Bulk-Water Behavior of Water Clusters Studied by Isotope Effects

The evaporative decay of mixed heavy and light water clusters was measured. The branching ratios of the three isotopologue molecules scale with the deuterium fraction from cluster size N=9 and up, including across the well-known shell closing at N=21, and are consistent with macroscopic surface values. Differences in the free energies of competing channels derived from the scaled distributions take the simplest possible form of a single energy parameter and the deuterium-protium mixing entropy. The observations here represent a direct observation of the isotopic mixing entropy. The energy parameter is consistent with zero point vibrational energy differences of the free and condensed water molecule for both H_{2}O/HDO and HDO/D_{2}O ratios.