Location of probe molecule in double-chain surfactant aggregates in absence and presence of water-soluble polymer by NMR

Abstract

The location of the aromatic molecule, benzyl alcohol, in two structurally similar surfactants, sodium bis(2-ethylhexyl) sulfosuccinate (AOT) and sodium bis(2-ethylhexyl) phosphate (NaDEHP), in the absence and presence of water-soluble polymer, poly(vinylpyrrolidone) (PVP) is investigated by NMR. According to the variation of the chemical shift of respective protons of AOT and NaDEHP molecules in the presence of probe molecules, it is interesting to find that most of the benzyl alcohol molecules are located around the surface of AOT micelles, while they mainly incorporate in the hydrophobic chain or close to the core of NaDEHP micelles. All the proton chemical shifts of the two studied surfactants moved to higher fields with the increasing benzyl alcohol concentration. The chemical shifts of different protons for the studied surfactant–PVP system with the increasing content of benzyl alcohol are also investigated. In the AOT–PVP system it is found that PVP is wrapped on the surface of AOT micelles due to the weak electrostatic interaction between PVP and AOT molecules, which makes the benzyl alcohol molecules enriched around the surface of the micelles. However, for the NaDEHP-PVP system, the results indicate that PVP has no distinct effect on the location of probe molecules, that is, most of the benzyl alcohol molecules solubilized close to the core of the NaDEHP micelles or in the hydrophobic chain of NaDEHP molecules in the presence of PVP. This is in agreement with our previous results from the surface tension and dynamic light scattering measurements. This study provides important insight into the probe molecules location in the double-chain surfactant micelles in the presence of water-soluble polymer and will be helpful for the application of amphiphile self-assembly in drug delivery.