Surface Behavior of Hydrated Guanidinium and Ammonium Ions: A Comparative Study by Photoelectron Spectroscopy and Molecular Dynamics

Through the combination of surface sensitive photoelectron spectroscopy and molecular dynamics simulation, the relative surface propensities of guanidinium and ammonium ions in aqueous solution are characterized. The fact that the N 1s binding energies differ between these two species was exploited...

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Bibliographic Details
Published in:The journal of physical chemistry. B 2014-06, Vol.118 (25), p.7119-7127
Main Authors: Werner, Josephina, Wernersson, Erik, Ekholm, Victor, Ottosson, Niklas, Öhrwall, Gunnar, Heyda, Jan, Persson, Ingmar, Söderström, Johan, Jungwirth, Pavel, Björneholm, Olle
Format: Article
Language:English
Subjects:
Ammonium Compounds - chemistry
Aqueous solutions
Chlorides
Fysik
Fysikalisk kemi
Gases - chemistry
Guanidine - chemistry
Ions - chemistry
Molecular dynamics
Molecular Dynamics Simulation
Monitors
Natural Sciences
Naturvetenskap
Photoelectron Spectroscopy
Photoemission
Physical Chemistry
Physical Sciences
Salts - chemistry
Simulation
Surface Properties
Water - chemistry
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Summary:Through the combination of surface sensitive photoelectron spectroscopy and molecular dynamics simulation, the relative surface propensities of guanidinium and ammonium ions in aqueous solution are characterized. The fact that the N 1s binding energies differ between these two species was exploited to monitor their relative surface concentration through their respective photoemission intensities. Aqueous solutions of ammonium and guanidinium chloride, and mixtures of these salts, have been studied in a wide concentration range, and it is found that the guanidinium ion has a greater propensity to reside at the aqueous surface than the ammonium ion. A large portion of the relative excess of guanidinium ions in the surface region of the mixed solutions can be explained by replacement of ammonium ions by guanidinium ions in the surface region in combination with a strong salting-out effect of guanidinium by ammonium ions at increased concentrations. This interpretation is supported by molecular dynamics simulations, which reproduce the experimental trends very well. The simulations suggest that the relatively higher surface propensity of guanidinium compared with ammonium ions is due to the ease of dehydration of the faces of the almost planar guanidinium ion, which allows it to approach the water-vapor interface oriented parallel to it.
ISSN:1520-6106
1520-5207
1520-5207
DOI:10.1021/jp500867w