EXPERIMENTS ON THE KINETICS OF FIELD EVAPORATION OF SMALL IONS FROM DROPLETS

摘要

The phenomenon of ion evaporation from charged liquid surfaces is at the basis of electrospray ionization; a source of a stunning variety of gas phase ions. It is studied here by producing a monodisperse cloud of charged droplets and measuring the charge q and diameter d(r) of the residue particles left after complete evaporation of the solvent. When the droplets contain small monovalent dissolved ions, the electric field E on the surface of their solid residues is found to be independent of d(r). One can thus argue that the source of small ions in electrospray ionization is field-emission, and not other proposed mechanisms such as Dole's charged residue model. A consequence of the observed independence of E on d(r) is that the rate of ion ejection is simply related to the rate of solvent; evaporation, estimated here as that fora clean surface of pure solvent. The reduction G(E) brought about by the electric field E in the activation energy for ion evaporation has thus been inferred asa function of the measured field E in the range 1.5