Micellization of selenium-containing cationic surfactants with different headgroups in aqueous solution

Redox-responsive surfactant based on selenium atom has recently attracted considerable interests. Herein, we have designed and synthesized three selenium-containing cationic surfactants with different headgroups, including N-(benzylselanyl-undecyl)-N,N,N-trimethyl ammonium bromide (BSeTAB)/pyridinium bromide (BSePyB)/methylpyrrolidinium bromide (BSeMPB). Their micellization in dilute solution was investigated by surface tension and conductivity, and was related to redox-induced change in molecular structure. Both ¹H NMR and ESI-MS spectra demonstrated that selenide in the reduced form is transformed into selenoxide after oxidation with H₂O₂, whatever the headgroup is. The formation of selenoxide not only increases the hydrophilicity of surfactant, resulting in a more negative \( \Delta {G}_{\mathrm{ads}}^o \), larger cmc, cmc/C₂₀ ratio, and G_min, compared to the reduced forms, but also provides a new polar headgroup, making it behave like bola-type structure and enhancing the hydrophilicity. The relatively smaller cmc, larger cmc/C₂₀ ratio, and more negative \( \Delta {G}_m^o \) value all revealed that the pyridinium or pyrrolidinium polar headgroup is more favorable for the micelle formation than the trimethylammonium group. The thermodynamic parameters showed that the micellization is transformed from entropy-driven mode to enthalpy-driven mode with increasing temperature and the transition temperature at which \( \Delta {H}_m^o \) equals to \( -\mathrm{T}\Delta {S}_m^o \) is lowest for BSeMPB, and highest for BSePyB.