Spectroscopic Characterization of Solubilized Water in Reversed Micelles and Microemulsions: Sodium Bis(2-ethylhexyl) Sulfosuccinate and Sodium Bis(2-ethylhexyl) Phosphate in <i>n</i>-Heptane

Abstract

The states and structure of the solubilized water in reversed micelles and microemulsions of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) and of sodium bis(2-ethylhexyl) phosphate (NaDEHP) in n-heptane have been characterized by FT-IR and NMR spectroscopic parameters. According to the four-component hydration model, the free, anion-bound, bulklike, and cation-bound water are present in reversed micelles and both of the water-in-oil (W/O) microemulsions formed by AOT and the bicontinuous microemulsions formed by NaDEHP in n-heptane. The observed chemical shifts (δ) of the water protons from NMR spectra were expressed as the weighted average of the in-core anion-bound, bulklike, and cation-bound water. Chemical shifts δ for individual components were evaluated from molar fractions, which were obtained by deconvolution of the O−H stretching vibrational absorption bands, and the observed δ. Results show that in W/O microemulsion of AOT in n-heptane and bicontinuous microemulsion of NaDEHP in n-heptane, the chemical shifts for individual components exhibit constant values, indicating stable microstructure for the given species, which can be considered as the criterion of either W/O or bicontinuous microemulsions. Results also show that in reversed micelles of both AOT and NaDEHP, the O−H bond strength and thereby the microstructure of different hydration species vary with water content, which can be explained by the interaction between electrical double layers. In transition of reversed micelles to microemulsions, the microstructures of water molecules transform from the variable state to a stable state.