On the evaporation of small ions from charged droplets

Experiments are briefly reported which show that small ions separate or ’’evaporate’’ from evaporating droplets carrying electrical charges. A theoretical argument is developed to find in which conditions this ion evaporation can occur; to that end solvation free enthalpies of cluster ions are derived from existing experimental data and the absolute reaction rate theory is applied. The conditions for ion evaporation are compared with those for Rayleigh instability and it is shown that the first process should occur only when the drop reaches sizes of the order of 10−6 cm. The ion evaporation process must be operative in the evaporation of highly electrified cloud droplets when their solute concentration is low.