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 +/- 3 K for (H2O)(48)(-) and 118 +/- 3 K for (H2O)(118)(-). At the transition temperature the heat capacity strongly increases, which marks the onset of melting.