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Fluorosurfactant self-assembly at solid/liquid interfaces
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface Chemistry.
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface Chemistry.
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface Chemistry.ORCID iD: 0000-0001-7496-1101
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Analytical Chemistry.ORCID iD: 0000-0002-3444-9987
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2002 (English)In: Langmuir, ISSN 0743-7463, E-ISSN 1520-5827, Vol. 18, no 21, 8085-8095 p.Article in journal (Refereed) Published
Abstract [en]

Fluorosurfactants have some unique properties that are advantageously used in a range of applications. Their solutions are commonly in contact with solid surfaces onto which the molecules adsorb. Despite this, the adsorption behavior of fluorosurfactants at solid/liquid interfaces is not sufficiently understood, and there is a need for more information. In this study we focus on cationic fluorosurfactant adsorption on negatively charged hydrophilic surfaces, especially with respect to the adsorbed layer structure, long-range interactions, and adhesion forces. To this end we combined results obtained from bimorph and interferometric surface force instruments and ellipsometry techniques. The initial adsorption to the oppositely charged surfaces occurs due to the electrostatic attraction between the charged headgroups and the surface. Further adsorption, driven by hydrophobic interactions, occurs readily as the surfactant concentration is increased. Surface force and ellipsometric experiments indicate that the surfactants self-assemble in the form of bilayer aggregates. The thickness of the bilayer aggregates was found to be consistent with the molecular structure. Further, ellipsometric measurements indicate that no complete bilayers were formed but rather that bilayer aggregates were present on the surface even at concentrations well above the cmc. Surface force data for low fluorosurfactant concentrations demonstrate that upon compression the bilayer aggregates assembled on the isolated surfaces are transformed, and as a result monolayer structures build up between the surfaces in contact. The force required to attain bilayer-bilayer contact increases with the surfactant bulk concentration due to an increase in the repulsive double-layer force. The force required to drive out surfactant molecules to achieve monolayer-monolayer contact also increases with surfactant concentration. Above the cmc some additional aggregates are present on top of the bilayer aggregates coating the surface. The adhesion found between the monolayer aggregates is an order of magnitude larger than between the bilayer aggregates. However, it is an order of magnitude lower than the corresponding value for Langmuir-Blodgett monolayer films of similar fluorosurfactants.

Place, publisher, year, edition, pages
2002. Vol. 18, no 21, 8085-8095 p.
Keyword [en]
Adhesion; Adsorption; Aggregates; Electrostatics; Ellipsometry; Hydrophilicity; Hydrophobicity; Langmuir Blodgett films; Molecular structure; Monolayers; Phase interfaces; Positive ions; Self assembly; Fluorosurfactants; Surface active agents
National Category
Physical Chemistry
URN: urn:nbn:se:kth:diva-7414DOI: 10.1021/la025989cISI: 000178589700049OAI: diva2:12435
QC 20100809Available from: 2009-08-30 Created: 2009-08-30 Last updated: 2012-02-10Bibliographically approved
In thesis
1. Interactions Between Biopolymers and Surfactants with Focus on Fluorosurfactants and Proteins
Open this publication in new window or tab >>Interactions Between Biopolymers and Surfactants with Focus on Fluorosurfactants and Proteins
2007 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

The aim of this thesis was to obtain a better understanding of the association between surfactants and biopolymers in bulk solutions and at solid/aqueous liquid interface. In order to do this, the interactions between surfactants and biopolymers were investigated with a variety of experimental techniques.

The main focus has been on the interactions between fluorosurfactants and proteins, which are important during electrophoresis of proteins in silica capillaries. Electrophoretic separation of positively charged proteins is often complicated by non-specific adsorption of protein onto capillary wall, while it was found to improve when cationic fluorosurfactants were added into the background buffer. We investigated the interactions between a cationic fluorosurfactant, FC134, and a positively charged protein, lysozyme. By employing Nuclear Magnetic Resonance (NMR) and tensiometry we could conclude that the cationic fluorosurfactant did not associate with positively charged lysozyme in bulk solutions. At the solid/aqueous liquid interface, the adsorption of fluorosurfactants and lysozyme onto silica was studied by the surface force technique (MASIF), ellipsometry, reflectrometry, Quartz Crystal Microbalance (QCM-D) and Atomic Force Microscopy (AFM). Cationic fluorosurfactant FC134 was found to adsorb onto the silica surface in a form of bilayer aggregates, which led to a charge reversal of the originally negatively charged substrate. The adsorption of lysozyme onto silica was also extensive and it corresponded to the more than monolayer coverage. When adsorbing from mixed solutions, the presence of the cationic fluorosurfactant in the solution led to an elimination of the lysozyme in the resulting adsorbed layer. For the lysozyme concentration of 0.2 mg/ml, which is typical for the electrophoretic separation, it was found that adsorption of protein was suppressed by more than 90% when only 30 μM of FC134 was added into the buffer. The presence of the low amounts of residual proteins in the adsorbed layers caused an enhancement of the adsorption of fluorosurfactants, which was attributed to adsorption of the fluorosurfactants between proteins in a form of large vesicles.

The interactions between a positively charged biopolymer chitosan and an anionic surfactant sodium dodecylsulfate (SDS) were studied with respect to the effect of the ionic strength of the background electrolyte, both in the bulk solution and at the silica/liquid interface. It was shown that SDS and chitosan form complexes in the bulk solution, which reverse their charge at higher SDS concentrations. At SDS concentrations above the critical micellar concentration, large aggregates were formed, which were trapped in long-lived nonequilibrium states at both high and low ionic strengths. SDS did not adsorb at the silica/liquid interface by itself. However, by employing QCM-D and ellipsometry we detected an extensive adsorption of SDS on the silica substrate, which has been modified by adsorbed chitosan. The structure of the chitosan layer on the lowly charged silica was strongly affected by the ionic strength of the solution from which the chitosan adsorption took place. The interactions between SDS and the pre-adsorbed chitosan were found to be similar on lowly charged silica and on highly charged mica.

A novel method based on the Bruggeman effective medium approximation was proposed for the evaluation of ellipsometric data characterizing composite adsorbed layers.

Finally, the effect of the adsorbed layer surface roughness on the QCM-D response in liquid was studied with focus on trapped water. It was found that QCM-D effectively senses water, which is mechanically trapped inside topographical structures with the size in nano-meter scale.

Place, publisher, year, edition, pages
Stockholm: KTH, 2007. 97 p.
Trita-CHE-Report, ISSN 1654-1081 ; 2007:54
National Category
Organic Chemistry
urn:nbn:se:kth:diva-4475 (URN)978-91-7178-739-2 (ISBN)
Public defence
2007-09-14, Salongen, KTH Biblioteket, Osquara backe 31, Stockholm, 09:00
QC 20100809Available from: 2009-08-30 Created: 2007-08-30 Last updated: 2010-08-09Bibliographically approved

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