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A Spectroscopic Study of Interfacial Films: Internal Structuring, Stability, and Hydration
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.ORCID iD: 0000-0003-2734-2847
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Monolayers of molecules are capable of completely changing the nature of surfaces and their interactions with the surroundings despite their almost negligible thickness. In the research presented in this doctoral thesis the surface specific technique Vibrational Sum Frequency Spectroscopy (VSFS) was combined with the Langmuir trough to investigate various manifestations of monolayers, such as biomimetic membranes, mono methyl branched long chain fatty acids in contact with air, and hydrophobic silane monolayers in contact with water.

VSFS was applied to in situ studies of the oxidative degradation of Langmuir monolayers of 1,2-diacylphosphocholines with identical C-18 chains featuring various degrees of unsaturation. The monolayer area and vinyl CH signal intensity were probed at constant surface pressure to monitor the degradation. The rapid degradation of the unsaturated lipids in contact with the ambient air is attributed to oxidation of the C=C bonds mediated by reactive species in the air and can be inhibited by purging the compartment surrounding the monolayer with nitrogen.

The molecular structure and order of Langmuir monolayers of 1,2-distearoyl-phosphocholine (18:0 PC) and their hydrating water were investigated at different surface pressures using VSFS. The monolayers are conformationally well ordered at all surface pressures and the signal intensity increases due to larger molecular number density at increasing surface pressures. Also, water signals with different vibrational frequencies are observed in different polarization combinations.

Additionally, a selection of common phospholipids (18:0 PC, 18:0 PC-D83 and 18:0 PS) were Langmuir-Blodgett (LB) deposited on CaF2 substrates and the CH- and OH-stretching regions as well as lower wavenumber regions were probed using VSFS. The orientation of the conformationally well ordered aliphatic chains was determined to be approximately perpendicular to the sample surface.

Monolayers of eicosanoic acid, its iso (19-MEA), and anteiso (18-MEA) analogues were investigated with VSFS, AFM imaging, and the Langmuir trough. The EA forms smooth, featureless monolayers when deposited on silica, while 19-MEA and 18-MEA form 10 -50 nm large domains with homogeneous size distribution. It was not possible to discriminate between the monolayers of racemic and chiral 18-MEA using any of the techniques employed

The influence of the experimental geometry on the SF spectral shape and en-hancement at and near total internal reflection conditions (TIR) were systematically investigated by comparing simulations with recorded data from a hydrophobized silica / water interface. The data agree qualitatively, but not quantitatively, with the simulations, and the reasons were critically discussed.

The water structure next to ordered and disordered hydrophobic silane monolayers on silica was investigated using VSFS. The results indicate that the structure of water next to a well ordered hydrophobic monolayer, with the exception of the first layer of water molecules, is not much different from that in the isotropic bulk. This is in contrast to the previous notion where such monolayers were assumed to induce a more ordered interfacial water structure.

Abstract [sv]

Trots sin begränsade tjocklek har monolager förmågan att totalt kunna förändra en ytas egenskaper och dess växelverkan med omgivningen. I forskningen som beskrivs i denna doktorsavhandling har den ytspecifika tekniken vibrationssumfrekvensspektroskopi (VSFS) kombinerats med Langmuir-tråget för att undersöka olika varianter av monolager, såsom biomimetiska membran, monometylgrenade långkedjiga fettsyror och hydrofoba silanmonolager i kontakt med vatten.

VSFS användes för att genomföra in situ undersökningar av den oxidativa nedbrytningen av Langmuir-monolager av 1,2-diacylfosfokoliner med identiska 18 kolatomer långa kedjor med varierande grader av omättnad. Monolagerarean och vinylCH-signalens intensitet mättes vid konstant yttryck för att övervaka nedbrytningen. Den snabba nedbrytningen av de omättade lipiderna i kontakt med den omgivande luften tillskrivs oxidation orsakad av reaktiva ämnen i luften och kan inhiberas fullständigt genom att genomblåsa utrymmet som omger monolagret med kvävgas.

Den molekylära strukturen och ordningen hos Langmuir-monolager av 1,2-distearoylfosfokolin (18:0 PC) och deras hydratiseringsvatten undersöktes vid olika yttryck med VSFS. Monolagerna har en välordnad konformation vid alla yttryck och signalintensiteten ökar vid ökande yttryck på grund av den större numerära molekyltätheten. Dessutom observeras vattensignaler med olika vibrationsfrekvenser i olika polarisationskombinationer.

Vidare LB-deponerades ett urval av vanligt förekommande fosfolipider (18:0 PC, 18:0 PC-D83 och 18:0 PS) på ett substrat av CaF2 varefter CH- och vattenregionen såväl som lågvågtalsområdet undersöktes med hjälp av VSFS. Orienteringen hos kolvätekedjorna, som hade en välordnad konformation, befanns vara ungefär vinkelrät mot provytan.

Monolager av eicosansyra, samt dess iso (19-MEA), och anteiso (18-MEA) analoger undersöktes med VSFS, AFM avbildning och Langmuirtråget. Eicosanssyran bildar jämna, strukturlösa monolager då den deponeras på kiseldioxid medan 19-MEA och 18-MEA formar 10 – 50 nm stora domäner med homogen storleksfördelning. Ingen av de använda teknikerna kunde särskilja racemisk och kiral 18-MEA.

Inverkan av försöksuppställningens geometri på SF-spektrumets form och förstärkning vid och i närheten av total internreflektion (TIR) undersöktes systematiskt genom att jämföra simuleringar med mätdata från en gränsyta mellan hydro-foberad kiseldioxid och vatten. Mätdata överensstämmer kvalitativt, men inte kvantitativt, med simuleringarna och orsakerna till detta diskuterades kritiskt.

Vattenstrukturen intill ordnade och oordnade hydrofoba silanmonolager undersöktes med VSFS. Resultaten visar att vattenstrukturen nära ett välordnat hydrofobt monolager, med undantag av det första lagret av vattenmolekyler, inte är särskilt olik den isotropa bulkfasens. Detta är i motsats till den tidigare uppfattningen att sådana monolager ansågs orsaka en mera välordnad vattenstruktur.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. , viii, 80 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2012:35
National Category
Physical Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-101081ISBN: 978-91-7501-436-4 (print)OAI: oai:DiVA.org:kth-101081DiVA: diva2:546321
Public defence
2012-09-21, KTH, Hörsal F3, Lindstedtsvägen 26, KTH, Stockholm, 13:30 (English)
Opponent
Supervisors
Note

QC 20120824

Available from: 2012-08-24 Created: 2012-08-23 Last updated: 2012-08-24Bibliographically approved
List of papers
1. Phospholipid Monolayers Probed by Vibrational Sum Frequency Spectroscopy: Instability of Unsaturated Phospholipids
Open this publication in new window or tab >>Phospholipid Monolayers Probed by Vibrational Sum Frequency Spectroscopy: Instability of Unsaturated Phospholipids
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2010 (English)In: Biophysical Journal, ISSN 0006-3495, E-ISSN 1542-0086, Vol. 98, L50-L52 p.Article in journal, Letter (Refereed) Published
Abstract [en]

The surface specific technique vibrational sum frequency spectroscopy has been applied to in situ studies of the degradation of Langmuir monolayers of 1,2-diacyl-phosphocholines with various degrees of unsaturation in the aliphatic chains. To monitor the degradation of the phospholipids, the time-dependent change of the monolayer area at constant surface pressure and the sum frequency intensity of the vinyl CH stretch at the carbon-carbon double bonds were measured. The data show a rapid degradation of monolayers of phospholipids carrying unsaturated aliphatic chains compared to the stable lipids carrying fully saturated chains when exposed to the ambient laboratory air. In addition, the degradation of the phospholipids can be inhibited by purging the ambient air with nitrogen. This instability may be attributed to spontaneous degradation by oxidation mediated by various reactive species in the air. To further elucidate the process of lipid oxidation in biological membranes artificial Langmuir monolayers probed by a surface specific spectroscopic technique as in this study can serve as a model system for studying the degradation/oxidation of cell membrane constituents.

Place, publisher, year, edition, pages
Cell Press, 2010
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-24456 (URN)10.1016/j.bpj.2010.02.009 (DOI)000277858400002 ()2-s2.0-77952778434 (Scopus ID)
Note

QC 20100924

Available from: 2010-09-09 Created: 2010-09-09 Last updated: 2017-12-12Bibliographically approved
2. The Structure of Model Membranes Studied by Vibrational Sum Frequency Spectroscopy
Open this publication in new window or tab >>The Structure of Model Membranes Studied by Vibrational Sum Frequency Spectroscopy
2010 (English)In: Progress in Colloid and Polymer Science, ISSN 0340-255X, E-ISSN 1437-8027, Vol. 137, 9-12 p.Article in journal (Refereed) Published
Abstract [en]

The structure and order of insoluble Langmuir monolayers consisting of 1,2-distearoyl-sn-glycero-3-phosphatidylcholine (DSPC or 18:0 PC) and the surrounding water molecules have been investigated by vibrational sum frequency spectroscopy (VSFS). At surface pressures of 1, 15, and 57 mN/m corresponding to molecular areas of 53, 50, and 43 Å2, respectively, the DSPC molecules formed a well ordered film. Both the VSF signal from the methyl stretching vibrations of the lipid and the surrounding water increased with enhanced surface pressure, as a result of the higher surface density and increased order of the system. Water molecules hydrating the polar parts of the headgroup and in close contact to the hydrocarbon groups of the lipid were observed in all three polarization combinations of the laser beams, and distinguished by their different vibrational frequencies.

Place, publisher, year, edition, pages
Berlin Heidelberg: Springer-Verlag, 2010
Keyword
Vibrational sum frequency spectroscopy, SFG, DSPC, Langmuir film, membrane
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-24458 (URN)10.1007/978-3-642-13461-6_3 (DOI)000295079000003 ()2-s2.0-79957827955 (Scopus ID)
Note

QC 20100924

Available from: 2010-09-09 Created: 2010-09-09 Last updated: 2017-12-12Bibliographically approved
3. Supported Phospholipid Monolayers: The Molecular Structure Investigated by Vibrational Sum Frequency Spectroscopy
Open this publication in new window or tab >>Supported Phospholipid Monolayers: The Molecular Structure Investigated by Vibrational Sum Frequency Spectroscopy
2011 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 115, no 21, 10617-10629 p.Article in journal (Refereed) Published
Abstract [en]

The molecular structure, packing properties, and hydrating water of Langmuir-Blodgett monolayers of the phospholipids 1,2-distearoyl-sn-glyercophosphatidylcholine (DSPC, 18:0 PC), its deuterated analogue (18:0 PC-d83), and 1,2-distearoyl-sn-glyerco-phosphatidylserine (DSPC, 18:0 PS) deposited on planar calcium fluoride (CaF2) substrates have been investigated using the surface-specific nonlinear optical technique vibrational sum frequency spectroscopy (VSFS). Compression isotherms were recorded before the deposition of the monolayers at a surface pressure of 35 mN/m, mimicking the conditions of biological cell membranes. The CH and CD stretch regions, the water region, and the lower wavenumber region, containing phosphate, ester, carboxylate, and amine signals, thus partly covering the fingerprint region, were probed to obtain a complete map of the molecules. The data indicate that all deposited monolayers formed a well-ordered and stable film, and probing the water region revealed significant differences in hydration for the different headgroups. In addition, the tilt angle of the aliphatic chains relative to the surface normal was estimated to be approximately 4 degrees to 10 degrees based on orientational analysis using the antisymmetric methyl stretching vibration. Orientational analysis of the ester C=O groups was also performed, and the result was consistent with the estimated tilt angle of the aliphatic chains.

Keyword
Vibrational sum frequency spectroscopy, SFG, DSPC, DSPS, Langmuir film, membrane
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-24464 (URN)10.1021/jp111587e (DOI)000290914700040 ()2-s2.0-79957845982 (Scopus ID)
Funder
Swedish Research Council
Note
QC 20100924. Uppdaterad från manuscript till published. Tidigare titel: Supported Phospholipid Monolayers: The Molecular Structure Investigated by Vibrational Sum Frequency Spectroscopy (VSFS)Available from: 2010-09-23 Created: 2010-09-09 Last updated: 2017-12-12Bibliographically approved
4. Self-assembly of long chain fatty acids: effect of a methyl branch
Open this publication in new window or tab >>Self-assembly of long chain fatty acids: effect of a methyl branch
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2014 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 16, no 33, 17869-17882 p.Article in journal (Refereed) Published
Abstract [en]

The morphology and molecular conformation of Langmuir Blodgett deposited and floating monolayers of a selection of straight chain (eicosanoic acid, EA), iso (19-methyl eicosanoic acid, 19-MEA), and anteiso (18-methyl eicosanoic acid, 18-MEA) fatty acids have been investigated by Vibrational Sum Frequency Spectroscopy (VSFS), AFM imaging, and the Langmuir trough. While the straight chain fatty acid forms smooth, featureless monolayers, all the branched chain fatty acids display 10-50 nm sized domains (larger for 19-MEA than the 18-MEA) with a homogeneous size distribution. A model is suggested to explain the domain formation and size in terms of the branched fatty acid packing properties and the formation of hemispherical caps at the liquid air interface. No difference between the chiral (S) form and the racemic mixture of the 18-MEA is observed with any of the utilized techniques. The aliphatic chains of the straight chain fatty acids appear to be oriented perpendicular to the sample surface, based on an orientational analysis of VSFS data and the odd/even effect. In addition, the selection of the subphase (neat water or CdCl2 containing water buffered to pH 6.0) used for the LB-deposition has a profound influence on the monolayer morphology, packing density, compressibility, and conformational order. Finally, the orientation of the 19-MEA dimethyl moiety is estimated, and a strategy for performing an orientational analysis to determine the complete molecular orientation of the aliphatic chains of 19-MEA and 18-MEA is outlined and discussed.

Place, publisher, year, edition, pages
Royal Society of Chemistry, 2014
Keyword
Sum-Frequency Spectroscopy, Air-Water-Interface, Generation Vibrational Spectroscopy, Normal-Alkyl Chains, H Stretching Modes, Molecular-Structure, 18-Methyleicosanoic Acid, Air/Water Interface, Eicosanoic Acid, Lipid-Bilayer
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-98091 (URN)10.1039/C4CP00512K (DOI)000341064800058 ()2-s2.0-84905453377 (Scopus ID)978-91-7501-436-4 (ISBN)
Funder
Swedish Foundation for Strategic Research Swedish Research Council
Note

QC 20140929

Manuscript in Thesis. QS 2012 with title "Domain Formation and Molecular Conformation in Monolayers of Branched Long Chain Fatty Acids: The Influence of ω-1 and ω-2 Methylation"

Available from: 2012-08-23 Created: 2012-06-19 Last updated: 2017-12-07Bibliographically approved
5. Vibrational sum frequency spectroscopy studies at solid/liquid interfaces: Influence of the experimental geometry in the spectral shape and enhancement
Open this publication in new window or tab >>Vibrational sum frequency spectroscopy studies at solid/liquid interfaces: Influence of the experimental geometry in the spectral shape and enhancement
2012 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 116, no 43, 22893-22903 p.Article in journal (Refereed) Published
Abstract [en]

The influence of the experimental geometry, specifically the angles of incidence (AOI) of the exciting beams, on the enhancement of the vibrational sum frequency spectroscopy (VSFS) spectra has been systematically investigated, particularly when approaching total internal reflection (TIR) conditions. Theoretical simulations of the spectral intensity as a function of the AOI and infrared wavelength at three different polarization combinations were critically compared to experimental data obtained from a hydrophobized silica/water interface. The spectra were recorded using a newly established femtosecond VSFS spectrometer designed to allow rapid and accurate changes of the angles of incidence. At the most favorable AOI, when the IR, visible, and/or emitted sum frequency beams were close to TIR conditions, the spectral enhancement could be up to several orders of magnitude. The effect from the IR beam was however less pronounced due to absorption from the aqueous phase. Furthermore, the relative enhancement of the spectral features across the CH and OH stretching regions varied significantly depending on the angles of incidence chosen. The uneven enhancement is a consequence of the large dispersion in the IR refractive index of water. Although the theoretical simulations qualitatively predict this trend, the quantitative agreement with the experimental data is poor. The simulations underestimate the amplification in the free OH range by 2−4 times as the IRbeam approaches TIR, and overestimates it as the visible beam approaches the critical angle. These observations may suggest a need to review the fundamental equations describing the angular dependence of the SF intensity. The practical advantages of selecting specific angles of incidence, as well as the expected variations for two other frequently encountered solid/liquid interfaces (CaF2/water and sapphire/water), are also discussed.

Keyword
SFG, VSFS, TIR-SFG, SFS, TIR, total internal reflection, angular dependence, water structure, hydrophobic surface, silane monolayer, silica, calcium fluoride, sapphire
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-101079 (URN)10.1021/jp306838a (DOI)000310482900029 ()2-s2.0-84868138282 (Scopus ID)
Funder
Swedish Research CouncilEU, FP7, Seventh Framework Programme, 239459
Note

QC 20121213

Available from: 2012-08-23 Created: 2012-08-23 Last updated: 2017-12-07Bibliographically approved
6. Water Structure Next to Ordered and Disordered Hydrophobic Silane Monolayers: A Vibrational Sum Frequency Spectroscopy Study
Open this publication in new window or tab >>Water Structure Next to Ordered and Disordered Hydrophobic Silane Monolayers: A Vibrational Sum Frequency Spectroscopy Study
2013 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 117, no 4, 1780-1790 p.Article in journal (Refereed) Published
Abstract [en]

Vibrational sum frequency spectroscopy (VSFS) has been used to study the structure of water incontact to fused silica surfaces, hydrophobically modified with series of alkyl trichlorosilanemonolayers of varying degrees of order. The interfacial molecular structural information wascomplemented using total internal reflection (TIR) Raman spectroscopy. The silane moleculesconsisted of octadecyltrichlorosilane (OTS) and its shorter chain analogue with eight carbonatoms. The VSFS spectra show a direct correlation between monolayer order and the intensity ofthe free OH mode, characteristic of straddling water molecules vibrating in direct contact to thehydrophobic layer, with a concurrent reduction of the bands associated with hydrogen bondedwater molecules. The results imply that the structure of water in the most ordered monolayers isnot much affected beyond the first layer of water molecules, with bulk isotropic propertiesbecoming apparent already at sub-nanometer distances from the surface. Contact anglemeasurements with both water and hexadecane were also performed in an effort to relatemacroscopically measureable parameters to the molecular surface structure provided by VSFSand TIR Raman. Only the receding contact angles, and in particular those with hexadecane, werefound to correlate with the monolayer order. Finally, to obtain an additional insight into the waterstructure in direct contact to an ordered hydrophobic surface, isotopic dilution experiments withD2O were also performed. They indicate that the vibrational coupling mechanism of watermolecules at the hydrophobized solid/water interface is different from what has been observed atthe liquid/vapor interface.

Keyword
TIR Raman, silica interface, contact angles, surface molecular structure, octadecyltrichlorosilane
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-101080 (URN)10.1021/jp310732f (DOI)000314492400030 ()2-s2.0-84874413657 (Scopus ID)
Funder
Swedish Research Council
Note

Uppdaterad från Manuskript till Artikel (20130308) QC 20130308

Available from: 2012-08-23 Created: 2012-08-23 Last updated: 2017-12-07Bibliographically approved

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