Homogeneous nucleation of H2O and D2O in comparison: The isotope effect

In the past century, the homogeneous nucleation of light water (H2O) has re peatedly been studied using various experimental techniques. Generally, the onset of nucleation was recorded, while less frequently, the actual nuclea tion rates were determined. In contrast, the nucleation of heavy water (D2O ) has been examined only in a single instance with no nucleation rates meas ured. Here, we report the first nucleation rate study of D2O along with nuc leation rate measurements for H2O, which we repeated for comparison under i dentical conditions. We find that the nucleation rates for H2O and D2O diff er by a factor of 2500, if compared at the same respective vapor pressure p (nu) and temperature T, whereas the comparison at the same supersaturation S shows an agreement within experimental scatter. Also, the numbers of mole cules in the critical clusters, which are determined from the slopes of the In J versus In S curves, are nearly the same for both isotopic waters. A s atisfactory agreement with previous nucleation rate measurements of H2O mad e by Viisanen et al. (Viisanen, Y.; Strey, R.; Reiss, H. J. Chem. Phys. 199 3, 99, 4680; 2000, 112, 8205) is observed, if the onset supersaturations So at nucleation rates of J(0) = 10(7) cm(-1) s(-1) are compared. Using the m ost recent expressions for temperature-dependent vapor pressures, we calcul ated surface tensions and densities predictions by the classical Becker-Dor ing nucleation theory. Around T = 240 K, the predictions quantitatively agr ee with the experimental data. However, as in the case of other systems (e. g., alcohols and alkanes), classical theory shows a stronger temperature de pendence than experimentally observed. A temperature-dependent correction o f the classical theory is developed which permits analytical calculation of nucleation rates as function of supersaturation and temperature over exten ded ranges.