Structural study of supercritical water. III. Rotational dynamics

Abstract

The rotational dynamics of water in super-and subcritical conditions is investigated by measuring the spin-lattice relaxation time T 1 of heavy water (D 2 O). The experimentally determined T 1 is shown to be governed by the quadrupolar relaxation mechanism even in the supercritical conditions and to provide the second-order reorientational correlation time 2R of the O-D axis of a single water molecule. It is then found that while 2R decreases rapidly with the temperature on the liquid branch of the saturation curve, it remains on the order of several tens of femtoseconds when the density is varied up to twice the critical at a fixed supercritical temperature of 400°C. The comparison of 2R with the angular momentum correlation time shows that the rotational dynamics is not diffusive in supercritical water. The dependence of 2R on the hydrogen bonding state is also examined in combination with molecular dynamics simulations, and the effect of the hydrogen bonding on the rotational dynamics in supercritical water is found to be weaker than but to be on the same order of magnitude as that in ambient water on the relative scale. Actually, although 2R is divergent in the limit of zero density, it is observed to increase with the density when the density is above ϳ1/3 of the critical.

Supercritical water: Local order and molecular dynamics

“…A progressive increase of the pressure (density) from 50 to 180 bar (from 0.05 to 0.1 gؒcm -3 ) shows the occurrence of dimers in the fluid. These findings are in agreement with previously reported results [10,[14][15][16]. In particular, we note that the density threshold from which dimers occur, at about 0.04-0.05 gؒcm -3 , is in qualitative agreement with other experimental [10] and theoretical work [14].…”

“…In particular, we note that the density threshold from which dimers occur, at about 0.04-0.05 gؒcm -3 , is in qualitative agreement with other experimental [10] and theoretical work [14]. In addition, such a density threshold is also seen from NMR data [16] and molecular dynamics results [15] on rotational relaxation time.…”

D/H fractionation in the H2–H2O system at supercritical water conditions: Compositional and hydrogen bonding effects

“…To a first approximation, D 2 O exhibits the same PVT properties as H 2 O (Fig. 6); an observation consistent with the electrostatic and density properties of D 2 O measured at the nearcritical and supercritical region (Okada et al, 1999;Matubayasi et al, 2001;Bazaev et al, 2003).…”

Effect of the local hydrogen bonding network on the reorientational and translational dynamics in supercritical water

“…Moreover, we have revealed that this behavior is more pronounced in the case of the first-order Legendre reorientational dynamics, whereas in the case of the second-order Legendre reorientational dynamics and especially for HBfree molecules, the corresponding reorientational times are not significantly affected by the bulk density. The sensitivity of the second-order reorientational times on the local HB network has also been mentioned in previous studies, 56 where it was also discussed that the effect of the local HB network is stronger in the liquid state than at supercritical conditions. Therefore, we may say that the first-order Legendre reorientational times could be considered as a better probe to analyze the effect of the local HB network on the reorientational dynamics in SCW.…”