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Fatty Acid Self-Assembly at the Air–Water Interface: Curvature, Patterning, and Biomimetics: A Study by Neutron Reflectometry and Atomic Force Microscopy
KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemistry, Surface and Corrosion Science.ORCID iD: 0000-0001-9197-4676
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

For more than a hundred years of interfacial science, long chain fatty acids have been the primary system for the study of floating monolayers at the air–water interface due to their amphiphilic nature and system simplicity: an insoluble hydrocarbon chain and a soluble carboxyl group at a flat air–water interface. Despite―or perhaps rather due to―the assumed simplicity of such systems and the maturity of the research field, the seemingly fundamentally rooted notion of a two-dimensional water surface has yet to be challenged.

The naturally occurring methyl-branched long chain fatty acid 18-methyleicosanoic acid and one of its isomers form monolayers consisting of monodisperse domains of tens of nanometres, varying in size with the placement of the methyl branch. The ability of domain-forming monolayers to three-dimensionally texture the air–water interface is investigated as a result of hydrocarbon packing constraints owing to the methyl branch.

In this work, neutron reflectometry has been used to study monolayers of branched long chain fatty acids directly at the air–water interface, which allowed precise probing of how a deformable water surface is affected by monolayer structure. Such films were also transferred by Langmuir–Blodgett deposition to the air–solid interface, and subsequently imaged by atomic force microscopy. Combined, the results unanimously―and all but unambiguously―show that the self-assembly of branched long chain fatty acids texture the air–water interface, inducing domain formation by a local curvature of the water surface, and thus controverting the preconceived notion of a planar air–water interface. The size and shape of the observed domains are shown to be tuneable using three different parameters: in mixed systems of branched and unbranched fatty acids, with varying hydrocarbon length of the straight chain, and altering subphase electrolyte properties. Each of these factors effectively allows changing the local curvature of the monolayer, much like analogous three-dimensional systems in bulk lyotropic crystals. This precise tuneability opens up for sustainable nanopatterning. Finally, the results lead to a plausible hypothesis of self-healing properties as to why the surface of hair and wool have a significant proportion of branched fatty acid.

Abstract [sv]

Under mer än hundra år av ytkemisk forskning har långa fettsyror utgjort ett standardsystem vid studier av monomolekylära skikt på fasgränsytan mellan luft och vatten. Här utnyttjas systemets enkelhet och fettsyrors amfifila egenskaper: de består av en hydrofil karboxylgrupp och en hydrofob kolkedja, vilket leder till att de adsorberas på en plan vatten–luftgränsyta. Systemets antagna enkelhet och fältets mognad till trots – eller snarare till följd av detta – har lett till att den förutfattade uppfattningen om en tvådimensionell vattenyta ännu ej ifrågasatts.

Den naturligt förekommande förgrenade fettsyran 18-metyleikosansyra och en av dess isomerer bildar monoskikt bestående av tiotals nanometer stora monodispersa domäner vars storlek varierar beroende på metylförgreningens placering på kolkedjan. Här undersöks hur dessa domäntäckta monoskikt strukturerar den underliggande vattenytan ut ur det tvådimensionella planet till följd av hur metylförgreningen begränsar intilliggande kolkedjors tätpackningsförmåga.

I avhandlingen har neutronspridning använts för att studera monoskikt av förgrenade fettsyror direkt på vatten–luftgränsytan. Metoden har möjliggjort att noggrant undersöka hur en formbar vattenyta påverkas av monoskiktets tredimensionella struktur. Sådana monoskikt har även överförts till fasta ytor med hjälp av Langmuir–Blodgettdeponering för att därefter karakteriseras med atomkraftsmikroskopi. Sammantaget har resultaten från dessa mättekniker enhälligt – om än allt utom strikt – bevisat att självassociering av grenade fettsyror kröker den underliggande vatten–luftgränsytan, vilket medför de uppvisade egenskaperna till domänformation. Detta bestrider föreställningen om en plan vattenyta. Form och storlek hos de observerade domänerna kan regleras genom att ändra kompositionen i blandsystem med förgrenade och raka fettsyror, variera längden på den raka fettsyran och genom att ändra subfasens elektrolytsammansättning. Vardera av dessa parametrar möjliggör lokal förändring av monoskiktets och därmed vattenytans krökning, vilket kan likställas med motsvarande självassocierande tredimensionella strukturer som miceller och flytande kristallina faser. Denna precisa styrning av domänformationen gör det möjligt att med hållbar kemi skapa varaktiga nanostrukturerade ytor. Slutligen har resultaten från den här avhandlingen lett fram till en hypotes relaterad till självläkande egenskaper, som beskriver varför den grenade fettsyran 18-metyleikosansyra står för en betydande del av fettsyrakompositionen på hårets yttersta gränsskikt.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2020. , p. 100
Series
TRITA-CBH-FOU ; 2020:26
Keywords [en]
self-assembly, branched fatty acids, Langmuir films, Langmuir–Blodgett, nanopatterning, AFM, neutron reflectometry
Keywords [sv]
grenade fettsyror, Langmuir monoskikt, Langmuir–Blodgett, självassociering, nanostrukturerade material, AFM, neutronspridning
National Category
Physical Chemistry
Research subject
Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-273240ISBN: 978-91-7873-533-4 (print)OAI: oai:DiVA.org:kth-273240DiVA, id: diva2:1429701
Public defence
2020-06-12, https://kth-se.zoom.us/webinar/register/WN_ELSPsgfwQXC1ueMnVXwIUg, Stockholm, 10:00 (English)
Supervisors
Funder
Swedish Research Council, VR 2013-04384
Note

QC 2020-05-15

Available from: 2020-05-15 Created: 2020-05-12 Last updated: 2020-05-20Bibliographically approved
List of papers
1. 3D texturing of the air–water interface by biomimetic self-assembly
Open this publication in new window or tab >>3D texturing of the air–water interface by biomimetic self-assembly
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2020 (English)In: Nanoscale Horizons, ISSN 2055-6756, no 5, p. 839-846Article in journal (Refereed) Published
Abstract [en]

A simple, insoluble monolayer of fatty acid is shown to induce 3D nanotexturing of the air–water interface. This advance has been achieved through the study of monolayers of a methyl-branched long chain fatty acid, analogous to those found on the surface of hair and wool, directly at the air–water interface. Specular neutron reflectometry combined with AFM probing of deposited monolayers shows pronounced 3D surface domains, which are absent for unbranched analogues and are attributed to hydrocarbon packing constraints. The resulting surface topographies of the water far exceed the height perturbation that can be explained by the presence of capillary waves of a free liquid surface. These have hitherto been considered the only source of perturbation of the flatness of a planar water interface under gravity in the absence of topographical features from the presence of extended, globular or particulate matter. This amounts to a paradigm shift in the study of interfacial films and opens the possibility of 3D texturing of the air–water interface.

National Category
Physical Chemistry
Research subject
Chemistry
Identifiers
urn:nbn:se:kth:diva-273214 (URN)10.1039/C9NH00722A (DOI)000531354100011 ()32364200 (PubMedID)
Note

QC 20200512

Available from: 2020-05-11 Created: 2020-05-11 Last updated: 2020-05-23Bibliographically approved
2. Tuneable Self-Assembly Curvature at the Air–Water Interface
Open this publication in new window or tab >>Tuneable Self-Assembly Curvature at the Air–Water Interface
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(English)In: Article in journal (Refereed) Submitted
Keywords
Self-assembly, Langmuir films, AFM, branched fatty acid, 18-MEA, air–water interface, curvature
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-273208 (URN)
Funder
Swedish Research Council, VR 2013- 04384
Available from: 2020-05-11 Created: 2020-05-11 Last updated: 2020-05-12
3. Texture and Topography of Fatty Acid Langmuir Films: Domain Stability and Isotherm Analysis
Open this publication in new window or tab >>Texture and Topography of Fatty Acid Langmuir Films: Domain Stability and Isotherm Analysis
(English)In: Article in journal (Refereed) Submitted
Keywords
Nanopatterning, Langmuir monolayer, LB deposition, surface texturing, Langmuir isotherms
National Category
Physical Chemistry
Research subject
Chemistry; Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-272480 (URN)
Funder
Swedish Research Council, VR 2013-04384
Available from: 2020-05-11 Created: 2020-05-11 Last updated: 2020-05-12
4. Self-Assembly Induced Patterning in Biomimetic Fatty Acid Monolayers
Open this publication in new window or tab >>Self-Assembly Induced Patterning in Biomimetic Fatty Acid Monolayers
(English)Manuscript (preprint) (Other academic)
Keywords
Self-assembly, Langmuir films, AFM, branched fatty acid, 18-MEA, air–water interface, curvature
National Category
Physical Chemistry
Research subject
Chemistry; Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-272482 (URN)
Funder
Swedish Research Council, VR 2013- 04384
Available from: 2020-05-11 Created: 2020-05-11 Last updated: 2020-05-12

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