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Molecular Insight into Ion-Specific Interactions: Vibrational Sum Frequency Study of the Carboxylic Acid Moiety
KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemistry, Surface and Corrosion Science.ORCID iD: 0000-0001-8624-3377
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Ion specific effects at charged interfaces find numerous applications in colloidal sciences and play a vital role in many biological processes. Despite having been studied for over a century, starting with the work of F. Hofmeister in the 1880s, a comprehensive molecular  understanding remains elusive. It is currently believed that specific molecular interactions between ions and the various chemical functional groups, including the disruption of the interfacial water structure, are the key underlying steps. The research presented in this doctoral thesis focuses on the carboxylic acid moiety which is one of the chemical functionalities most frequently encountered at biological interfaces. Vibrational sum frequency spectroscopy (VSFS), a non-linear optical technique with an exquisite surface specificity, was used to investigate the interactions between the carboxylic acid moiety of a fatty acid Langmuir monolayer with monovalent (Li+, Na+, K+, Cs+), divalent (Ca2+, Mg2+, Mn2+, Ni2+, Co2+), and trivalent (Y3+, La3+) cations. The studies also focused on understanding the remarkable effect of negatively charged co-ions (Cl-, Br-, I-, SCN-) on the cation-carboxylate interactions. Another key result of this work is the identification of resolved spectral features linked to the Eigen-like hydronium (H3O+) cation at the charged carboxylic acid interface. VSFS allowed quantifying the surface charge, type of cation binding, and structural changes in the interfacial water molecules upon changes of the ion identity, concentration, and pH. The findings demonstrate that the physical-chemical properties of the interfacial layers reflect a subtle balance between molecular and electrostatic competitive interactions, providing new experimental quantitative insights for testing the suitability of extended new theories on charged interfaces and ion specific interactions.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2019. , p. 94
Series
TRITA-CBH-FOU ; 2019:40
Keywords [en]
specific ion effects, carboxylic acid, fatty acid Langmuir monolayer, vibrational sum frequency spectroscopy, VSFS, Electrical double layer
National Category
Physical Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-257762ISBN: 978-91-7873-279-1 (print)OAI: oai:DiVA.org:kth-257762DiVA, id: diva2:1348213
Public defence
2019-09-27, Hörsal D2, Lindstedtsvägen 9, stockholm, 14:00 (English)
Opponent
Supervisors
Note

QC 2019-09-05

Available from: 2019-09-05 Created: 2019-09-03 Last updated: 2019-09-05Bibliographically approved
List of papers
1. Molecular insight into carboxylic acid-alkali metal cations interactions: reversed affinities and ion-pair formation revealed by non-linear optics and simulations
Open this publication in new window or tab >>Molecular insight into carboxylic acid-alkali metal cations interactions: reversed affinities and ion-pair formation revealed by non-linear optics and simulations
2019 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 21, no 21, p. 11329-11344Article in journal (Refereed) Published
Abstract [en]

Specific interactions between the carboxylic acid moiety and the monovalent salts CsCl, NaCl, and LiCl, have been investigated in Langmuir monolayers using vibrational sum frequency spectroscopy (VSFS) and complemented with coarse grained and all-atom molecular dynamics simulations. By exploiting VSFS's intrinsic surface specificity, an emphasis was made on targeting headgroup vibrations of both its charged and uncharged forms as well as water molecules in the interfacial layer. The degree of deprotonation of the monolayer as a function of cation concentration and pH was experimentally determined and theoretically rationalized. Starting from 100 mM, the surface charge was overestimated by the Gouy-Chapman model and varied depending on the identity of the cation, highlighting the appearance of ion specific effects. Agreement could be found using a modified Poisson-Boltzmann model that takes into account steric effects, with a fitted effective ion-size compatible with the hydrated ion diameters. The relative affinity of the cations to the carboxylic acid moiety was pH dependent: at pH 4.5 they arranged in the order Cs+ 4 Na+ 4 Li+, but fully reversed (Li+ 4 Na+ 4 Cs+) at pH 9. Simulations yielded microscopic insight into the origin of this behavior, with the cations showing contrasting interaction preferences for either the uncharged carboxylic acid or the charged carboxylate. Sum frequency spectra also provided evidence that all cations remained hydrated when interacting with the charged headgroup, forming solvent-separated or solvent-shared ion pairs. However, for the specific case of 1 M Li+ at pH 9, contact ion pairs were formed. Finally, the remarkable effect of trace metal multivalent cations in the interpretation of experiments is briefly discussed. The results provide exciting new insights into the complex interactions of alkali metal cations with the biophysically relevant carboxylic acid moiety.

Place, publisher, year, edition, pages
Royal Society of Chemistry, 2019
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-254505 (URN)10.1039/c9cp00398c (DOI)000471025900047 ()31107479 (PubMedID)
Note

QC 20190715

Available from: 2019-07-15 Created: 2019-07-15 Last updated: 2019-09-03Bibliographically approved
2. Interactions of Na+ Cations with a Highly Charged Fatty Acid Langmuir Monolayer: Molecular Description of the Phase Transition
Open this publication in new window or tab >>Interactions of Na+ Cations with a Highly Charged Fatty Acid Langmuir Monolayer: Molecular Description of the Phase Transition
2019 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455Article in journal (Other (popular science, discussion, etc.)) Published
Abstract [en]

Vibrational sum frequency spectroscopy has been used to study the molecular properties upon compression of a highly charged arachidic acid Langmuir monolayer, which displays a first-order phase transition plateau in the surface pressure - molecular area (π-A) isotherm. By targeting vibrational modes from the carboxylic acid headgroup, alkyl chain, and interfacial water molecules, information regarding the surface charge, surface potential, type of ion pair formed, and conformational order of the monolayer could be extracted. The monolayer was found to be fully charged before reaching the 2D-phase transition plateau, where partial reprotonation, as well as the formation of COO⎺ Na+ contact-ion pairs, started to take place. After the transition, three headgroup species, mainly hydrated COO⎺, COOH, and COO⎺ Na+ contact-ion pairs could be identified and their proportions quantified. Comparison with theoretical models shows that predictions from the Gouy Chapman model are only adequate for the lowest surface charge densities (<-0.1 C/m2). In contrast, a modified Poisson-Boltzmann (MPB) model that accounts for finite-size of the cation, captures many of the experimental observables, including the partial reprotonation, and surface potential changes upon compression. The experimental results provide a quantitative molecular insight that could be used to test potential extensions to the theory.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2019
Keywords
Langmuir monolayer; VSFS; Vibrational Sum Frequency Spectroscopy
National Category
Physical Chemistry
Research subject
Chemistry
Identifiers
urn:nbn:se:kth:diva-256048 (URN)10.1021/acs.jpcc.9b06435 (DOI)000487349600035 ()2-s2.0-8507270841 (Scopus ID)
Note

QC 20190827

QC 20191015

Available from: 2019-08-16 Created: 2019-08-16 Last updated: 2019-10-15Bibliographically approved
3. Anion specific effects at negatively charged interfaces:the influence of Cl⎺, Br⎺, I⎺, and SCN⎺ on the interactionsof Na+ with the carboxylic acid moiety
Open this publication in new window or tab >>Anion specific effects at negatively charged interfaces:the influence of Cl⎺, Br⎺, I⎺, and SCN⎺ on the interactionsof Na+ with the carboxylic acid moiety
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The effect of SCN⎺ and the halide co-ions in the interactions of Na+ with carboxylic acid Langmuir monolayers has been investigated using vibrational sum frequency spectroscopy (VSFS). At 1 M concentrations in the subphase, the identity of the anion is shown to have a remarkable effect on the degree of deprotonation of the monolayer, with ions ordering in the sequence I⎺ > SCN⎺ > Cl⎺≈Br⎺. The same trend is observed both at pH 6 and pH 9 when the monolayer is intrinsically more charged. Evidence for the presence of SCN⎺ in the interfacial region, albeit at low to negligible concentrations, was found after identifying the C≡N stretch just above the detection limits. The results contradict electrostatic theories on charged interfaces where co-ions are not expected to play any significant role. The higher propensity for the large polarizable anions to deprotonate the monolayer is explained in terms of their ability to change the surface hydronium ion concentration.

National Category
Physical Chemistry
Research subject
Chemistry
Identifiers
urn:nbn:se:kth:diva-256568 (URN)
Note

QC 20190905

Available from: 2019-08-28 Created: 2019-08-28 Last updated: 2019-09-05Bibliographically approved
4. La3+ and Y3+ interactions with the carboxylic acid moiety at theliquid/vapor interface: identification of binding complexes, chargereversal, and detection limits.
Open this publication in new window or tab >>La3+ and Y3+ interactions with the carboxylic acid moiety at theliquid/vapor interface: identification of binding complexes, chargereversal, and detection limits.
Show others...
(English)Manuscript (preprint) (Other academic)
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-256569 (URN)
Note

Part of a thesis

QC 20190906

Available from: 2019-08-28 Created: 2019-08-28 Last updated: 2019-09-06Bibliographically approved
5. Vibrational Spectral Signature of Eigen-like hydrated protonsat negatively charged surfaces
Open this publication in new window or tab >>Vibrational Spectral Signature of Eigen-like hydrated protonsat negatively charged surfaces
(English)Manuscript (preprint) (Other academic)
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-256570 (URN)
Note

QC 20190906

Available from: 2019-08-28 Created: 2019-08-28 Last updated: 2019-09-06Bibliographically approved

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