Comparative Study on the Structure of Reverse Micelles. 2. FT-IR, ¹H NMR, and Electrical Conductance of H₂O/AOT/NaDEHP/n-Heptane Systems

Abstract

The microstructure of mixed reverse micelles stabilized with surfactants of sodium bis(2-ethylhexyl)sulfosuccinate (aerosol OT, or AOT) and/or sodium bis(2-ethylhexyl)phosphate (NaDEHP) has been studied by means of Fourier transform infrared spectroscopy (FT-IR), proton nuclear magnetic resonance (1H NMR), and electrical conductometry. The O−H stretching band in IR and the chemical shift in 1H NMR of water solubilized in the reverse micelles change concomitantly with both water content (W 0, the number of solubilized water molecules per amphipathic molecule) and relative content of surfactants (AOT and NaDEHP). As in the reverse micelles stabilized with AOT and AOT + NaDEHP (50:50), the maxima of conductivity were observed in respective conductivity vs water content (κ−W 0) curves below the percolation threshold, while the conductivity of NaDEHP-formed reverse micelles changed proportionally with water content. The striking point is that, at a certain water content, the conductivity of the H2O/AOT/NaDEHP/n-heptane system decreases with increasing temperature from 0 to 55 °C, a rare phenomenon in solution. The changes are attributed to the hydration of surfactant ions plus the morphology change of surfactant aggregates in respective systems.

Comparison between Two Anionic Reverse Micelle Interfaces: The Role of Water–Surfactant Interactions in Interfacial Properties

“…At W 0 = 1 the n OH values obtained were 3495, 3368, and 3417 cm À1 for AOT/toluene, NaDEHP/toluene, and bulk water, respectively. Similar values were obtained at W 0 = 1 for AOT/nheptane and NaDEHP/n-heptane systems by Li et al [18,20] The fact that the n OH value is higher in AOT/toluene RMs than in bulk water confirms that the hydrogen-bond network present in pure water is destroyed upon encapsulation in AOT RMs, as is already known. [3,6,21] On the other hand, the n OH value is lower in the NaDEHP/toluene systems than in bulk water, thus suggesting that the water hydrogen bond is enhanced upon confinement by NaDEHP RMs.…”

“…[20] The 1 H NMR water signal shows a downfield shift in the water/ AOT/n-heptane RMs, whereas it exhibits an upfield chemical shift in the NaDEHP system with an increase of the water content. The phenomenon of opposite tendency is also observed in the infrared spectra.…”

“…Li et al performed a comparative study of water/NaDEHP/nheptane and water/AOT/n-heptane by means of FTIR [18][19][20] and 1 H NMR spectroscopy, [19,20] and electrical conductimetry. [20] The 1 H NMR water signal shows a downfield shift in the water/ AOT/n-heptane RMs, whereas it exhibits an upfield chemical shift in the NaDEHP system with an increase of the water content.…”

The Effect of Water on the Microstructure of 1‐Butyl‐3‐methylimidazolium Tetrafluoroborate/TX‐100/Benzene Ionic Liquid Microemulsions

“…[29,34,[53][54][55] The characteristics of the water molecules incorporated into reverse micelles depend strongly on the water content and the nature of the surfactant headgroups. [56] Generally, solubilized water molecules in microemulsions have three distinct states: trapped water, bound water, and free water. The trapped water, with the O À H stretching vibration at about 3600 cm À1 , is defined as the water species dispersed among the long hydrocarbon chains of the surfactant molecules.…”

Electrical Conductivity of Water/Sodium Bis(2-ethylhexyl) Sulfosuccinate/n-Heptane and Water/Sodium Bis(2-ethylhexyl) Phosphate/n-Heptane Systems: The Influences of Water Content, Bis(2-ethylhexyl) Phosphoric Acid, and Temperature

“…Thermodynamic and spectroscopic properties of surfactant aggregates have received extensive study by employing a variety of physico-chemical techniques, such as IR (3)(4)(5), NMR (6,7), and fluorescence spectroscopy (8,9), electrical conductometry (10)(11)(12), etc. Among these, electrical conductometry provides a particularly convenient and useful tool for probing the structure and phase phenomena of reverse micelles/microemulsions.…”