Thermochemistry and Kinetics of Evaporation and Condensation for Small Water Clusters

Abstract

Summary. The evaluation of thermochemical properties of small water clusters (e.g., consisting of 2 -10 water molecules) is complicated by their dissociative nature. At temperatures for which dissociation of the cluster into molecular fragments has significant probability, an "operational definition" of a cluster is required to restrict the phase space and evaluate a finite value for the partition function or cluster property of interest. We will review a theoretical approach, Dynamical Nucleation Theory (DNT), for evaluating rate constants for cluster evaporation and condensation, which allows for a unique definition of clusters and their thermochemical properties. We will also discuss the relevance of DNT to homogeneous vapor-to-liquid nucleation of water. IntroductionEquilibrium properties of water clusters are important in a variety of areas. Free energies of formation of water clusters from gas-phase water molecules determine equilibrium cluster populations, which are important in the at mosphere [1][2][3][4]. Thermochemical properties, such as enthalpies and free energies of cluster formation and virial coefficients [5][6][7][8][9][10][11], also provide important benchmarks for intermolecular interactions [12][13][14] that are difficult to probe directly by experiment. Dynamical properties of water clusters, such as rate constants for evaporation and condensation of water clusters, are also important. The kinetics of cluster condensation and evaporation determine timedependent cluster populations and their energy content in expansions [15] and vapor-to-liquid nucleation rates are extremely sensitive to the kinetics of cluster condensation and evaporation [16].Thermochemical properties of water clusters can be determined from statistical mechanical simulations based upon a molecular interaction potential. Although dynamical properties of water clusters require extensions of statistical mechanical simulation techniques beyond simple equilibrium methods, the accurate treatment of partition functions provides the foundation for these extensions. Molecular-Ievel approaches to determining thermochemical and dynamical properties of clusters require (i) accurate descriptions of the intermolecular interactions, (ii) consistent theories connecting the molecular interaction potentials with observable properties, and (iii) effective simulation methods, which allow efficient sampling of those regions of phase space that V. Buch et al. (eds.), Water in Confining Geometries

Proton dynamics in molecular solvent clusters as an indicator for hydrogen bond network strength in confined geometries

“…The constituent molecules of the cluster would therefore attempt to counteract any stretching of intermolecular O-O distances, leading to a buildup of internal pressure within the cluster, which increases with decreasing cluster size and radius of curvature. 39 Internal pressure deforms the hydrogen bond network in a way analogous to applying external pressure on the system, which has been shown to weaken hydrogen bonding. 40 In the case of methanol, molecules at the interface arrange themselves with the methyl group pointing towards vacuum, 5,6 which allows them to maintain a number of hydrogen bonds similar to that found in the bulk, as indicated in Fig.…”