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Ekholm, Victor
Publications (10 of 14) Show all publications
Ekholm, V., Vazdar, M., Mason, P. E., Bialik, E., Walz, M.-M., Ohrwall, G., . . . Björneholm, O. (2018). Anomalous surface behavior of hydrated guanidinium ions due to ion pairing. Journal of Chemical Physics, 148(14), Article ID 144508.
Open this publication in new window or tab >>Anomalous surface behavior of hydrated guanidinium ions due to ion pairing
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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.

National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-353201 (URN)10.1063/1.5024348 (DOI)000430128600034 ()29655316 (PubMedID)
Funder
Swedish Research Council
Available from: 2018-06-13 Created: 2018-06-13 Last updated: 2018-09-02Bibliographically approved
Ekholm, V. (2018). Ion pairing and Langmuir-like adsorption at aqueous surfaces studied by core-level spectroscopy. (Doctoral dissertation). Uppsala: Acta Universitatis Upsaliensis
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
Werner, J., Persson, I., Björneholm, O., Kawecki, D., Saak, C.-M., Walz, M.-M., . . . Prisle, N. L. (2018). Shifted Equilibria of Organic Acids and Bases in the Aqueous Surface Region. Physical Chemistry, Chemical Physics - PCCP, 20(36), 23281-23293
Open this publication in new window or tab >>Shifted Equilibria of Organic Acids and Bases in the Aqueous Surface Region
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2018 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 20, no 36, p. 23281-23293Article in journal (Refereed) Published
Abstract [en]

Acid-base equilibria of carboxylic acids and alkyl amines in the aqueous surface region were studied using surface-sensitive X-ray photoelectron spectroscopy and molecular dynamics simulations. Solutions of these organic compounds were examined as a function of pH, concentration and chain length to investigate the distribution of acid and base form in the surface region as compared to the aqueous bulk. Results from these experiments show that the neutral forms of the studied acid-base pairs are strongly enriched in the aqueous surface region. Moreover, we show that for species with at least four carbon atoms in their alkyl-chain, their charged forms are also found to be abundant in the surface region. Using a combination of XPS and MD results, a model is proposed that effectively describes the surface composition. Resulting absolute surface concentration estimations show clearly that the total organic mole fractions in the surface region change drastically as a function of solution pH. The origin of the observed surface phenomena, hydronium/hydroxide concentrations in the aqueous surface region and why standard chemical equations, used to describe equilibria in dilute bulk solution are not valid in the aqueous surface region, are discussed in detail. The reported results are of considerable importance especially for the detailed understanding of properties of small aqueous droplets that can be found in the atmosphere.

National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-341965 (URN)10.1039/c8cp01898g (DOI)000447370600011 ()30191936 (PubMedID)
Funder
Swedish Research Council, 2013-3940EU, Horizon 2020, 717022
Available from: 2018-02-16 Created: 2018-02-16 Last updated: 2019-01-22Bibliographically approved
Ekholm, V., Caleman, C., Bjärnhall Prytz, N., Walz, M.-M., Werner, J., Öhrwall, G., . . . Björneholm, O. (2018). Strong Enrichment of Atmospherically Relevant Organic Ions at the Aqueous Interface: The Role of Ion Pairing and Cooperative Effects. Physical Chemistry, Chemical Physics - PCCP, 20(42), 27185-27191
Open this publication in new window or tab >>Strong Enrichment of Atmospherically Relevant Organic Ions at the Aqueous Interface: The Role of Ion Pairing and Cooperative Effects
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2018 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 20, no 42, p. 27185-27191Article in journal (Refereed) Published
Abstract [en]

Surface affinity, orientation and ion pairing are investigated in mixed and single solute systems of aqueous sodium hexanoate and hexylammonium chloride. The surface sensitive X-ray photoelectron spectroscopy technique has been used to acquire the experimental results, while the computational data have been calculated using molecular dynamics simulations. By comparing the single solute solutions with the mixed one, we observe a non-linear surface enrichment and reorientation of the organic ions with their alkyl chains pointing out of the aqueous surface. We ascribe this effect to ion paring between the charged functional groups on the respective organic ion and hydrophobic expulsion of the alkyl chains from the surface in combination with van der Waals interactions between the alkyl chains. These cooperative effects lead to a substantial surface enrichment of organic ions, with consequences for aerosol surface properties.

National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:uu:diva-357368 (URN)10.1039/c8cp04525a (DOI)000451351500042 ()30339167 (PubMedID)
Funder
Swedish Research Council, 2013-3940Swedish Research Council, 2014-04518Carl Tryggers foundation
Available from: 2018-08-15 Created: 2018-08-15 Last updated: 2019-01-07Bibliographically approved
Marinho, R. R. T., Walz, M.-M., Ekholm, V., Ohrwall, G., Björneholm, O. & de Brito, A. N. (2017). Ethanol Solvation in Water Studied on a Molecular Scale by Photoelectron Spectroscopy. Journal of Physical Chemistry B, 121(33), 7916-7923
Open this publication in new window or tab >>Ethanol Solvation in Water Studied on a Molecular Scale by Photoelectron Spectroscopy
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2017 (English)In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 121, no 33, p. 7916-7923Article in journal (Refereed) Published
Abstract [en]

Because of the amphiphilic properties of alcohols, hydrophobic hydration is important in the alcohol water system. In the present paper we employ X-ray photoelectron spectroscopy (XPS) to investigate the bulk and surface molecular structure of ethanol water mixtures from 0.2 to 95 mol %. The observed XPS binding energy splitting between the methyl C is and hydroxymethyl C is groups (BES_[CH3-CH2OH]) as a function of the ethanol molar percentage can be divided into different regions: one below 35 mol % with higher values (about 1.53 eV) and one starting at 60 mol % up to 95 mol % with 1.49 eV as an average value. The chemical shifts agree with previous quantum mechanics/molecular mechanics (QM/MM) calculations [Loytynoja, T.; et al. J. Phys. Chem. B 2014, 118, 13217]. According to these calculations, the BES_[CH3-CH2OH] is related to the number of hydrogen bonds between the ethanol and the surrounding molecules. As the ethanol concentration increases, the average number of hydrogen bonds decreases from 2.5 for water-rich mixtures to 2 for pure ethanol. We give an interpretation for this behavior based on how the hydrogen bonds are distributed according to the mixing ratio. Since our experimental data are surface sensitive, we propose that this effect may also be manifested at the interface. From the ratio between the XPS C is core lines intensities we infer that below 20 mol % the ethanol molecules have their hydroxyl groups more hydrated and possibly facing the solution's bulk. Between 0.1 and 14 mol %, we show the formation of an ethanol monolayer at approximately 2 mol %. Several parameters are derived for the surface region at monolayer coverage.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2017
National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-335233 (URN)10.1021/acs.jpcb.7b02382 (DOI)000408598300020 ()28715892 (PubMedID)
Available from: 2017-12-05 Created: 2017-12-05 Last updated: 2017-12-05Bibliographically approved
Walz, M.-M., Werner, J., Ekholm, V., Prisle, N. L., Öhrwall, G. & Björneholm, O. (2016). Alcohols at the Aqueous Surface: Chain Length and Isomer Effects. Physical Chemistry, Chemical Physics - PCCP, 18(9), 6648-6656
Open this publication in new window or tab >>Alcohols at the Aqueous Surface: Chain Length and Isomer Effects
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2016 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 18, no 9, p. 6648-6656Article in journal (Refereed) Published
Abstract [en]

Surface-active organic molecules at the liquid-vapor interface are of great importance in atmospheric science. Therefore, we studied the surface behavior of alcohol isomers with different chain lengths (C4-C6) in aqueous solution with surface- and chemically sensitive X-ray photoelectron spectroscopy (XPS), which reveals information about the surface structure on a molecular level. Gibbs free energies of adsorption and surface concentrations are determined from the XPS results using a standard Langmuir adsorption isotherm model. The free energies of adsorption, ranging from around -15 to -19 kJ/mol (C4-C6), scale linearly with the number of carbon atoms within the alcohols with ΔGAds/CH2 ≈ -2 kJ/mol. While for the linear alcohols, surface concentrations lie around 2.4 x 1014 molecules/cm2 at the bulk concentrations where monolayers are formed, the studied branched alcohols show lower surface concentrations of around 1.6 x 1014 molecules/cm2, both of which are in line with the molecular structure and their orientation at the interface. Interestingly, we find that there is a maximum in the surface enrichment factor for linear alcohols at low concentrations, which is not observed for the shorter branched alcohols. This is interpreted in terms of a cooperative effect, which we suggest to be the result of more effective van der Waals interactions between the linear alcohol alkyl chains at the aqueous surface, making it energetically even more favorable to reside at the liquid-vapor interface. 

National Category
Physical Chemistry Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-265221 (URN)10.1039/c5cp06463e (DOI)000371139400030 ()26868637 (PubMedID)
Funder
Swedish Research CouncilSwedish Foundation for Strategic Research Carl Tryggers foundation
Available from: 2015-10-26 Created: 2015-10-26 Last updated: 2018-09-02Bibliographically approved
Couto, R. C., Guarise, M., Nicolaou, A., Jaouen, N., Chiuzbaian, G. S., Luening, J., . . . Simon, M. (2016). Anomalously strong two-electron one-photon X-ray decay transitions in CO caused by avoided crossing. Scientific Reports, 6, Article ID 20947.
Open this publication in new window or tab >>Anomalously strong two-electron one-photon X-ray decay transitions in CO caused by avoided crossing
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2016 (English)In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 6, article id 20947Article in journal (Refereed) Published
Abstract [en]

The unique opportunity to study and control electron-nuclear quantum dynamics in coupled potentials offered by the resonant inelastic X-ray scattering (RIXS) technique is utilized to unravel an anomalously strong two-electron one-photon transition from core-excited to Rydberg final states in the CO molecule. High-resolution RIXS measurements of CO in the energy region of 12-14 eV are presented and analyzed by means of quantum simulations using the wave packet propagation formalism and ab initio calculations of potential energy curves and transition dipole moments. The very good overall agreement between the experimental results and the theoretical predictions allows an in-depth interpretation of the salient spectral features in terms of Coulomb mixing of "dark" with "bright" final states leading to an effective two-electron one-photon transition. The present work illustrates that the improved spectral resolution of RIXS spectra achievable today may call for more advanced theories than what has been used in the past.

National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:uu:diva-279560 (URN)10.1038/srep20947 (DOI)000369829300001 ()26860458 (PubMedID)
Funder
Swedish National Infrastructure for Computing (SNIC), SNIC 2015/1-69Swedish National Infrastructure for Computing (SNIC), SNIC 023/07-18Swedish Research CouncilKnut and Alice Wallenberg FoundationSwedish Research Council
Available from: 2016-03-02 Created: 2016-03-02 Last updated: 2017-11-30Bibliographically approved
Couto, R. C., Guarise, M., Nicolaou, A., Jaouen, N., Chiuzbaian, G. S., Luening, J., . . . Kimberg, V. (2016). Coupled electron-nuclear dynamics in resonant 1 sigma -> 2 pi x-ray Raman scattering of CO molecules. PHYSICAL REVIEW A, 93(3), Article ID 032510.
Open this publication in new window or tab >>Coupled electron-nuclear dynamics in resonant 1 sigma -> 2 pi x-ray Raman scattering of CO molecules
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2016 (English)In: PHYSICAL REVIEW A, ISSN 2469-9926, Vol. 93, no 3, article id 032510Article in journal (Refereed) Published
Abstract [en]

We present a detailed experimental-theoretical analysis of O K-edge resonant 1 sigma-2 pi inelastic x-ray scattering (RIXS) from carbon monoxide with unprecedented energy resolution. We employ high-level ab initio calculations to compute the potential energy curves of the states involved in the RIXS process and simulate the measured RIXS spectra using the wave-packet-propagation formalism, including Coulomb coupling in the final-state manifold. The theoretical analysis allows us to explain all the key features of the experimental spectra, including some that were not seen before. First, we clearly show the interference effect between different RIXS channels corresponding to the transition via orthogonal (1)Pi(x) and (1)Pi(y) core-excited states of CO. Second, the RIXS region of 13 eV energy loss presents a triple structure, revealed only by the high-resolution measurement. In previous studies, this region was attributed solely to a valence state. Here we show a strong Coulomb mixing of the Rydberg and valence final states, which opens the forbidden RIXS channels to the "dark" final Rydberg states and drastically changes the RIXS profile. Third, using a combination of high-resolution experiment and high-level theory, we improve the vertical bar 4 sigma(-1)2 pi(1)> final-state potential-energy curve by fitting its bottom part with the experiment. Also, the coupling constants between Rydberg and valence states were refined via comparison with the experiment. Our results illustrate the large potential of the RIXS technique for advanced studies of highly excited states of neutral molecules.

National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:uu:diva-286660 (URN)10.1103/PhysRevA.93.032510 (DOI)000372399100006 ()
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation
Available from: 2016-04-27 Created: 2016-04-21 Last updated: 2016-04-27Bibliographically approved
Ohrwall, G., Prisle, N. L., Ottosson, N., Werner, J., Ekholm, V., Walz, M.-M. & Björneholm, O. (2015). Acid-Base Speciation of Carboxylate Ions in the Surface Region of Aqueous Solutions in the Presence of Ammonium and Aminium Ions. Journal of Physical Chemistry B, 119(10), 4033-4040
Open this publication in new window or tab >>Acid-Base Speciation of Carboxylate Ions in the Surface Region of Aqueous Solutions in the Presence of Ammonium and Aminium Ions
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2015 (English)In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 119, no 10, p. 4033-4040Article in journal (Refereed) Published
Abstract [en]

The acid base speciation of surface-active carboxylate ions in the surface region of aqueous solutions was studied with synchrotron-radiation-based photoelectron spectroscopy. The protonated form was found at an extraordinarily large fraction compared to that expected from the bulk pH. When adding salts containing the weak acid NH4+ to the solution, the fraction of the acidic form at the surface increases, and to a Much greatet extent than expected from the bulk pH of the solution. We show that ammonium ions also are overrepresented in the surface region, and propose that the interaction between the surface-active anionic carboxylates and cationic ammonium ions creates a carboxylateammonium bilayer close to the surface, which increases the probability of the protonation of the carboxylae ions. By comparing the situation when a salt of the less volatile amine diethanolatnine is used, We also show that the observed evaporation of ammonia that occurs after such an event only affects the equilibrium marginally.

National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-251803 (URN)10.1021/jp509945g (DOI)000351188300016 ()25700136 (PubMedID)
Available from: 2015-04-28 Created: 2015-04-24 Last updated: 2017-12-04Bibliographically approved
Walz, M.-M., Caleman, C., Werner, J., Ekholm, V., Lundberg, D., Prisle, N. L., . . . Björneholm, O. (2015). Surface behavior of amphiphiles in aqueous solution: a comparison between different pentanol isomers. Physical Chemistry, Chemical Physics - PCCP, 17(21), 14036-14044
Open this publication in new window or tab >>Surface behavior of amphiphiles in aqueous solution: a comparison between different pentanol isomers
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2015 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 17, no 21, p. 14036-14044Article in journal (Refereed) Published
Abstract [en]

Position isomerism is ubiquitous in atmospheric oxidation reactions. Therefore, we have compared surface-active oxygenated amphiphilic isomers (1- and 3-pentanol) at the aqueous surface with surface- and chemically sensitive X-ray photoelectron spectroscopy (XPS), which reveals information about the surface structure on a molecular level. The experimental data are complemented with molecular dynamics (MD) simulations. A concentration-dependent orientation and solvation of the amphiphiles at the aqueous surface is observed. At bulk concentrations as low as around 100 mM, a monolayer starts to form for both isomers, with the hydroxyl groups pointing towards the bulk water and the alkyl chains pointing towards the vacuum. The monolayer (ML) packing density of 3-pentanol is approx. 70% of the one observed for 1-pentanol, with a molar surface concentration that is approx. 90 times higher than the bulk concentration for both molecules. The molecular area at ML coverage (approximate to 100 mM) was calculated to be around 32 +/- 2 angstrom(2) per molecule for 1-pentanol and around 46 +/- 2 angstrom(2) per molecule for 3-pentanol, which results in a higher surface concentration (molecules per cm(2)) for the linear isomer. In general we conclude therefore that isomers - with comparable surface activities - that have smaller molecular areas will be more abundant at the interface in comparison to isomers with larger molecular areas, which might be of crucial importance for the understanding of key properties of aerosols, such as evaporation and uptake capabilities as well as their reactivity.

National Category
Physical Chemistry Physical Sciences
Identifiers
urn:nbn:se:uu:diva-256562 (URN)10.1039/c5cp01870f (DOI)000354946200029 ()25953683 (PubMedID)
Available from: 2015-06-24 Created: 2015-06-24 Last updated: 2018-09-02Bibliographically approved
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