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Exploring the Surface of Aqueous Solutions: X-ray photoelectron spectroscopy studies using a liquid micro-jet
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy. Swedish University of Agricultural Science, Department of Chemistry and Biotechnology. (Division for molecular and condensed matter physics)
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 [en]
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: urn:nbn:se:uu:diva-265210ISBN: 978-91-554-9399-8 (print)OAI: oai:DiVA.org:uu-265210DiVA: diva2:865742
Public defence
2015-12-18, Häggsalen, Ångströmslaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
Opponent
Supervisors
Available from: 2015-11-27 Created: 2015-10-25 Last updated: 2016-01-13
List of papers
1. Succinic acid in aqueous solution: connecting microscopic surface composition and macroscopic surface tension
Open this publication in new window or tab >>Succinic acid in aqueous solution: connecting microscopic surface composition and macroscopic surface tension
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2014 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 16, no 39, 21486-21495 p.Article in journal (Refereed) Published
Abstract [en]

The water vapor interface of aqueous solutions of succinic acid, where pH values and bulk concentrations were varied, has been studied using surface sensitive X-ray photoelectron spectroscopy (XPS) and molecular dynamics (MD) simulations. It was found that succinic acid has a considerably higher propensity to reside in the aqueous surface region than its deprotonated form, which is effectively depleted from the surface due to the two strongly hydrated carboxylate groups. From both XPS experiments and MD simulations a strongly increased concentration of the acid form in the surface region compared to the bulk concentration was found and quantified. Detailed analysis of the surface of succinic acid solutions at different bulk concentrations led to the conclusion that succinic acid saturates the aqueous surface at high bulk concentrations. With the aid of MD simulations the thickness of the surface layer could be estimated, which enabled the quantification of surface concentration of succinic acid as a multiple of the known bulk concentration. The obtained enrichment factors were successfully used to model the surface tension of these binary aqueous solutions using two different models that account for the surface enrichment. This underlines the close correlation of increased concentration at the surface relative to the bulk and reduced surface tension of aqueous solutions of succinic acid. The results of this study shed light on the microscopic origin of surface tension, a macroscopic property. Furthermore, the impact of the results from this study on atmospheric modeling is discussed.

National Category
Chemical Sciences
Identifiers
urn:nbn:se:uu:diva-236531 (URN)10.1039/c4cp02776k (DOI)000343072900050 ()25182698 (PubMedID)
Available from: 2014-11-26 Created: 2014-11-19 Last updated: 2017-12-05Bibliographically approved
2. Surface behavior of amphiphiles in aqueous solution: a comparison between different pentanol isomers
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, 14036-14044 p.Article 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: 2017-12-04Bibliographically approved
3. Alcohols at the Aqueous Surface: Chain Length and Isomer Effects
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, 6648-6656 p.Article 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: 2017-12-01Bibliographically approved
4. Surface Behavior of Hydrated Guanidinium and Ammonium Ions: A Comparative Study by Photoelectron Spectroscopy and Molecular Dynamics
Open this publication in new window or tab >>Surface Behavior of Hydrated Guanidinium and Ammonium Ions: A Comparative Study by Photoelectron Spectroscopy and Molecular Dynamics
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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.

National Category
Physical Sciences Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-229444 (URN)10.1021/jp500867w (DOI)000338184400014 ()
Available from: 2014-08-08 Created: 2014-08-07 Last updated: 2017-12-05Bibliographically approved
5. Surface Partitioning in Organic-Inorganic Mixtures Contributes to the Size-Dependence of the Phase-State of Atmospheric Nanoparticles
Open this publication in new window or tab >>Surface Partitioning in Organic-Inorganic Mixtures Contributes to the Size-Dependence of the Phase-State of Atmospheric Nanoparticles
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2016 (English)In: Environmental Science & Technology, ISSN 0013-936X, Vol. 50, no 14, 7434-7442 p.Article in journal (Refereed) Published
Abstract [en]

Atmospheric particulate matter is one of the main factors governing the Earth's radiative budget, but its exact effects on the global climate are still uncertain. Knowledge on the molecular-scale surface phenomena as well as interactions between atmospheric organic and inorganic compounds is necessary for understanding the role of airborne nanoparticles in the Earth system. In this work, surface composition of aqueous model systems containing succinic acid and sodium chloride or ammonium sulfate is determined using a novel approach combining X-ray photoelectron spectroscopy, surface tension measurements and thermodynamic modeling. It is shown that succinic acid molecules are accumulated in the surface, yielding a 10-fold surface concentration as compared with the bulk for saturated succinic acid solutions. Inorganic salts further enhance this enrichment due to competition for hydration in the bulk. The surface compositions for various mixtures are parametrized to yield generalizable results and used to explain changes in surface tension. The enhanced surface partitioning implies an increased maximum solubility of organic compounds in atmospheric nanoparticles. The results can explain observations of size-dependent phase-state of atmospheric nanoparticles, suggesting that these particles can display drastically different behavior than predicted by bulk properties only.

National Category
Atom and Molecular Physics and Optics Physical Chemistry Meteorology and Atmospheric Sciences
Identifiers
urn:nbn:se:uu:diva-265223 (URN)10.1021/acs.est.6b00789 (DOI)000380295700019 ()27326704 (PubMedID)
External cooperation:
Funder
Swedish Research CouncilSwedish Foundation for Strategic Research Carl Tryggers foundation EU, European Research Council, ERC-StG-27882 ATMOGAIN
Available from: 2015-10-29 Created: 2015-10-26 Last updated: 2016-09-13Bibliographically approved

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