Heat capacities of freely evaporating charged water clusters

We report on evaporation studies on positively charged water clusters $(H+(H2O)N)$ and negatively charged mixed clusters $(X−(H2O)N)$ with a small core ion $X$ (⁠$X=O2$⁠, $CO3$⁠, or $NO3$⁠), in the size range $N=5–300$⁠. The clusters were produced by corona discharge in ambient air, accelerated to 50 keV and mass selected by an electromagnet. The loss of monomers during the subsequent 3.4 m free flight was recorded. The average losses are proportional to the clusters’ heat capacities and this allowed the determination of size-dependent heat capacities. The values are found to increase almost linearly with clusters size for both species, with a rate of $6kB–8kB$ per added molecule. For clusters with $N<21$ the heat capacities per molecule are lower but the incremental increase higher. For $N>21$ the values are intermediate between the bulk liquid and the solid water $0°C$ values.