Effect of surfactant headgroups on the oil/water interface: An interfacial tension measurement and simulation study

In the article, four anionic surfactants with different headgroups and same alkyl tail, sodium dodecyl sulfonate (SDSn), sodium dodecyl sulfate (SDS), sodium dodecyl benzene sulfonate (SDBS), and sodium dodecyl-di(oxyethylene) ether sulfate (AES), are adopted to investigate the influence of headgroup structure on oil–water interfacial tensions. The measured experimental results present that the capability of the four surfactants reducing interfacial tension follows the order of AES > SDBS > SDS > SDSn. Furthermore, molecular dynamic simulation (MD) is conducted to investigate the interfacial property of the four surfactants, and three parameters, interface formation energy, interfacial thickness, interaction between surfactant and water, are proposed to reveal the effecting mechanism of molecular structure on interfacial tension. And then, the polarity is studied by quantum mechanics calculation (QM) to investigate the interaction between headgroup and water molecule. The researched results indicate the addition of oxygen, benzene ring and oxyethyl group would enhance the polarity of surfactant, which induce the increase of interaction between headgroup and water molecule. The inferred interfacial tensions from MD and QM follow the order of AES > SDBS > SDS > SDSn, which is according with the experimental results. The researches explore the correlation between interfacial tensions and different headgroup structures, and these results maybe have some references for designing of high-efficient surfactant.