Use of NMR to probe the structure of water at interfaces of organized assemblies

This review addresses the use of NMR spectroscopy to probe the structure of interfacial water of organized assemblies: aqueous micelles, reverse micelles, RMs, and water-in-oil microemulsions, W/O mu Es. For aqueous micelles, the dependence of the H-1 NMR chemical shift of water on [surfactant] is measured in H2O-D2O mixtures. In case of RMs and W/O mu Es, one determines the dependence of H-1 NMR chemical shift of solubilized H2O-D2O, and/or H-1 and C-13 chemical shifts of the surfactant headgroup on the deuterium content of solubilized water. The measured deuterium isotope effect on the appropriate chemical shift is then used to calculate the so called ''deuterium/protium fractionation factor, phi'' for interfacial water. Values of phi thus obtained are rationalized in terms of effects of the interface on the structure of its water of hydration, relative to that of bulk water. The important conclusions of this review are: (1) Effects of simple ions (e.g., butylsulfate or butyltrimethylammonium) on the structure of water are different from those of micellized ions (e.g., dodecylsulfate or cetyltrimethylammonium plus the associated counterions), this difference is due to electrostriction of water by the charged interface; (2) Perturbation of the structure of interfacial water is larger for ionic micelles than for the corresponding zwitterionic ones; (3) For the same class of surfactants, e.g., cationic or zwitterionic, the micelle-induced enhancement of the structure of interfacial water (relative to that of bulk water) increases as a function of increasing the hydrophobic character of the surfactant headgroup; (4) Water solubilized by RMs and W/O mu Es does not seem to coexist in ''layers'' of different structures within the micellar water ''pool''. (C) 1997 Elsevier Science B.V.