced from Huabei oilfield. Through the optimization of oil washing process parameters, the oil removal rate reached the maximum value (95.66%, residual oil rate 1.98%) at the condition of heating temperature of 45 °C, detergents concentration of 500 mg/L, washing time of 3 h, liquid/solid mass ratio of 1:4, stirring speed of 300 r/min, and washing 4 times. The factors affecting the oil washing effect were analyzed from the composition and performance characteristics of oily sludge samples, washing oil system and washing process parameters. The results showed that low oil content of oily sludge, small specific surface area, strong wetting and solubilization of the oil-washing system all can increase the oil-washing effect and the washing time and temperature had a great influence on the oil-washing effect. Compared with the results of other researchers, the oil washing temperature and the concentration of oil washing agent were significantly lower and high oil removal rate and low residual oil rate were obtained in this study. It was confirmed that thermal oil washing method using RT/SL binary bio-surfactant mixing system was proved to a high-efficiency, low-consumption and wide range of applications technology.