Gas-Phase Hydration of Alkali and Halide Ions

The alkali and halide ions have played an important role in studies of ionic solutions. The reasons for this are obvious. The alkali halides form simple salts which are soluble in water. The ions have spherical symmetry and vary sufficiently in size, thus permitting studies of the dependence of solvation on ionic diameter. 

Evidence for the existence of alkali hydrates such as Na (OH2) in the gas phase was first obtained by Munson and Hoselitz on the basis of gas-phase measurements of the mobility of the alkali ions in the presence of water vapor. 

The potassium hydrate K OH2 was detected by Chupka in a mass spectrometric study of ions emerging from a heated Knudsen cell containing KCl salt, and water as an impurity. The ion clusters Ar OH2, where M = Li, Na, and K, were also mass spectrometrically observed in a study of ions in flames by Hayhurst and Sugden. The ions were produced by spraying an aqueous solution of the alkali chlorides in a premixed H2, 02,N2 flame at 1 atm. The authors also attempted to measure the ionic equilibria -h H2O = M OH2. However, their data are inconsistent, which is not surprising, considering the complexity of the experimental conditions and arrangement. 

A comprehensive study of the alkali hydrates was made by Searles and co-workers. Some experimental difficulties were encountered initially since the alkali ions had to be created in the presence of a few Torr of water vapor and conditions of thermal and clustering equilibrium had to be achieved. Successful measurements could be made after development of suitable thermionic ion sources.  

The relative ion intensities of the sodium ion hydrates measured with such apparatus at various water pressures are shown in Fig. 9. At equilibrium the relative intensities should reflect the stabilities of the clusters. The results show that two, three, or even four clusters with different number of water molecules may coexist with concentrations of the same order of magnitude. Increase of water pressure leads to increase of cluster size. The number of coexisting clusters also increases with pressure, showing that the stabilities of clusters become more similar as the number of molecules in the cluster increases. The regular change of cluster intensities with pressure indicates that the number of ligands is not restricted to any fixed coordination number like four, six, or eight. 

---