title: Acid-base speciation of carboxylate ions in the surface region of aqueous solutions in the presence of ammonium and aminium ions.
abstract: The acid-base speciation of surface-active carboxylate ions in the surface region of aqueous solutions was studied with synchrotron-radiation-based photoelectron spectroscopy. The protonated form was found at an extraordinarily large fraction compared to that expected from the bulk pH. When adding salts containing the weak acid NH4(+) to the solution, the fraction of the acidic form at the surface increases, and to a much greater extent than expected from the bulk pH of the solution. We show that ammonium ions also are overrepresented in the surface region, and propose that the interaction between the surface-active anionic carboxylates and cationic ammonium ions creates a carboxylate-ammonium bilayer close to the surface, which increases the probability of the protonation of the carboxylate ions. By comparing the situation when a salt of the less volatile amine diethanolamine is used, we also show that the observed evaporation of ammonia that occurs after such an event only affects the equilibrium marginally.
authors.affiliation: MAX-lab, Lund University , P.O. Box 118, SE-221 00 Lund, Sweden.
authors.forename: Gunnar
authors.initials: G
authors.lastname: Öhrwall
authors.forename: Nønne L
authors.initials: NL
authors.lastname: Prisle
authors.forename: Niklas
authors.initials: N
authors.lastname: Ottosson
authors.forename: Josephina
authors.initials: J
authors.lastname: Werner
authors.forename: Victor
authors.initials: V
authors.lastname: Ekholm
authors.forename: Marie-Madeleine
authors.initials: MM
authors.lastname: Walz
authors.forename: Olle
authors.initials: O
authors.lastname: Björneholm