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Interactions of Na+ Cations with a Highly Charged Fatty Acid Langmuir Monolayer: Molecular Description of the Phase Transition
KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemistry, Surface and Corrosion Science. kth.ORCID iD: 0000-0001-8624-3377
KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemistry, Surface and Corrosion Science.
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 [en]
Langmuir monolayer; VSFS; Vibrational Sum Frequency Spectroscopy
National Category
Physical Chemistry
Research subject
Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-256048DOI: 10.1021/acs.jpcc.9b06435ISI: 000487349600035Scopus ID: 2-s2.0-8507270841OAI: oai:DiVA.org:kth-256048DiVA, id: diva2:1343507
Note

QC 20190827

QC 20191015

Available from: 2019-08-16 Created: 2019-08-16 Last updated: 2019-10-15Bibliographically 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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