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Surface Behavior of Hydrated Guanidinium and Ammonium Ions: A Comparative Study by Photoelectron Spectroscopy and Molecular Dynamics
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and condensed matter physics.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and condensed matter physics.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and condensed matter physics.
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2014 (English)In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 118, no 25, 7119-7127 p.Article in journal (Refereed) Published
Abstract [en]

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 Is 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.

Place, publisher, year, edition, pages
2014. Vol. 118, no 25, 7119-7127 p.
National Category
Physical Sciences Physical Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-229444DOI: 10.1021/jp500867wISI: 000338184400014OAI: oai:DiVA.org:uu-229444DiVA: diva2:736687
Available from: 2014-08-08 Created: 2014-08-07 Last updated: 2017-12-05Bibliographically approved
In thesis
1. Exploring the Surface of Aqueous Solutions: X-ray photoelectron spectroscopy studies using a liquid micro-jet
Open this publication in new window or tab >>Exploring the Surface of Aqueous Solutions: X-ray photoelectron spectroscopy studies using a liquid micro-jet
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The surface behavior of biologically or atmospherically relevant chemical compounds in aqueous solution has been studied using surface-sensitive X-ray photoelectron spectroscopy (XPS). The aim is to provide information on the molecular-scale composition and distribution of solutes in the surface region of aqueous solutions. In the first part, the distribution of solutes in the surface region is discussed, where in particular single molecular species are studied. Concentration-dependent studies on succinic acid and various alkyl-alcohols, where also parameters such as pH and branching are varied, are analyzed using different approaches that allow the quantification of surface concentrations. Furthermore, due to the sensitivity of XPS to the chemical state, reorientation of linear and branched alkyl-alcohols at the aqueous surface as a function of concentration is observed. The results are further discussed in terms of hydrophilic and hydrophobic interactions in the interfacial region, where the three-dimensional hydrogen bonded water structure terminates. In the second part, mixed solutions of compounds, both ionic and molecular, are inspected. Again concentration, but also co-dissolution of other chemical compounds, are varied and differences in the spatial distribution and composition of the surface region are discussed. It is found that the guanidinium ion has an increased propensity to reside at the surface, which is explained by strong hydration in only two dimensions and only weak interactions between the aromatic π-system and water. Ammonium ions, on the other hand, which require hydration in three dimensions, are depleted from the surface region. The presence of strongly hydrated electrolytes out-competes neutral molecules for hydrating water molecules leading to an enhanced abundance of molecules, such as succinic acid, in the interfacial region. The partitioning is quantified and discussed in the context of atmospheric science, where the impact of the presented results on organic loading of aerosol particles is emphasized.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2015. 88 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1313
Keyword
X-ray Photoelectron spectroscopy, liquid micro-jet, air-water interface, inorganic salt, carboxylic acid, alcohol, isomers, hydration.
National Category
Atom and Molecular Physics and Optics Condensed Matter Physics Chemical Sciences
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-265210 (URN)978-91-554-9399-8 (ISBN)
Public defence
2015-12-18, Häggsalen, Ångströmslaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
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Supervisors
Available from: 2015-11-27 Created: 2015-10-25 Last updated: 2016-01-13

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Werner, JosephinaEkholm, VictorSöderström, JohanBjörneholm, Olle

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