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Molecular Sinkers: X-ray Photoemission and Atomistic Simulations of Benzoic Acid and Benzoate at the Aqueous Solution/Vapor Interface
Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Yt- och gränsskiktsvetenskap.
Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Yt- och gränsskiktsvetenskap.
Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Fysiska sektionen, Institutionen för fysik och astronomi, Yt- och gränsskiktsvetenskap.
Vise andre og tillknytning
2012 (engelsk)Inngår i: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 116, nr 43, s. 13017-13023Artikkel i tidsskrift (Fagfellevurdert) Published
Abstract [en]

In this work we, for the first time, provide a detailed microscopic picture of the behavior of benzoic acid at the aqueous solution/vapor interface in its neutral as well as in its dissociated form (benzoate). This is achieved through a combination of highly surface-sensitive X-ray photoelectron spectroscopy experiments and fully atomistic molecular simulations. We show that drastic changes occur in the interface behavior of the neutral acid upon release of the proton. The benzoic acid molecules are found to be strongly adsorbed in the interface layer with the planes of the aromatic rings oriented almost parallel to the water surface. In contrast, in the benzoate form the carboxylate group shows a sinker-like behavior while the aromatic ring acts as a buoy, oriented nearly perpendicular to the surface. Furthermore, a significant fraction of the molecular ions move from the interface layer into the bulk of the solution. We rationalize these findings in terms of the very different hydration properties of benzoic acid's carboxylic group in the two charge states. The molecule has an amphiphilic nature and the deprotonation thus changes the hydrophobic/hydrophilic balance between the nonpolar aromatic and the polar carboxylic parts of the molecule. That, consequently, leads to a pronounced reorientation of the molecule at the interface.

sted, utgiver, år, opplag, sider
2012. Vol. 116, nr 43, s. 13017-13023
HSV kategori
Forskningsprogram
Fysik
Identifikatorer
URN: urn:nbn:se:uu:diva-151434DOI: 10.1021/jp300956jISI: 000310482800014OAI: oai:DiVA.org:uu-151434DiVA, id: diva2:409927
Tilgjengelig fra: 2011-04-11 Laget: 2011-04-11 Sist oppdatert: 2017-12-11bibliografisk kontrollert
Inngår i avhandling
1. Aqueous Solutions as seen through an Electron Spectrometer: Surface Structure, Hydration Motifs and Ultrafast Charge Delocalization Dynamics
Åpne denne publikasjonen i ny fane eller vindu >>Aqueous Solutions as seen through an Electron Spectrometer: Surface Structure, Hydration Motifs and Ultrafast Charge Delocalization Dynamics
2011 (engelsk)Doktoravhandling, med artikler (Annet vitenskapelig)
Abstract [en]

In spite of their high abundance and importance, aqueous systems are enigmatic on the microscopic scale. In order to obtain information about their geometrical and electronic structure, simple aqueous solutions have been studied experimentally by photo- and Auger electron spectroscopy using the novel liquid micro-jet technique in conjunction with synchrotron radiation. The thesis is thematically divided into three parts.

In the first part we utilize the surface sensitivity of photoelectron spectroscopy to probe the distributions of solutes near the water surface. In agreement with recent theoretical predictions we find that large polarizable anions, such as I- and ClO4-, display enhanced surface propensities compared to smaller rigid ions. Surface effects arising from ion-ion interactions at higher electrolyte concentrations and as function of pH are investigated. Studies of linear mono-carboxylic acids and benzoic acid show that the neutral molecular forms of such weak acids are better stabilized at the water surface than their respective conjugate base forms.

The second part examines what type of information core-electron spectra can yield about the chemical state and hydration structure of small organic molecules in water. We demonstrate that the method is sensitive to the protonation state of titratable functional groups and that core-level lineshapes are dependent on local water hydration configurations. Using a combination of photoelectron and X-ray absorption spectroscopy we also show that the electronic re-arrangement upon hydrolysis of aldehydes yields characteristic fingerprints in core-level spectra.

In the last part of this thesis we study ultrafast charge delocalization dynamics in aqueous solutions using resonant and off-resonant Auger spectroscopy. Intermolecular Coulombic decay (ICD) is found to occur in a number of core-excited solutions where excess energy is transferred between the solvent and the solute. The rate of ultrafast electron delocalization between hydrogen bonded water molecules upon oxygen 1s resonant core-excitation is found to decrease upon solvation of inorganic ions.

The presented work is illustrative of how core-level photoelectron spectroscopy can be valuable in the study of fundamental phenomena in aqueous solutions.

sted, utgiver, år, opplag, sider
Uppsala: Acta Universitatis Upsaliensis, 2011. s. 118
Serie
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 828
Emneord
Water, Aqueous solutions, Ions, Molecular Hydration, Electron dynamics, Atmospheric Chemistry, Hydrolysis, Acid-Base Chemistry, Interatomic Coulombic Decay, ICD, Liquid Micro-Jet, X-ray Photoelectron Spectroscopy, XPS, Auger Electron Spectroscopy, AES, MAX-lab, BESSY
HSV kategori
Forskningsprogram
Fysik med inriktning mot atom- molekyl- och kondenserande materiens fysik
Identifikatorer
urn:nbn:se:uu:diva-151435 (URN)978-91-554-8083-7 (ISBN)
Disputas
2011-06-01, Polhemssalen, Ångström Laboratory, Lägerhyddsvägen 1, Uppsala, 10:15 (engelsk)
Opponent
Veileder
Tilgjengelig fra: 2011-05-11 Laget: 2011-04-11 Sist oppdatert: 2011-07-01bibliografisk kontrollert

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