Sodium Fluoride at the Air/Water Interface

Abstract

The zeta potentials of a single N2 bubble in near neutral aqueous solutions of 10–5 to 10–2 M NaF have been measured with a rotating cell zetameter. The zeta potentials in the NaF solutions are slightly more negative than those of comparable NaI concentrations. A simple interpretation of this observation is that the small size of the fluoride ion leads to its stronger adsorption at the interface, despite its smaller dielectric decrement. This would not be consistent with independent electrospray results from 100 µM solutions that bromide ion is less strongly adsorbed than iodide ion, i.e. that surface affinity increases with ion size. An alternative interpretation of the fluoride experiment is that the fluoride surface affinity is the consequence of stronger ion pairing with the NaOH that spontaneously forms at the interface.