Evaporation kinetics of tetraalkylammonium ions from charged formamide drops.

The rate of ion evaporation from the surface of electrically charged liquid drops may be inferred from observations of the minimum drop charge q present on drops of a given radius R. This critical relation q(R) is measured here from the fossil solid residues left by the drops after complete solvent evaporation. We obtain mobility distributions of singly charged clusters formed by charge-reduced electrosprays of tetra-n-alkylammonium salts (C(n)()H(2)(n)()(+1))(4)N(+) (n = 2-10) dissolved in formamide. These distributions exhibit modulated structures, with each wave being associated with an initial charge state of the clusters prior to charge reduction, from which critical q(R) relations follow. For n from 4 to 7, the behavior is weakly dependent on the length of the alkyl chain. Above n = 7, there is a marked increase in solvation energy of the alkylammonium ions, but drop curvature effects contribute a compensating reduction of the energy barrier for ionization. This curvature effect increases monotonically with n and is probably associated with surface activity. Few clear modulations are seen for n < 3, perhaps because of the decreased role of surface activity in transferring solute into very small drops during the Coulombic breakup of larger drops. For this reason, extension of this technique to small inorganic salts is problematic.