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Anomalous surface behavior of hydrated guanidinium ions due to ion pairing
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and Condensed Matter Physics. Uppsala Univ, Dept Phys & Astron, POB 516, SE-75120 Uppsala, Sweden.
Rudjer Boskovic Inst, Bijenicka Cesta 54, Zagreb 10000, Croatia.
Acad Sci Czech Republ, Inst Organ Chem & Biochem, Flemingovo Nam 2, CR-16610 Prague 6, Czech Republic.
Lund Univ, Dept Chem, Phys Chem, POB 124, SE-22100 Lund, Sweden.
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2018 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 148, no 14, article id 144508Article in journal (Refereed) Published
Abstract [en]

Surface affinity of aqueous guanidinium chloride (GdmCl) is compared to that of aqueous tetrapropylammonium chloride (TPACl) upon addition of sodium chloride (NaCl) or disodium sulfate (Na2SO4). The experimental results have been acquired using the surface sensitive technique X-ray photoelectron spectroscopy on a liquid jet. Molecular dynamics simulations have been used to produce radial distribution functions and surface density plots. The surface affinities of both TPA(+) and Gdm(+) increase upon adding NaCl to the solution. With the addition of Na2SO4, the surface affinity of TPA(+) increases, while that of Gdm(+) decreases. From the results of MD simulations it is seen that Gdm(+) and SO42- ions form pairs. This finding can be used to explain the decreased surface affinity of Gdm(+) when co-dissolved with SO42- ions. Since SO42- ions avoid the surface due to the double charge and strong water interaction, the Gdm(+)-SO42- ion pair resides deeper in the solutions' bulk than the Gdm(+) ions. Since TPA(+) does not form ion pairs with SO42-, the TPA(+) ions are instead enriched at the surface.

Place, publisher, year, edition, pages
2018. Vol. 148, no 14, article id 144508
National Category
Physical Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-353201DOI: 10.1063/1.5024348ISI: 000430128600034PubMedID: 29655316OAI: oai:DiVA.org:uu-353201DiVA, id: diva2:1217443
Funder
Swedish Research CouncilAvailable from: 2018-06-13 Created: 2018-06-13 Last updated: 2018-09-02Bibliographically approved
In thesis
1. Ion pairing and Langmuir-like adsorption at aqueous surfaces studied by core-level spectroscopy
Open this publication in new window or tab >>Ion pairing and Langmuir-like adsorption at aqueous surfaces studied by core-level spectroscopy
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Surface-bulk equilibria for solutes in aqueous solutions are studied using X-ray Photoelectron Spectroscopy (XPS) with high surface and chemical sensitivity. In the first part, the results show a reduction of the biochemically relevant guanidinium ions’ surface propensity with the addition of disodium sulphate due to ion pairing with the strongly hydrated sulphate ion, which could have implications for protein folding. Thereafter, the work considers amphiphilic organic compounds related to atmospheric science where the surface propensities, orientations at the surface and solute-solute and solute-solvent interactions are investigated. In the second part, two linear organic ions are investigated both as single solutes and in mixture. Both organic ions are surface enriched on their own and even more in the mixed solute solution. Due to hydrophobic expulsion of the alkyl chains, ion pairing between the organic ions and van der Waals interaction, the organic ions seem to assemble in clusters with their alkyl chains pointing out of the surface. The third part also covers linear organic compounds but one at a time probing the surface concentration as a function of bulk concentration. A Langmuir-like adsorption behavior was observed and Gibb’s free energy of surface adsorption (ΔGAds) values were extracted. An empiric model for deriving values for ΔGAds is proposed based upon the seemingly linear change in ΔGAds per carbon when comparing alcohols of different chain lengths. The fourth part investigates the acid/base fraction at the surface as function of bulk pH. The most important factor for this fraction seems to be how the surface propensity varies with the charge state of the acid or base instead of a possible difference in pH or pKa value at the surface. In the fifth part the oxygen K-edge of aqueous carbonate and bicarbonate is probed with the bulk-sensitive Resonant Inelastic X-ray Scattering (RIXS) technique.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2018. p. 62
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1715
Keywords
XPS, Surface, Aqueous, Liquid jet, Langmuir
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:uu:diva-357369 (URN)978-91-513-0432-8 (ISBN)
Public defence
2018-10-19, Polhemsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
Opponent
Supervisors
Available from: 2018-09-27 Created: 2018-09-02 Last updated: 2018-10-16

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Ekholm, VictorWalz, Marie-MadeleineWerner, JosephinaRubensson, Jan-ErikBjörneholm, Olle

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