Comparative Study on Structure of Solubilized Water in Reversed Micelles. 1. FT-IR Spectroscopic Evidence of Water/AOT/<i>n</i>-Heptane and Water/NaDEHP/<i>n</i>-Heptane Systems

Abstract

The interactions of water with the polar headgroups in the reversed micelles of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) and sodium bis(2-ethylhexyl) phosphate (NaDEHP) have been studied by means of Fourier transform infrared spectroscopy (FT-IR). O−H stretching bands of water in both systems varied with water content (W0, the molar ratio of water to surfactant). When W0 increased from 1 to 20, the O−H stretching frequency shifted from 3493 to 3416 cm-1 in AOT formed reverse micellar system and from 3385 to 3417 cm-1 in NaDEHP system, approaching the frequency of the bulk water. As a response to water solubilization, frequency decreases of sulfonate and phosphate stretching mode in AOT and NaDEHP reverse micellar solutions have also been observed, indicating hydrations of the two surfactants. Results showed that the water molecules encapsulated in reversed micelles are similar to that in the bulk form when W0 > 16 in AOT system and W0 > 12 in NaDEHP system. In addition, the splitted peaks of the asymmetric sulfonate stretching band were found shifting to lower frequencies upon hydration, due to the weakening of the sodium−sulfonate interaction after hydration and an associated increase in their spatial separation.