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Ion association in aqueous and non-aqueous solutions probed by diffusion and electrophoretic NMR
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.
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2015 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 17, no 5, 3402-3408 p.Article in journal (Refereed) Published
Abstract [en]

The results of diffusion and electrophoretic NMR (eNMR) measurements are reported for a series of tetramethylammonium (TMA) electrolytes (with sulphate, fluoride, acetate, chloride, bromide, nitrate, iodide and perchlorate as anions) in deuterated solvents such as water, dimethylsulphoxide (DMSO), acetonitrile, methanol and ethanol. In addition, similar data are presented for aqueous solutions of tetraalkylammonium salts with increasing alkyl chain length. The combination of diffusion NMR and eNMR yields the effective charge for the TMA cation. Relative to the nominal charge of z(nom) = 1 of TMA, the effective charge in the different solvents is found to be progressively smaller in the order water > DMSO > methanol > acetonitrile > ethanol. A part of this observed trend is ascribed to regular ion-ion interactions incorporated in the Onsager limiting law. Indeed, in solvents with high dielectric constants such as water, DMSO and methanol, the Onsager limiting law describes well the observations for all tetraalkylammonium ions. For ethanol and acetonitrile, there is a significant difference between the experimental data and the expected limiting-law behavior that is attributed to ion association (ion pairing) not taken into consideration by the Onsager limiting law.

Place, publisher, year, edition, pages
2015. Vol. 17, no 5, 3402-3408 p.
National Category
Physical Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-160404DOI: 10.1039/c4cp04446kISI: 000348203200049Scopus ID: 2-s2.0-84921628743OAI: oai:DiVA.org:kth-160404DiVA: diva2:790384
Funder
Knut and Alice Wallenberg FoundationSwedish Research Council
Note

QC 20150224

Available from: 2015-02-24 Created: 2015-02-19 Last updated: 2017-12-04Bibliographically approved
In thesis
1. Ions interacting with macromolecules: NMR studies in solution
Open this publication in new window or tab >>Ions interacting with macromolecules: NMR studies in solution
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Specific ion effects, identified for more than hundred years, play an important role in a wide range of phenomena and applications. Several mechanisms such as direct ion interaction with molecules have been suggested to explain these effects, but quantitative experimental evidence remains scarce. Electrophoretic NMR (eNMR) has been emerging as a very powerful tool for studying molecular association and ionic transport in a variety of systems. Yet its potential in studying specific ion effect has been unexplored. In this thesis, eNMR was in part developed further as an analytical method and was in part used as one of the main techniques to study ions interacting with macromolecules in aqueous and non-aqueous solutions.

The complexation of a large group of cations with poly ethylene oxide (PEO) in methanol was studied with eNMR. The binding of monovalent ions was demonstrated not to follow the Hofmeister order; multivalent cations except barium all showed negligible complexation. As a unifying feature, only cations with surface charge density below a threshold value were able to bind suggesting that ion solvation is critical. The binding mechanism was examined in greater detail for K+ and Ba2+ with oligomeric PEO of different chain lengths. Those two cations exhibited different binding mechanisms. K+ was found to bind to PEO by having at least 6 repeating units wrap around it while retaining the polymer flexibility. On the other hand, Ba2+ (and, to some extent, (BaAnion)+) needs a slightly shorter section to bind, but the molecular dynamics at the binding site slow considerably.

The binding of anions with poly (N-isopropylacrylamide) in water was quantified at low salt concentration with eNMR and the binding affinity, though very weak, followed the Hofmeister order. This result indicates the non-electrostatic nature of this specific ion effects. The increase of binding strength with salt concentration is well described by a Langmuir isotherm.

The specific ion binding to a protein, bovine serum albumin (BSA), was also studied at pH values where BSA has either net positive and negative charges. Our results show that anions have the same binding affinity irrespective of the surface charge while the binding strength of cations is reversed with the change in net surface charge. This indicates different binding mechanisms for cations and anions.

The ionization of cellobiose in alkaline solutions was measured quantitatively by eNMR. The results show a two-step deprotonation process with increasing alkaline strength. Supported by results from 1H-13C HSQC NMR and MD simulation, ionization was proposed to be responsible for the improved solubility of cellulose in alkaline solution. eNMR was also used to characterize the effective charge of tetramethylammonium ions in a variety of solvents. In solvents of high polarity, the results agree well with predictions based on Onsager’s limiting law but for nonpolar solvents deviations were found that were attributed to ion pair formation. 

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2017. 61 p.
Series
TRITA-CHE-Report, ISSN 1654-1081 ; 2017 : 12
Keyword
electrophoretic NMR, diffusion NMR, specific ion effects, Hofmeister, ion binding
National Category
Physical Chemistry
Research subject
Chemistry
Identifiers
urn:nbn:se:kth:diva-201732 (URN)978-91-7729-282-1 (ISBN)
Public defence
2017-03-22, F3, Lindstedtsvägen 26, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20170216

Available from: 2017-02-16 Created: 2017-02-15 Last updated: 2017-05-30Bibliographically approved

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