How many molecules make a solution?

Water clusters are studied in order to investigate the evolution of solution phase chemistry from the molecular level to bulk. When water clusters are studied in a Fourier transform ion cyclotron resonance (FI-ICR) mass spectrometer, their fragmentation under the influence of infrared black-body radiation must be considered and can be used as a tool to monitor the destabilization of particular arrangements as a function of cluster size. As examples for solution phase chemistry, dissolution of acids, metal ion oxidation and metal halide precipitation, acid-base catalysis and hydrolysis are discussed. All these solution phase reactions proceed on the single-ion level in small water clusters. In accordance with spectroscopic and thermochemical data, gas phase ion chemistry of small water clusters rapidly approaches bulk behaviour, if the ion high concentration and pH value of the cluster are taken into account. Examination of competing reaction pathways as a function of cluster size in hydrated electron clusters reveals the crucial influence of entropy. Recent photodissociation experiments by Metz and Cocrorteers provide experimental evidence for the salt bridge mechanism for charge separation in hydrated dication clusters.