Nanostructure of Open Water-Channel Reversed Micelles. I. 1H NMR Spectroscopy and Molecular Modeling

Abstract

A recently proposed model for the rodlike reversed micelles of nickel(II) bis(2-ethylhexyl)phosphate is examined in greater detail using 1H NMR spectroscopy and molecular modeling. 1H NMR spectra show that the solubilized water molecules are situated in a different environment compared with the water molecules in classical (AOT) reversed micelles. Geometry optimization and molecular dynamics simulation clearly indicate that the water molecules are not located in the interior core of the reversed micelles, but instead the water molecules exist in compartments or channels in the surface of these rodlike reversed micelles, thereby confirming the open water-channel model of reversed micelles. Molecular modeling was also employed to examine the effects of surfactant molecular structure, cosurfactant, solvent aromaticity, and temperature on the nanostructure of the reversed micellar aggregates. It is significant that molecular modeling provides an interpretation of the nanostructure of reversed micellar aggregates that is consistent with a variety of known experimental observations reported in the liquid/liquid extraction literature. These findings show that the structure of reversed micelles is much richer at the nanoscale level than previously recognized.