Tracing the micro-process of co-aggregation between binary surfactants in aqueous solutions using 1H NMR

Abstract

The micellization process of binary mixed surfactants of different structures, where hexadecyl trimethyl ammonium bromide (CTAB)/polyethylene glycol (23) lauryl ether (Brij-35), CTAB/triton X-100 (TX-100), Sodium dodecylsulfate (SDS)/Brij-35 and SDS/TX-100 binary surfactants in aqueous solutions were studied by 1H NMR. The critical aggregation concentrations (CAC, i.e. the concentration at which the proton chemical starts to vary) of hydrophilic groups and hydrophobic groups of each individual component in the binary mixed surfactant solutions were gained by tracing the first inflection points on the 1D 1H NMR spectra of chemical shift with concentration, which were compared with those in their single-state systems. Results indicate that in contrast to those in their single states, where self-aggregation of the terminal methyl protons of the hydrophobic side chains start first, mixing CTAB with TX-100, and SDS mixed with Brij-35, showed that the hydrophilic head groups of the pair mixed surfactants co-aggregate synchronously first, forming mixed pre-micelles, due to the attractive electrostatic interaction between the positively and negatively charged hydrophilic groups of these pair mixed surfactants. It shows that the electrostatic attraction is more effective than the hydrophobic interaction in lowering the free energy of these binary mixed surfactant solutions. However, when CTAB is mixed with Brij-35 and SDS is mixed with TX-100, where the hydrophilic groups in the pair mixed surfactants are of the same charge, the electro-static repulsion forces the hydrophobic interaction of the terminal methyl groups of the surfactant, having low CAC, self-aggregate first, forming self-aggregated pre-micelles, i.e. the hydrophobic interaction is effective in lowering the free energies of these mixed surfactant solutions. The fact that although both TX-100 and Brij-35 belong to non-ionic surfactants, they showed different behaviors in mixing with CTAB and with SDS, shows that the traditional concept “type of surfactant” cannot be the only category to describe the behavior of a surfactant. A full description about the micro-process of aggregation between binary surfactants of different structures is thus obtained.