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Assessing Surfactant Binding to Carbon Nanotubes via Competitive Adsorption: Binding strength and critical coverage
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry. Centro de Investigação em Química, Department of Chemistry and Biochemistry, Faculty of Science, University of Porto, Rua do Campo Alegre, s/n, Porto, Portugal.ORCID iD: 0000-0003-1775-8160
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.
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2015 (English)Manuscript (preprint) (Other academic)
Abstract [en]

The displacement of a nonionic polymeric dispersant, Pluronic F127, adsorbed at the surface of single-walled carbon nanotubes, by low molecular-weight ionic dispersants (surfactants) is studied in aqueous dispersion. The method applied is diffusion NMR spectroscopy that can accurately measure the fraction of F127 molecules adsorbed at the tube surface because of the slow exchange (over the experimental time scale) of F127 between bulk and surface. In a series of surfactants with varying chain length and headgroups, we find that anionic surfactants replace in general more nonionic F127 than do cationic surfactants. The data collected show a strong correlation with the critical dispersibility concentration of the different surfactants, a parameter that signifies the concentration at which one obtains significant dispersed nanotube concentration by ultrasonication. We posit that this finding indicates the existence of a threshold surface coverage for dispersants that constitutes a necessary condition for de-bundling by ultrasonication. The results are discussed in relation to previous findings in the literature. 

Place, publisher, year, edition, pages
2015.
National Category
Physical Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-176436OAI: oai:DiVA.org:kth-176436DiVA, id: diva2:866856
Note

QC 20180103

Available from: 2015-11-04 Created: 2015-11-04 Last updated: 2018-01-03Bibliographically approved
In thesis
1. Dispersing Carbon Nanotubes: Towards Molecular Understanding
Open this publication in new window or tab >>Dispersing Carbon Nanotubes: Towards Molecular Understanding
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Carbon nanotubes (CNTs) exhibit unique and fascinating intrinsic electrical, optical, thermal or mechanical properties that lead to a plethora of potential applications in composite materials, electronics, energy storage, medicine, among others. However, the manipulation of nanotubes is not trivial and there are significant difficulties to overcome before achieving their full potential in applications. Because of their high aspect ratio and strong tube-to-tube van der Waals interactions, nanotubes form bundles and ropes that are difficult to disperse in liquids. In this thesis, the topic of dispersing carbon nanotubes in water was addressed by several experimental methods such as nuclear magnetic resonance (NMR) diffusometry and light/electron microcopy. The main goal was to obtain molecular information on how the dispersants interact with carbon nanotubes.

In dispersions of single-walled carbon nanotubes (SWNTs) in water, only a small fraction of the polymeric dispersant (Pluronic F127) was shown to be adsorbed at the CNT surface. Regarding dynamic features, the residence time of F127 on the SWNT surface was measured to be in the order of hundred milliseconds, and the lateral diffusion coefficient of the polymer along the nanotube surface proved to be an order of magnitude slower than that in the solution. The surface coverage of SWNTs by F127 was also investigated and the competitive adsorption of F127 and the protein bovine serum albumin, BSA, was assessed. F127 was found to bind stronger to the CNT surface than BSA does.

Low molecular weight dispersants, viz. surfactants, were also investigated. Using carefully controlled conditions for the sonication and centrifugation steps, reproducible sigmoidal dispersibility curves were obtained, that exhibited an interesting variation with molecular properties of the surfactants. Various metrics that quantify the ability of different surfactants to disperse CNTs were obtained. In particular, the concentration of surfactant required to attain maximal dispersibility depends linearly on alkyl chain length, which indicates that the CNT-surfactant association, although hydrophobic in nature, is different from a micellization process. No correlation between dispersibility and the critical micellization concentration, cmc, of the surfactants was found. For gemini surfactants of the n-s-n type with spacer length s and hydrophobic tail length n, the dispersibility of multiwalled carbon nanotubes (MWNTs) also followed sigmoidal curves that were compared to those obtained with single-tailed homologues. The increase in spacer length caused an increase in the dispersion efficiency. The observations indicate a loose type of monolayer adsorption rather than the formation of micelle-like aggregates on the nanotube surface. With the future goal of embedding nanotubes in liquid crystal (LC) phases and thereby creating nanocomposites, the effect of the spacer length on the thermotropic behavior of the gemini 12-s-12 surfactant was investigated. Different mesophases were observed and a non-monotonic effect of the spacer length was found and rationalized within a model of the surfactant packing in the solid state.

The relative binding strength of simple surfactants to CNTs was assessed by the amount of F127 they displace from the CNT surface upon addition. Anionic surfactants were found to replace more F127, which was interpreted as a sign of stronger binding to CNT. The data collected for all surfactants showed a good correlation with their critical dispersibility concentration that suggests the existence of a surface coverage threshold for dispersing nanotubes.

On the macroscopic scale, the formation of weakly bound CNT aggregates in homogeneous dispersions was found to be induced by vortex-shaking. These aggregates could quickly and easily be re-dispersed by mild sonication. This counterintuitive behavior was related to the type of dispersant used and of the duration of mechanical agitation and was explained as a result of loose coverage by the dispersant. 

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. p. xii, 77
Series
TRITA-CHE-Report, ISSN 1654-1081 ; 2015:60
Keywords
carbon nanotubes, dispersion, surfactants, polymers, adsorption, liquid crystals, nuclear magnetic resonance, self-diffusion
National Category
Physical Chemistry
Research subject
Chemistry
Identifiers
urn:nbn:se:kth:diva-176443 (URN)978-91-7595-713-5 (ISBN)
Public defence
2015-11-26, F3, Lindstedtsvägen 26, KTH, Stockholm, 10:10 (English)
Opponent
Supervisors
Note

This Ph.D thesis was completed under the Thesis Co-supervision Agreement between KTH Royal Institute of Technology and the University of Port. QC 20151105

Available from: 2015-11-04 Created: 2015-11-04 Last updated: 2015-11-04Bibliographically approved
2. Adsorption, aggregation and phase separation in colloidal systems
Open this publication in new window or tab >>Adsorption, aggregation and phase separation in colloidal systems
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The thesis presents work regarding amphiphilic molecules associated in aqueous solution or at the liquid/solid interface. Two main topics are included: the temperature-dependent behavior of micelles and the adsorption of dispersants on carbon nanotube (CNT) surfaces. Various NMR methods were used to analyze those systems, such as chemical shift detection, spectral intensity measurements, spin relaxation and, in particular, self-diffusion experiments. Besides this, small angle X-ray scattering (SAXS) was also applied for structural characterization.

 

A particular form of phase transition, core freezing, was detected as a function of temperature in micelles composed by a single sort of Brij-type surfactants. In mixed micelles, that phase transition still occurs accompanied by a reversible segregation of different surfactants into distinct aggregates. Adding a hydrophobic solubilizate shifts the core freezing point to a lower temperature. Upon lowering the temperature to the core freezing point, the solubilizate is released. The temperature course of the release curves with different initial solubilizate loadings is rationalized in terms of a temperature-dependent loading capacity.

 

The behavior of amphiphilic dispersant molecules in aqueous dispersions of carbon nanotubes (CNTs) has been investigated with a Pluronic-type block copolymer as frequent model dispersant. Detailed dispersion curves were recorded and the distribution of the dispersant among different available environments was analyzed. The amount of dispersed CNT was shown to be defined by a complex interplay of several factors during the dispersion process such as dispersant concentration, sonication time, centrifugation and CNT loading. In the dispersion process, high amphiphilic concentration is required because the pristine CNT surfaces made available by sonication must be rapidly covered by dispersants to avoid their re-attachment. In the prepared dispersions, the competitive adsorption of possible dispersants was investigated that provided information about the relative strength of the interaction of those with the nanotube surfaces. Anionic surfactants were found to have a strong tendency to replace Pluronics, which indicates a strong binding of those surfactants.

 

CNTs were dispersed in an epoxy resin to prepare nanotube-polymer composites. The molecular mobility of epoxy was investigated and the results demonstrated the presence of loosely associated CNT aggregates within which the molecular transport of epoxy is slow because of strong attractive intermolecular interactions between epoxy and the CNT surface. The rheological behavior is dominated by aggregate-aggregate jamming.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2017. p. 62
Series
TRITA-CHE-Report, ISSN 1654-1081 ; 2017:88
Keywords
NMR, chemical shift, spin relaxation, self-diffusion, micelle, core freezing, segregation, solubilization, release, adsorption, binding, surfactant, carbon nanotube, block copolymer, dispersion, competitive adsorption, nanocomposite
National Category
Physical Chemistry
Research subject
Chemistry
Identifiers
urn:nbn:se:kth:diva-220669 (URN)978-91-7729-647-8 (ISBN)
Public defence
2018-02-09, Sal F3, Lindstedtsvägen 26, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20180103

Available from: 2018-01-03 Created: 2017-12-29 Last updated: 2018-01-03Bibliographically approved

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Fernandes, Ricardo M.F.Furo, Istvan

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