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.