The Effects of Dynamic Noncovalent Interaction between Surfactants and Additional Salt on the pH-Switchable Interfacial Tension Variations

The dynamic noncovalent interaction between the anionic surfactant sodium dodecyl benzene sulfonate (SDBS) and 1,3-diphenylguanidine (DPG) was employed to control the interfacial activity of the surfactant. At high HCl concentration (1000 mg L⁻¹), the SDBS/DPGⁿ⁺ system could reduce the water/oil interfacial tension (IFT) to 10⁻⁴ mN m⁻¹ order of magnitude, which was much lower than the IFT values in the SDBS/DPG⁺ system with a low HCl concentration (100 mg L⁻¹) and the individual SDBS system by three and four orders of magnitude, respectively. The pH-switchable protonation of amido groups in DPG molecules determines the SDBS/DPG molecular interaction and the amplitude of IFT reduction, which was confirmed by control experiments using two other surfactants (sodium dodecyl sulfate SDS] and dodecyl trimethylammonium bromide DTAB]). Moreover, the investigation of the NaCl and temperature effects on the IFT indicated the intensity of mixed SDBS/DPGⁿ⁺ adsorption layers at the water/oil interface.