Spectroscopic Characterization of Solubilized Water in Reversed Micelles and Microemulsions:  Sodium Bis(2-ethylhexyl) Sulfosuccinate and Sodium Bis(2-ethylhexyl) Phosphate in n-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.

The effect of water on the microstructure and properties of benzene/[bmim][AOT]/[bmim][BF4] microemulsions

“…Besides, a small amount of water dissolved in nonpolar solvent is also considered to be trapped water. 52 As the trapped water molecules are matrix-isolated dimers or monomeric in nature, they absorb in the high frequency region. 51 The bound water molecules are hydrogen bonded with the polar headgroups of surfactants, which results in absorption in the low-frequency region of the IR spectrum.…”

The interaction between poly(vinylpyrrolidone) and reversed micelles of water/AOT/ n -heptane

“…The bound water has some unusual properties, such as high microviscosity, no freezing point, and blocking of the formation of hydrogen bonds [1]. The water in a reversed micelle is similar to the water in a biomembrane in many aspects [11], and hence much attention has been paid to the state and structure of the water in reversed micelles and microemulsions [12,13,14,15,16,17]. People hope to mimic biomembranes and to disclose the biological physics and biological chemistry phenomena by investigating the water state in the reversed micelle and microemulsion.…”

Study of Ionic Liquid Microemulsions: Ethylammonium Nitrate/TritonX-100/Cyclohexane

“…The unusual behavior of the water in reverse microemulsions is attributed to its strong interaction with the hydrophilic groups of the surfactant molecule and the disruption of the three-dimensional hydrogen bonded network. The hydration affinity of surfactant head groups can be regarded as the driving force in the formation of microemulsion aggregates [30]. In aqueous microemulsions with the nonionic surfactant TX-100, an increase in water content induces the generation of an extended network of OE-water and water-water hydrogen bonds.…”