Different Submicellar Solubilization Mechanisms Revealed by 1H NMR and 2D Diffusion Ordered Spectroscopy (DOSY)

Different submicellar solubilization mechanisms of two systems Triton X-100 (TX-100)/tetradecane and sodium dodecyl sulfate (SDS)/butyl methacrylate are revealed on the molecular scale by 1H NMR spectroscopy and 2D diffusion ordered spectroscopy (DOSY). It is evident that both the apparent solubility of tetradecane and butyl methacrylate are enhanced, even at very lower surfactant concentrations than CMCs. Solubilized solutes also contribute to the early formation of surfactant micelles. In general, the molar solubilization ratios (MSRs) of both solutes linearly increase as the surfactant concentration increase. However, variations in MSRs of two systems are different at below and above the CMC, which are probably related to the different solubilization mechanisms. For TX-100/tetradecane, as the TX-100 concentration increases, the tetradecane resonance in the monomeric state transforms into the aggregated state one and the corresponding evolution of diffusions are shown in 2D DOSY spectra. These results demonstrate that below the CMC, tetradecane is first solubilized in TX-100 solutions, and then solubilized in TX-100 micelles above the CMC. For SDS/butyl methacrylate, the appearance of oligomeric SDS resonances below the CMC indicates butyl methacrylate is mainly solubilized in SDS oligomers. Then, when the CMC is reached, the dominant, monomeric SDS molecules aggregate into oligomers, the similar diffusivity trend of butyl methacrylate with that of SDS indicate that a proportion of butyl methacrylate molecules are solubilized in it. Finally, the fusing of SDS resonances in two states and the tendency of co-diffusion of SDS and butyl methacrylate indicate all SDS molecules gradually aggregate into micelles, and almost all butyl methacrylate molecules are solubilized in them. In conclusion, above CMCs, the solubilization manners of these two systems are similar. However, they are different below CMCs. The solubilization of tetradecane by TX-100 is driven by the intermolecular hydrophobic interaction, i.e., molecular-pair formation. However, the polar interaction between functional groups of butyl methacrylate and the polar head of SDS contributes to the solubilization of butyl methacrylate. The different submicellar solubilization mechanisms are mainly caused by different properties of solutes and surfactants, which also results in different solubilization sites in micelles.