Cluster ion thermodynamic properties: The liquid drop model revisited

The liquid drop (LD) model is revisited to assess the reliability of its pr edictions for thermodynamic properties of cluster ions and to examine the r ate of convergence of such properties to their bulk counterparts. The model predictions are in very good agreement with both available experimental da ta and simulation results for Na+(H2O)(n) clusters, surprisingly for all cl uster sizes, and the stepwise cluster thermodynamic properties are found to converge only slowly to their bulk counterparts. The LD model allows a nat ural partitioning of the cluster ion thermodynamic properties into various components, one of which (the solvation part) is of prime importance in con necting cluster solvation properties to the bulk limit. The latter LD model component is found to be entirely analogous to the so-called dielectric sp here theory, and as implied in earlier work, the results of dielectric mode ls suggest that ion solvation is also a very slow process to converge to th e bulk limit. In addition, a form alternative to the customary interior ion LD model is proposed, where the ion resides at the surface of a solvent dr oplet, and the resulting model successfully predicts that surface ion I-(H2 O), clusters are thermodynamically very likely and that large halide ions t end to be located, if not at the surface, very close to it in large cluster s of polar solvent molecules. Conversely, small ions such as Na+ are predic ted to be interior in water clusters. Further, large ions such as I- are pr edicted to have interior but near-surface locations in acetonitrile cluster s. Even though it seems to work better for smaller ions and solvents such a s water, the LD model, despite its simplicity, generally appears to properl y describe cluster ion thermodynamic properties over a wide range of cluste r sizes and even for relatively small cluster sizes.