Seminars Archive

Time-Resolved Spectroscopy of Water confined in Vycor

Abstract
Transient grating experiments are powerful tools for investigate the relaxation dynamics of liquids. A particular kind of TG experiment offers the possibility to study, at one time, the acoustic propagation at around 1 GHz, the density relaxation and thermal diffusion processes. The very broad time window covered by this experiment, typically from 10-9 to 10-3 s, gives access to a dynamic range hardly explored by other methods. In this talk we present heterodyne detected transient grating (HD-TG) measurements on water confined in Vycor 7930. The study has been performed in two different temperature regimes: from 90 up to -15 °C and from -30 up to -75 °C. In the high temperature range, the data show interesting aspects due partly to the confinement effects and partly to the peculiarities of bulk water. In particular the signals present a rise at middle times related to the water viscous flow through the pores. This diffusive mode is well reproduced by a single exponential whose time constant shows a q-2 dependence. We found that this relaxation is fully described by a macroscopic hydrodynamic approach in spite of the nanoscale confinement. Furthermore, from the analysis of the temperature behaviour of the signal intensity, we extracted the temperature value where the thermal expansion coefficient of water vanishes giving thus the temperature of water density maximum. In particular we found a density maximum at -1.5 °C instead of at 4 °C as in the bulk water. At -16 °C, water inside the tubules partially crystallizes while first few layers of water adjacent to the pore walls retain a liquid/amorphous structure. Below this temperature, the transient grating signal quite changes but shows again a density rearrangement at middle times. In this case, the rearrangement is no longer due to the viscous water flow but it has to be addressed to the structural dynamics of the unfreezable water. The relaxation is, in fact, properly described by a stretched exponential. (Rif. C. Masciovecchio)