The Interfacial Dilational Rheology of Surfactant Solutions with Low Interfacial Tension

In this paper, the spinning drop method was used to measure the oil-water interfacial dilational modulus of four different types of surfactants with low interfacial tension (IFT), including the anionic surfactant sodium dodecyl sulfate (SDS), the nonionic surfactant Triton X-100 (TX100), the zwitterionic surfactant alkyl sulfobetaine (ASB), and the extended surfactant alkyl polyoxypropyl ether sodium sulfate (S-C₁₃PO₁₃S). Based on the experimental results, we found that the spinning drop method is an effective means of measuring the interfacial dilational modulus of the oil-water interface with an IFT value of lower than 10 mN/m. For common surfactants SDS and TX100, the interfacial dilational modulus decreases rapidly to near zero with an increase in concentration when the IFT is lower than 1 mN/m. On the other hand, ASB has the highest interfacial dilatation modulus of 50 mN/m, which comes from the flatness of its unique hydrophilic group structure. The interfacial dilational modulus of S-C₁₃PO₁₃S showed a moderate plateau value of 30 mN/m with a broader concentration change. This is due to the fact that the main relaxation process dominating the interfacial film properties comes from the long helical polyoxypropyl chain. Through the large-size hydrophilic groups in betaine molecules and the long PO chains in the extended surfactant molecules, an interfacial film with controllable strength can be formed in a low IFT system to obtain a higher interfacial dilational modulus. This is of great significance in improving the emulsification and oil displacement of chemical flooding in reservoir pores.