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 ${\mathrm{H}}_2\mathrm{O}/\mathrm{HDO}$ and $\mathrm{HDO}/{\mathrm{D}}_2\mathrm{O}$ ratios.

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• Atomic & molecular clusters
• Atomic & molecular structure
• Thermal properties