Surface Chemical Properties and Micellization of Disodium Hexadecyl Diphenyl Ether Disulfonate in Aqueous Solution

Abstract

The surface tension of disodium hexadecyl diphenyl ether disulfonate (C₁₆-MADS) was measured at different NaCl concentrations (0.00–0.50 mol L⁻¹) and temperatures (298.0–318.0 K) using the drop-volume method. The results show that, with increasing temperature, the critical micelle concentration (CMC) of C₁₆-MADS increases slightly, but the maximum surface adsorption capacity (Γ _max) at the air–water interface decreases. When the concentration of NaCl was increased from 0.00 to 0.50 mol L⁻¹, the CMC of C₁₆-MADS decreased from 1.45 × 10⁻⁴ to 4.10 × 10⁻⁵ mol L⁻¹, but the surface tension at the CMC (γ _cmc) was not affected. When the concentration of NaCl was increased at 298.0 and 303.0 K, the Γ _max of C₁₆-MADS increased. When the temperature was increased from 308.0 to 318.0 K, the surface excess concentration (Γ _max) of C₁₆-MADS abnormally decreased from 2.26 to 1.41 μmol m⁻² with increasing NaCl concentration. The micellization free energy ($\Delta G_{\text{m}}^{ \circ }$) decreased from −63.98 to −76.20 kJ mol⁻¹ with increase of temperature and NaCl concentration. The micellar aggregation number (N _m) of disodium hexadecyl diphenyl ether disulfonate (C₁₆-MADS) was determined using the molecule fluorescence probe method with pyrene as probe and benzophenone as quencher. The results show that an appropriate N _m could be measured only at surfactant concentration above the CMC. The N _m increased with an increase in C₁₆-MADS concentration, but the micropolarity in the micelle nucleus decreased. The temperature had little effect on N _m. Compared with typical single hydrophilic headgroup surfactants, aggregates of C₁₆-MADS exhibit different properties.