Water dynamics and aggregate structure in reversed micelles at sub-zero temperatures. A deuteron spin relaxation study

Abstract

The microemulsion phase of the system AOT–D₂O–2,2,4-trimethylpentane (TMP) has been studied by water ²H longitudinal and transverse spin relaxation measurements at three frequencies and at temperatures down to –29 °C. Additional information was obtained from n.m.r. signal intensities and quadrupolar line splittings, differential scanning calorimetry and small-angle X-ray scattering. The results establish the existence of reversed micelles with unfrozen aqueous cores containing between 2 and 4 water molecules per AOT. As the temperature is lowered, these micelles grow into very long rods. The rate of water reorientation in the core, obtained directly from the frequency-dependence of the longitudinal relaxation rate, is two orders of magnitude lower than that of bulk water at the same temperature.