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.