Calorimetric Observation of the Melting of Free Water Nanoparticles at Cryogenic Temperatures

We present an experimental study of the thermodynamics of free, size-selected water cluster anions consisting of 48 and 118 molecules. The measured caloric curves of the clusters are bulklike at low temperatures but show a well-defined, particle-size specific transition at $93\pm 3\mathrm{K}$ for $({\mathrm{H}}_2\mathrm{O}{)}_{48}^{-}$ and $118\pm 3\mathrm{K}$ for $({\mathrm{H}}_2\mathrm{O}{)}_{118}^{-}$. At the transition temperature the heat capacity strongly increases, which marks the onset of melting.

Figure 1

Experimental results for water cluster anions consisting of 48 and 118 water molecules. (a) The mean inner energy per molecule of an ensemble of $({\mathrm{H}}_2\mathrm{O}{)}_n^{-}$ clusters as a function of the heat bath temperature. Black, full dots: $n=48$; gray, open dots: $n=118$. At lower temperatures, the experimental points represent the caloric curve of the mass-selected clusters. The solid, thick lines are cubic spline fits to the caloric curves; the dashed-dotted line is the caloric curve of bulk ice [12]. The large gray dots indicate the evaporative ensemble energies and temperatures that result from the extrapolations of the caloric curves up to the evaporative ensemble (thin black solid lines). (b) The specific heat per molecule of the clusters calculated by the differentiation of the spline in (a). The sudden increase marks the onset of the melting transition.