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Molecular insight into carboxylic acid-alkali metal cations interactions: reversed affinities and ion-pair formation revealed by non-linear optics and simulations
KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemistry.
Lund Univ, Sweden.
Lund Univ Sweden.ORCID iD: 0000-0001-8178-8175
KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemistry.
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. Vol. 21, no 21, p. 11329-11344
National Category
Chemical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-254505DOI: 10.1039/c9cp00398cISI: 000471025900047PubMedID: 31107479OAI: oai:DiVA.org:kth-254505DiVA, id: diva2:1337454
Note

QC 20190715

Available from: 2019-07-15 Created: 2019-07-15 Last updated: 2019-09-03Bibliographically approved
In thesis
1. Molecular Insight into Ion-Specific Interactions: Vibrational Sum Frequency Study of the Carboxylic Acid Moiety
Open this publication in new window or tab >>Molecular Insight into Ion-Specific Interactions: Vibrational Sum Frequency Study of the Carboxylic Acid Moiety
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
specific ion effects, carboxylic acid, fatty acid Langmuir monolayer, vibrational sum frequency spectroscopy, VSFS, Electrical double layer
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-257762 (URN)978-91-7873-279-1 (ISBN)
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

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