流体のダイナミクス 超臨界水の動的構造解析

Abstract

The rotational dynamics of water in super- and subcritical conditions is investigated by measuring the spin-lattice relaxation time T1 of heavy water (D2O). The experimentally determined T1 is shown to be governed by the quadrupolar mechanism even in the supercritical conditions and to provide the second-order reorientational correlation time τ2R of the O-D axis. It is then found that while τ2R decreases rapidly with the temperature on the saturation curve, it remains on the order of several tens of femtoseconds when the density is varied at a temperature above the critical. The comparison of τ2R with the angular momentum correlation time shows that the inertial effect is operative in the rotational dynamics of supercritical water. The dependence of τ2R on the hydrogen bonding state is also examined in combination with computer 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 τ2R is divergent in the limit of zero density, it is observed to increase with the density when the density is above ∼1/3 of the critical.