Heat capacities of freely evaporating charged water clusters

Abstract

We report on evaporation studies on positively charged water clusters (H(+)(H(2)O)(N)) and negatively charged mixed clusters (X(-)(H(2)O)(N)) with a small core ion X (X=O(2), CO(3), or NO(3)), 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 6k(B)-8k(B) 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 degrees C values.

Thermal effects on energetics and dynamics in water cluster anions (H2O)n−

“…As formulated by Klots, the cluster temperature within this limit depends only on the solvent binding energy, and is estimated to be 160 K for water clusters, based on a binding energy of 320 meV. 64 Hence, this model predicts the same temperature for water cluster anions formed in Ar or Ne, whereas our data point to warmer clusters in Ne. This apparent discrepancy can be resolved by realizing that the cluster temperature from the evaporative ensemble model is taken as an upper limit.…”

Insights into the dynamics of evaporation and proton migration in protonated water clusters from Large‐scale Born–Oppenheimer direct dynamics

“…These considerations show that evaporation is a “fast” process, considerably faster than estimated by Sundén et al on the base of simple models applied to the results of mass‐spectrometric measurements. [53] In fact, according to their data, the rate coefficient is estimated to be 10 5 –10 6 s −1 . The large discrepancy can be rationalized as follows.…”

Average Sequential Water Molecule Binding Enthalpies of M(H2O)19−1242+ (M = Co, Fe, Mn, and Cu) Measured with Ultraviolet Photodissociation at 193 and 248 nm

“…To determine the best effective binding enthalpy term to be used in the TLDM (i.e., the y-axis offset) for clusters in this size range, the deviation between the values obtained from the TLDM and the experimental values were minimized, resulting in an effective enthalpy Figure 3) as a function the average cluster size (〈n〉 -〈x〉/2). Values for sequential water molecule binding enthalpies to M(H 2 O) n 2+ , calculated using the discrete implementation of the TLDM, 44 the TLDM without surface tension (TLDM-ST), the TLDM modified to include an additional ion-dipole term (Dipole TLDM) 68 parametrized to 298 K, the TLDM parametrized to 273 K (TLDM 273 K), 44 and the liquid drop expansion model (Drop Expansion), 48 are plotted as a function of n. Error bars represent uncertainty in the average number of water molecules lost propagated from the noise in the corresponding mass spectra. value of 11.17 kcal/mol using parameters for liquid water at 273 K. This effective enthalpy for clusters this size may account for other uncertainties in the model, or it may indicate that the clusters have some partial ice-like structures.…”

“…Photodissociation experiments at 1064 nm have also been used to obtain the internal energy content of (H 2 O) 48 -and (H 2 O) 118 -and cluster heat capacities as a function of the initial ion temperatures. 42 The onset of sharp increases in the cluster heat capacities with increasing cluster temperature at 93 and 118 K, for (H 2 O) 48 -and (H 2 O) 118 -, respectively, were attributed to the onset of cluster melting.…”