The Structure and Properties of Supercritical Water

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1 Netsu Sokutei The Structure and Properties of Supercritical Water Masaru Nakahara (Received November 21, 2003; Accepted December 25, 2003) First we explain the importance of the investigation on the structure and properties of supercritical water in relation to the chemical evolution and the environmental and energy issues of the 21 st century. Molecular interpretations are given to the temperature dependence of the density, dielectric constant, and viscosity, hydrogen-bonding structure, and dynamics of supercritical water. The three-dimensional network structure, characteristic of ambient water, is broken down in hot expanded water at temperatures higher than 200 and densities lower than 0.9gcm 3. The number of hydrogen bonds per molecule has been determined by the NMR method combined with computer simulation; it decreases from 4 for ambient water to 1-2 for supercritical water at 400 and the critical density ( 0.32 g cm 3 ). The NMR rotational correlation time (τ 2R) for supercritical water at the medium densities is in the range of fs, two orders of magnitude smaller than the ambient value (2 ps). Supercritical water is shown to be an alternative to hazardous organic solvents; there are being found new hydrothermal organic reactions without catalyst for the development of green chemistry ) H 2COHCOOH COCO 2 C1 H 2 H 2O 12) 2004 The Japan Society of Calorimetry and Thermal Analysis. 14

2 21 SCWO80 C-C C-H C-OH 1 20 Table 1 Critical temperature, pressure, and density for widely used substances. Species T c/ P c/mpa ρ c/g cm 3 Water Methanol Ethanol Ethane Carbon Dioxide Benzene ,13-17) 18) T c P c ρ c Table 1 CO 2 H 2O CO 2 T c P c ρ cvan der Waals aba/b 19,20) 1,3,4,20) 15

3 ST2 MCY TIP4P SPC SPC/E 3 H-O-H Lennard-Jones 2 δ 2δ 2 2δ SPC SPC/E ST2 1.8 Debye OH OH 1/20 1/30 k BT 10 subnanometer scale 1 1/2 nanometer scale2 Debye micrometer scale 80 Fig.1 Water sealed in a capillary made of quartz; (a) ambient and (b) high-temperature conditions. The arrow indicates the vapor-liquid meniscus. For the in-situ observation of microscopic properties of hot water, the capillary is set in an NMR tube containing DEMNUM (Daikin, perfluoroether) as the high-temperature heat medium and mounted on a high-temperature NMR probe. The NMR observation is thus carried out under isochoric conditions as a function of the temperature. Filling factor y is defined as v / v 0, where v and v 0 are the sample (water) and quartz capillary volumes, respectively. In the case of water, the density is 1 and ρ in g cm 3 in ambient supercritical conditions, respectively. Hence the density of supercritical water is equal to the filling factor in number (y ρ ), because the supercritical water fills the whole volume of the capillary. Fig MPa ) Fig.1(b) 22) P ρ T P -ρ Fig.2 ρ -T Fig.3 b y ρ/g cm 3 Fig.2 ρ c ρ c ρ G 16

4 P / MPa Density / g cm 3 Density / kg m 3 T / K Fig.2 The isothermal pressure of H 2O plotted against the density at several temperatures above, at, and below the critical temperature. The broken line like a mountain indicates the liquid-gas transition, and the horizontal dotted line shows the coexistence tie line. ρ L ρ G ρ L Fig.1 1 gcm 3 ρ y 2 Fig.2 Fig.3 The densities of the liquid and the gas in equilibrium plotted against the temperature. Fig.3 Cailletet- Mathias's law; (ρ L ρ G)/2 a bt 22) Fig.4 Fig.3 Kirkwood g K g K 1 g K NMR 10,11,23) 10 % 17

5 Dielectric Constant Viscosity / m Pa s T / K T / K Fig.4 The temperature dependence of the dielectric constants of the liquid and gas phases of water (H 2O) in equilibrium up to the critical temperature and the isochoric values under the supercritical conditions at the different densities. Fig.5 The viscosity coefficients for the liquid and gas phases of water (H 2O) in equilibrium plotted against the temperature and the isochoric values under the supercritical conditions at the different densities. NMR 24) Born Kirkwood 20) 25,26) Debye τ Dτ D ps 22) Fig.5 Fig.3 Fig.4 Einstein-Stokes-Debye Hubbard-Onsager 27,28) 18

6 (a) (b) δ / ppm δ / ppm T / T / δ / ppm Fig.6 Proton NMR spectrum for water as a function of temperature. The proton chemical shift δ is relative to that for an isolated water at 200 (reference). The supercritical water is at the density of 0.4 g cm 3 at ) 29) IAPWS SF-95 30) 22) Nature 31) g cm 3 NMR ) 5-7) 32) 0.8 mm 0.6 mm Fig.7 Temperature and density dependence of the proton chemical shifts of hot water including the supercritical. Part (b) is the expanded portion of the supercritical region of part (a); the numbers indicate the densities in g cm 3. The upper and the lower portions of part (a) correspond to the liquid and the gas phases, respectively, in the subcritical conditions. Fig.1 NMR NMR 21) Fig.6 δ /ppm Fig ppm δ 400 ρ c 1.3 ppm 0 ppm 1) NMR 21) OH 13 C NMR 33) 13 C Fig.7 19

7 N HB τ 2R / ps ρ / g cm 3 ρ / g cm 3 Fig.8 The hydrogen bond number N HB. The points at 400 are for the supercritical water at the corresponding densities. Fig.9 Rotational correlation times τ 2R for D 2O as a function of the density. The data in the density range of 0 to 0.6 g cm 3 are for the supercricial (heavy) water at Boltzmann k BT NMR 31) O-H 2,4,10,11) 23) Fig g cm 3 2 HF 2 D 2O NMR T 1 1,34-36) T 1 Jonas 37) H 2O H 2O NMR T 1 spin-rotation T 1 τ J D 2O D T 1 spin-rotation T 1 D 2O T 1 OD 2 τ 2R 0 t OD θ (t) τ 2R 3 τ 2R dt cos 2 θ (t ) τ D Debyeτ 2R 3 τ 2R ρ H 2O 20

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