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Interactions of Na+ Cations with a Highly Charged Fatty Acid Langmuir Monolayer: Molecular Description of the Phase Transition
KTH, Skolan för kemi, bioteknologi och hälsa (CBH), Kemi, Yt- och korrosionsvetenskap. kth.ORCID-id: 0000-0001-8624-3377
KTH, Skolan för kemi, bioteknologi och hälsa (CBH), Kemi, Yt- och korrosionsvetenskap.
2019 (Engelska)Ingår i: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455Artikel i tidskrift (Övrig (populärvetenskap, debatt, mm)) 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.

Ort, förlag, år, upplaga, sidor
American Chemical Society (ACS), 2019.
Nyckelord [en]
Langmuir monolayer; VSFS; Vibrational Sum Frequency Spectroscopy
Nationell ämneskategori
Fysikalisk kemi
Forskningsämne
Kemi
Identifikatorer
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
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QC 20190827

QC 20191015

Tillgänglig från: 2019-08-16 Skapad: 2019-08-16 Senast uppdaterad: 2019-10-15Bibliografiskt granskad
Ingår i avhandling
1. Molecular Insight into Ion-Specific Interactions: Vibrational Sum Frequency Study of the Carboxylic Acid Moiety
Öppna denna publikation i ny flik eller fönster >>Molecular Insight into Ion-Specific Interactions: Vibrational Sum Frequency Study of the Carboxylic Acid Moiety
2019 (Engelska)Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
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.

Ort, förlag, år, upplaga, sidor
Stockholm: KTH Royal Institute of Technology, 2019. s. 94
Serie
TRITA-CBH-FOU ; 2019:40
Nyckelord
specific ion effects, carboxylic acid, fatty acid Langmuir monolayer, vibrational sum frequency spectroscopy, VSFS, Electrical double layer
Nationell ämneskategori
Fysikalisk kemi
Identifikatorer
urn:nbn:se:kth:diva-257762 (URN)978-91-7873-279-1 (ISBN)
Disputation
2019-09-27, Hörsal D2, Lindstedtsvägen 9, stockholm, 14:00 (Engelska)
Opponent
Handledare
Anmärkning

QC 2019-09-05

Tillgänglig från: 2019-09-05 Skapad: 2019-09-03 Senast uppdaterad: 2019-09-05Bibliografiskt granskad

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