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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, Skolan för kemi, bioteknologi och hälsa (CBH), Kemi.
Lund Univ, Sweden.
Lund Univ Sweden.ORCID-id: 0000-0001-8178-8175
KTH, Skolan för kemi, bioteknologi och hälsa (CBH), Kemi.
2019 (Engelska)Ingår i: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 21, nr 21, s. 11329-11344Artikel i tidskrift (Refereegranskat) 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.

Ort, förlag, år, upplaga, sidor
Royal Society of Chemistry, 2019. Vol. 21, nr 21, s. 11329-11344
Nationell ämneskategori
Kemi
Identifikatorer
URN: urn:nbn:se:kth:diva-254505DOI: 10.1039/c9cp00398cISI: 000471025900047PubMedID: 31107479OAI: oai:DiVA.org:kth-254505DiVA, id: diva2:1337454
Anmärkning

QC 20190715

Tillgänglig från: 2019-07-15 Skapad: 2019-07-15 Senast uppdaterad: 2019-09-03Bibliografiskt 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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