Real-Time Kinetic Measurements of the Condensation and Evaporation of D2O Molecules on Ice at 140 K < T < 220 K

The kinetics of condensation of D218O water vapor on D216O ice was studied in the temp. range 140-220 K. The measurements have been performed in real time using the pulsed valve technique in a low-pressure flow reactor. Ice samples were prepd. by several different methods. The uptake coeff. (g) was obsd. to decrease with increasing surface temp., and varied from g = 0.06 to 0.8. A significant dependence of g on the method of prepn. of the ice has been found. At 180 K, g = 0.13 for single-crystal ice, 0.18 for ice condensed from the vapor phase, and approx. 0.25 for bulk ice obtained from freezing liq. D216O. The uptake coeff. attained a value of g = 0.8 at 140 K for cubic ice prepd. by vapor condensation at 140 K. The activation energy for evapn. of D2O at low temps. (140-190 K) has been measured as 12.2 kcal/mol, whereas it decreases to 8.3 kcal/mol at higher temps. (190-220 K). The rate of evapn. at 200 K corresponds to the loss of approx. 70 +- 10 formal monolayers per s. The exptl. results suggest the formation of loosely bound water adsorbed to the surface of ice whose bond energy is estd. to be 4.0 +- 0.4 kcal/mol, independent of the type of ice.