Effect of selenium-position on the redox responsivity of isomeric selenium-containing anionic surfactants

The effect of Se-position on the redox responsivity of three isomeric selenium-containing surfactants (CmSeCnSO4Na, n = 3, 8, and 11, m + n = 15) was investigated under the conditions of monomer, micelle, and adsorbed layer at the liquid paraffin-water interface. The divalent selenide group in CmSeCnSO4Na (the reduction form, CmSeCnSO4Na-Re) was oxidized to a more hydrophilic tetravalent selenoxide by the oxidation with H2O2 to afford CmSeOCnSO4Na (the oxidation form, CmSeCnSO4Na-Ox), and the selenoxide group of CmSeCnSO4Na-Ox could be reduced to selenide by the reduction with Na2SO3 to form CmSeCnSO4Na-Re again. In the oxidation process of the monomeric CmSeCnSO4Na in water, no obvious difference in the oxidation responsivity among three Se-position isomers was observed. In contrast, a remarkable effect of the Se-position on the oxidation responsivity was observed in the cases of CmSeCnSO4Na self-assemblies, including micelles and adsorbed layers at the liquid paraffin-water interface. Both the steric hindrance of the well-organized self-assemblies and the hydrophobic microenvironment of the selenide group in the self-assemblies hindered the Se atom from reacting with H2O2. The greater n, the harder was the oxidation of Se with H2O2. In the reduction process of CmSeCnSO4Na-Ox, the hydrophilic selenoxide group was exposed outright to water, allowing the selenoxide group to readily react with Na2SO3 regardless of whether CmSeCnSO4Na-Ox was self-assembled or not.