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Anion specific effects at negatively charged interfaces:the influence of Cl⎺, Br⎺, I⎺, and SCN⎺ on the interactionsof Na+ with 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
KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemistry.
(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: urn:nbn:se:kth:diva-256568OAI: oai:DiVA.org:kth-256568DiVA, id: diva2:1346732
Note

QC 20190905

Available from: 2019-08-28 Created: 2019-08-28 Last updated: 2019-09-05Bibliographically 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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Sthoer, Adrien

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