Thermal Properties of Deuterium Oxide near the Triple Point Predicted from Nonequilibrium Evaporation Fei Duan,* A. Crivoi, and B. He Division of Thermal and Fluids Engineering, School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore 639798 Statistical rate theory (SRT) was applied to predict the saturation pressure of D 2 O numerically near the triple point based on the interfacial liquid-phase temperature, the interfacial vapor-phase temperature, and the local evaporation flux from 102 local measures in a series of nonequilibrium steady-state droplet evaporation experiments. An analytical expression of the saturation pressure, P sat , was obtained from the predicted values. Following the thermodynamic relations, the specific entropy of evaporation, h fg , and the liquid-phase specific heat at constant pressure, C p L , were computed. The agreement that the calculated values of these properties with those obtained from the independent measurements indicates that the SRT expression accurately predicts the thermal properties of D 2 O near the triple point. Introduction The accurate measurement of thermal properties near the triple point is always a challenge for a liquid due to the possible ice formation. Deuterium oxide, D 2 O, a heavy water which has the same structure as H 2 O, is normally applied in the operation of nuclear power reactors as a moderator or a heat transfer agent. Solvents created with the heavy water are used to help researchers determine the structure of complex organic chemicals in life science. Additionally and importantly, from the point of view of physics and chemistry, the steady-state experiments near the triple point on evaporating D 2 O droplets help us to validate a method to predict the thermal properties (the saturation pressure, P sat , the specific entropy of evaporation or condensation, h fg , and the specific heat at constant pressure in the liquid, C p L ) on the basis of the statistical rate theory (SRT) approach intro- duced below. By definition, the saturation pressure, P sat , at its isothermal liquid temperature, is determined at a flat surface under the equilibrium conditions. The pressure of D 2 O was experimentally measured above the triple point at 276.97 K, 1-4 whereas the experiments were seldom reported in measuring P sat directly below the triple point. Bottomley measured the vapor-pressure difference between the metastable liquid and the stable solid of D 2 O by using two 0.5 g samples in two connected glass bulbs, respectively, as the temperature was lowered to 261.35 K. 5 However, the assumption of thermal equilibrium might be difficult to reach in the experiments. A mass transport was expected due to the pressure difference in the connected tube between the water sample at a higher vapor pressure and the ice sample at a lower vapor pressure during the measure- ment. The values of P sat reported by Bottomley are plotted in Figure 1 with his smoothed fitting curve. A detectable deviation could be found there. The measurement values are 4.1 % at 2.0 °C and 11.1 % at 261.35 K higher than the fitting curve. Kraus and Greer directly measured the vapor pressure of the metastable liquid D 2 O in the range (257.75 to 276.35) K above the small dew droplets condensed from the hot vapor under the assumed equilibrium conditions. 6 They could not directly observe the droplets during the measurement, so some droplets might be frozen at lower temperatures which would affect their readings. The measurements of Kraus and Greer and their fitting equation are plotted in Figure 1 as well. The measured values are also higher than the fitting curve. The difference is 4.6 Pa at 257.75 K, while it is 27.3 Pa at 276.35 K. There is a variation between the two sets of experimental data below the triple point of D 2 O. At 261.35 K, the value of P sat reported by Kraus and Greer is 4.7 Pa greater than that of Bottomley. Pupezin et al. provided an empirical saturation pressure equation, P sat,P , based on their experimental data for D 2 O from (270 to 373) K, 1 which partially covers the metastable range below the triple point. If it is extended into a lower temperature, the equation surprisedly has a close agreement with the data reported by Bottomley 5 within a derivation of 1 %, by Kraus and Greer 6 within a derivation of 3 %. As shown in Figure 2, the difference of the equation in Pupezin et al. from the polynomial fitting of the measures of Kraus and Greer, P sat,KG(f) , 7 * Corresponding author. E-mail: feiduan@ntu.edu.sg. Figure 1. Comparison of the experimental vapor-phase pressures of D 2 O with the existing fitting expressions of the saturation pressure from Bottomley (black dotted line), 5 Kraus and Greer (blue solid line), 6 Kraus and Greer (f) (green dashed line), 7 and Pupezin et al. (red solid line). 1 Data from: 9, Kraus and Greer; 6 [, Bottomley. 5 J. Chem. Eng. Data 2010, 55, 3674–3679 3674 10.1021/je100226g  2010 American Chemical Society Published on Web 06/15/2010