pH-Switchable IFT variations and emulsions based on the dynamic noncovalent surfactant/salt assembly at the water/oil interface

Abstract

Additional HCl can facilely control the dynamic noncovalent interaction between anionic surfactant sodium dodecyl benzene sulfonate (SDBS) and additional organic matter, 4,4′-oxydianiline (ODA), at the water/oil interface. At low HCl concentration (ODA/HCl molar ratio (r) = 1 : 1.5, [ODA] = 250 mg L⁻¹), the ODA⁺ ions effectively enhanced the SDBS ability to reduce the water/oil interfacial tension (IFT) by about two orders of magnitude, while the (SDBS)₂/ODA²⁺ gemini-like surfactants could be constructed at a relatively high HCl concentration (r = 1 : 4, [ODA] = 250 mg L⁻¹), which could largely reduce the IFT to 1.19 × 10⁻³ mN m⁻¹. Molecular simulation was employed to explore the interfacial activity of ODAⁿ⁺ (ODA⁺/ODA²⁺) ions and the SDBS/ODAⁿ⁺ interaction. The control experiments used another three surfactants to verify the proposed model. The pH-switchable gradual protonation of amino groups in ODA molecules determined the SDBS/ODA interfacial assembly, which was responsible for the reversal of IFT variations and the related emulsion behaviors.