TUTCRIS

TY - JOUR

T1 - Surface behavior of hydrated guanidinium and ammonium ions

T2 - A comparative study by photoelectron spectroscopy and molecular dynamics

AU - Werner, Josephina

AU - Wernersson, Erik

AU - Ekholm, Victor

AU - Ottosson, Niklas

AU - Öhrwall, Gunnar

AU - Heyda, Jan

AU - Persson, Ingmar

AU - Söderström, Johan

AU - Jungwirth, Pavel

AU - Björneholm, Olle

PY - 2014/6/26

Y1 - 2014/6/26

N2 - 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.

AB - 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.

UR - http://www.scopus.com/inward/record.url?scp=84903466740&partnerID=8YFLogxK

U2 - 10.1021/jp500867w

DO - 10.1021/jp500867w

M3 - Article

VL - 118

SP - 7119

EP - 7127

JO - Journal of Physical Chemistry Part B

JF - Journal of Physical Chemistry Part B

SN - 1520-6106

IS - 25

ER -