Structure and dynamics of water in nanoscopic spheres and tubes

We study the reorientation dynamics of liq. water confined in nanometer-​sized reverse micelles of spherical and cylindrical shape. The size and shape of the micelles are characterized in detail using small-​angle x-​ray scattering, and the reorientation dynamics of the water within the micelles is investigated using GHz dielec. relaxation spectroscopy and polarization-​resolved IR pump-​probe spectroscopy on the OD-​stretch mode of dil. HDO:H2O mixts. The GHz dielec. response of both the spherical and cylindrical reverse micelles can be well described as a sum of contributions from the surfactant, the water at the inner surface of the reversed micelles, and the water in the core of the micelles. The Debye relaxation time of the core water increases from the bulk value τH2O of 8.2 ± 0.1 ps for the largest reverse micelles with a radius of 3.2 nm to 16.0 ± 0.4 ps for the smallest micelles with a radius of 0.7 nm. For the nano-​spheres the dielec. response of the water is approx. ∼6 times smaller than expected from the water vol. fraction and the bulk dielec. relaxation of water. The dielec. response of nano-​spheres is more attenuated than that of nanotubes of identical compn. (water-​surfactant ratio)​, whereas the reorientation dynamics of the water hydroxyl groups is identical for the 2 geometries. We attribute the attenuation of the dielec. response compared to bulk water to a local anti-​parallel ordering of the mol. dipole moments. The difference in attenuation between nano-​spheres and nano-​cylinders indicates that the anti-​parallel ordering of the water dipoles is more pronounced upon spherical than upon cylindrical nanoconfinement.