| Nuclear magnetic resonance(NMR)method has become to an important tool to study surfactant solubilization because of its advantages of multi-parameter,multimethod and non-invasive.Due to the amphiphilic nature,surfactants can solubilize hydrophobic organic compounds(HOCs),which leads to many important applications in areas of drug solubilization,soil remediation,and enhanced oil recovery.There has been a lot of research on its solubilization properties above the critical micelle concentration(critical micelle concentration,CMC),but the solubilization behavior below the CMC is still lacking at the molecular level.In this paper,we reveal two different submicellar solubilization mechanism systems of Triton X-100(TX100)/tetradecane and SDS/butyl methacrylate at the molecular scale by 1H NMR spectroscopy,2D diffusion ordered spectroscopy(DOSY)and NOESY.The main works are as follows:First,we summarize the effect of micellar solubilization and its application,and introduces the applications of NMR methods—chemical shift,2D DOSY and NOESY in micellar solubilization and research status of micellar solubilization of HOCs.Second,based on NMR methods,the solubilization mechanism of tetradecane in the non-ionic surfactant TX-100 solution was explored.The results indicate that even when the TX-100 concentration is much lower than CMC,the apparent solubility of tetradecane will increase,and tetradecane exhibits different molar solubilization ratios before and after TX-100 CMC,suggesting that there may be different solubilization mechanism.Tetradecane also contributes to the early formation of TX-100 micelles.With the formation of TX-100 micelles,the free-state tetradecane resonance peak gradually changed to an aggregated state,indicating that when TX-100 formed micelles,tetradecane was also added to the micelles.Below CMC,the solubilized tetradecane may be solubilized in the TX-100 solution by forming a molecular-pair with TX-100.Finally,the solubilization sites of tetradecane in TX-100 micelles were discussed through the cross-peak information of NOESY spectrum.Third,the solubilization mechanism of butyl methacrylate(Bu)in anionic surfactant SDS solution was studied based on NMR method.In the case where the SDS concentration is much lower than CMC,the apparent solubility of Bu will also increase.Different apparent solubility changes from TX-100/tetradecane indicate that SDS/Bu solubilization mechanism may be different.Similarly,the addition of Bu also led to the early formation of SDS micelles.The difference is that due to the addition of Bu,below the CMC,in addition to the resonance peak in the monomer state,the resonance peak in the aggregation state also appears in the SDS solution,indicating that a small amount of Bu is solubilized in a small portion of the SDS oligomers.When CMC is reached,the apparent diffusion coefficient of micelles formed by SDS decreases,and the downward trend of Bu’s apparent diffusion is similar to that of SDS,indicating that a certain proportion of Bu is solubilized in SDS micelles.Unlike tetradecane,Bu may be solubilized between the polar head of the SDS micelle and the barrier layer,which also leads to the instability of its solubilizing structure and is consistent with the relatively independent diffusion results of the two components. |
