Viscoelastic micellar solution formed by a Se-based ionic liquid surfactant and its response to redox changes

Abstract

Redox-responsive colloidal systems based on selenium-containing molecules have recently attracted considerable attention. However, the effects of a Se atom and its site in a hydrophobic chain are yet to be fully characterized. In this study, we synthesized three ionic liquid surfactants with similar molecular structures (C_mSeC_nMPB, m = 14, 12, and 10, n = 2, 4, and 6), in which the spacers between the Se atom and the polar headgroup comprise 2, 4, and 6 CH₂ units, respectively. Their abilities to form worm-like micelles (WLMs) in the presence of sodium salicylate (NaSal) were investigated and compared to that of N-cetyl-N-methylpyrrolidinium bromide (C₁₆MPB) with no Se atom in its hydrophobic chain. The results showed that the incorporation of a Se atom endows the molecules with redox-sensitivity, and that its site also plays a crucial role. In the presence of NaSal, all C_mSeC_nMPB analogues self-assemble into viscoelastic WLMs. The viscosity enhancing abilities of the compounds followed an order of C₁₄SeC₂MPB-NaSal > C₁₂SeC₄MPB-NaSal ≈ C₁₆MPB-NaSal > C₁₀SeC₆MPB-NaSal, coinciding with the variation trend of micelle formation in one-component C_mSeC_nMPB solutions. Upon the addition of H₂O₂, the selenide was oxidized to selenoxide, enhancing the hydrophilicity and changing the molecular structure, and thus the viscoelastic WLM network was disrupted. Due to the different geometry adopted after oxidation, the critical concentration of H₂O₂ that was required to induce the gel-to-sol transition was also distinct. Only 0.1 equiv. of H₂O₂ was required for C₁₂SeC₄MPB-NaSal or C₁₀SeC₆MPB-NaSal, but 0.4 equiv. of H₂O₂ was required for C₁₄SeC₂MPB-NaSal. These viscoelastic fluids represent good candidates for drug delivery.