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.