Change search
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Dispersing Carbon Nanotubes: Towards Molecular Understanding
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.ORCID iD: 0000-0003-1775-8160
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. , xii, 77 p.
Series
TRITA-CHE-Report, ISSN 1654-1081 ; 2015:60
Keyword [en]
carbon nanotubes, dispersion, surfactants, polymers, adsorption, liquid crystals, nuclear magnetic resonance, self-diffusion
National Category
Physical Chemistry
Research subject
Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-176443ISBN: 978-91-7595-713-5 (print)OAI: oai:DiVA.org:kth-176443DiVA: diva2:866863
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
List of papers
1. Lateral Diffusion of Dispersing Molecules On Nanotubes As Probed by NMR
Open this publication in new window or tab >>Lateral Diffusion of Dispersing Molecules On Nanotubes As Probed by NMR
Show others...
2014 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 118, no 1, 582-589 p.Article in journal (Refereed) Published
Abstract [en]

Noncovalent dispersion of carbon nanotubes is essential to most applications but still poorly understood at the molecular level. The interaction of the dispersing molecule with the nanotube, wrapping or nonwrapping, still awaits consensus. Herein, we have studied by H-1 NMR diffusometry some features of molecular dynamics in the system of carbon nanotubes dispersed by triblock copolymer Pluronics F127 in water. The diffusional decays obtained at different diffusion times, Delta, are not single-exponential and have a complex Delta-dependent profile, ultimately implying that the polymer is observed in two states: free (in unimeric form) and nanotube-bound. Fitting a two-site exchange model to the data indicates that at any instant, only a small fraction of polymers are adsorbed on the nanotubes, with polydisperse residence times in the range of 100-400 ms. Most significantly, we further provide an estimate of D = (3-8) x 10(-12) m(2) s(-1) the coefficient of lateral diffusion of the polymer along the nanotube surface, which is an order of magnitude slower than the corresponding self-diffusion coefficient in water. The emerging picture is that of a nonwrapping mode for the polymer-nanotube interaction.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2014
Keyword
Walled Carbon Nanotubes, Nuclear-Magnetic-Resonance, Angle Neutron-Scattering, Rod-Like Macromolecules, Monte-Carlo-Simulation, Field Gradient, Concentrated-Solution, Aqueous Dispersions, Block-Copolymers, Rodlike Polymers
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-141296 (URN)10.1021/jp4114046 (DOI)000329678200063 ()2-s2.0-84892578577 (Scopus ID)
Funder
Swedish Research Council
Note

QC 20140213. QC 20160222

Available from: 2014-02-13 Created: 2014-02-13 Last updated: 2017-12-06Bibliographically approved
2. Surface Coverage and Competitive Adsorption on Carbon Nanotubes
Open this publication in new window or tab >>Surface Coverage and Competitive Adsorption on Carbon Nanotubes
Show others...
2015 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, The Journal of Physical Chemistry C, ISSN 1932-7447, Vol. 119, no 38, 22190-22197 p.Article in journal (Refereed) Published
Abstract [en]

The binding strength of dispersants to the surface of carbon nanotubes is of crucial importance for the efficiency of the dispersion process and for potential applications, yet data are scarce on this subject. Here we present the results of diffusion NMR experiments in dispersions of single-walled carbon nanotubes (SWNTs) prepared by either the polymer Pluronics F127 or the protein bovine serum albumin (BSA). The experiments detect the amount of F127 molecules adsorbed onto the SWNT surface. This quantity is recorded (i) in F127-SWNT dispersions to which BSA molecules are added and (ii) in BSA-SWNT dispersions to which F127 molecules are added. The data clearly show that F127 replaces BSA adsorbed at the SWNT surface, while BSA leaves the adsorbed F127 coverage intact. Consequently, F127 binds to the nanotube surface more strongly than BSA. Hence, we provide a way to categorize dispersants by adsorption strength. We also provide evidence showing that the nanotubes dispersed by BSA form loose aggregates where a large part of the surface is not in direct contact with the surrounding liquid. The results are discussed in relation to previous findings in the literature.

National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-176433 (URN)10.1021/acs.jpcc.5b06685 (DOI)000361921600051 ()2-s2.0-84942540248 (Scopus ID)
Funder
Swedish Research Council
Note

QC 20151105

Available from: 2015-11-04 Created: 2015-11-04 Last updated: 2017-12-01Bibliographically approved
3. Dispersing Carbon Nanotubes with Ionic Surfactants under Controlled Conditions: Comparisons and Insight
Open this publication in new window or tab >>Dispersing Carbon Nanotubes with Ionic Surfactants under Controlled Conditions: Comparisons and Insight
Show others...
2015 (English)In: Langmuir, ISSN 0743-7463, E-ISSN 1520-5827, Vol. 31, no 40, 10955-10965 p.Article in journal (Refereed) Published
Abstract [en]

A fundamental understanding of the mechanisms involved in the surfactant-assisted exfoliation and dispersion of carbon nanotubes (CNTs) in water calls for well-controlled experimental methodologies and reliable comparative metrics. We have assessed the ability of several ionic surfactants to disperse single and multiwalled carbon nanotubes, resorting to a stringently controlled sonication-centrifugation method for the preparation of the dispersions. The CNT concentration was accurately measured for a wide range of surfactant concentration, using combined thermogravimetric analysis and UV–vis spectroscopy. The obtained dispersibility curves yield several quantitative parameters, which in turn allow for the effects of nanotube morphology and surfactant properties (aromatic rings, chain length, headgroup charge, andcmc) to be assessed and rationalized, both in terms of dispersed nanotube mass and surface area. The data also indicate that the CNT-surfactant association follows patterns that are markedly different from other equilibrium processes governed by hydrophobicity (such as micellization); in particular, the surfactant concentration needed for maximum dispersibility,cs,max, and the number of surfactant molecules per unit CNT area at cs,max are shown to depend linearly on chain length. The results further suggest that the presence of micelles in the exfoliation process is not a key factor either for starting CNT dispersibility or attaining its saturation value.

Keyword
CNT, dispersion, surfactants, TGA
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-176435 (URN)10.1021/acs.langmuir.5b02050 (DOI)26390187 (PubMedID)2-s2.0-84944128629 (Scopus ID)
Note

QC 20151105

Available from: 2015-11-04 Created: 2015-11-04 Last updated: 2017-12-01Bibliographically approved
4. Assessing Surfactant Binding to Carbon Nanotubes via Competitive Adsorption: Binding strength and critical coverage
Open this publication in new window or tab >>Assessing Surfactant Binding to Carbon Nanotubes via Competitive Adsorption: Binding strength and critical coverage
Show others...
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. 

National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-176436 (URN)
Note

QC 20180103

Available from: 2015-11-04 Created: 2015-11-04 Last updated: 2018-01-03Bibliographically approved
5. Mechanical Agitation Induces Aggregation of Pre-Dispersed Carbon Nanotubes
Open this publication in new window or tab >>Mechanical Agitation Induces Aggregation of Pre-Dispersed Carbon Nanotubes
Show others...
2015 (English)Manuscript (preprint) (Other academic)
Abstract [en]

Mechanical agitation is typically used to fragment and disperse insoluble materials in a solvent. We report here an aggregation process that, contrary to expectation, is induced by mechanical agitation: when aqueous dispersions of single-walled carbon nanotubes (SWNTs) are subject to vortex-shaking, weakly bound micron-sized aggregates are formed. The SWNT dispersions are prepared by adding various dispersants employing a sonication followed by centrifugation approach. While surfactant adsorption to the SWNTs during sonication results in stabilized exfoliated tubes and thin bundles, we find that vortex-shaking the fresh dispersions for short periods (10-60 s) results in re-aggregation into flocs in the 1-102 µm range. The aggregation is reversible: if the vortexed dispersions are mildly sonicated, the flocs break down and re-dispersal occurs. Imaging at different resolutions shows that the aggregates consist of loose networks of intertwined tubes and bundles. The data further indicate that the average aggregate size increases logarithmically with vortex time and is critically influenced by dispersant type (ionic or nonionic), centrifugation time (prior to vortexing) and initial concentration of dispersed SWNTs. These results are relevant if stabilization or destabilization of dispersions is sought for, i.e., in drug delivery or sensing applications, and could also be of interest for chiral sorting of SWNTs and percolation conductivity.

National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-176437 (URN)
Note

QS 2015

Available from: 2015-11-04 Created: 2015-11-04 Last updated: 2015-11-04Bibliographically approved
6. Gemini Surfactants as Dispersants of Multiwalled Carbon Nanotubes: a Systematic Study on the Role of Molecular Structure
Open this publication in new window or tab >>Gemini Surfactants as Dispersants of Multiwalled Carbon Nanotubes: a Systematic Study on the Role of Molecular Structure
Show others...
2015 (English)Manuscript (preprint) (Other academic)
Abstract [en]

Surfactants have been widely used as non-covalent dispersants of carbon nanotubes and yet a deeper and systematic understanding of the role of their molecular properties on dispersibility still awaits consensus. Herein, we report on the dispersibility of multiwalled carbon nanotubes (MWNTs) using a set of dicationic gemini surfactants of the n-s-n type, where both the length of the covalent spacer (s) that bridges the two cationic headgroups and the length of the tails (n) are systematically varied. Thus, 12-s-12 gemini with s = 2, 6, and 12 are studied together with 16-s-16 (s = 2 and 12). In addition, the single-tailed homologues dodecyltrimethylammonium bromide, DTAB (n = 12), and cetyltrimethylammonium bromide, CTAB (n = 16), are employed for comparisons. High precision dispersion curves (dispersed NT vs. surfactant concentration) are presented, obtained through a well-controlled sonication/centrifugation procedure combined with an accurate determination of MWNT concentration. The gemini amphiphiles, despite being double-tailed and double-charged, are found to be less effective dispersants than CTAB and roughly as effective as DTAB. Among the gemini, the following pattern emerges as concerning dispersion behavior. (i) The tail length, n, is less influential than spacer length, s, in dispersing ability, implying that the spacer hydrophobicity rather than that of the tail may govern the affinity for the nanotube surface. (ii) In the 12-s-12 series, the surfactant concentration needed for maximum MWNT dispersibility depends linearly on s, while it is known that the neat cmc depends non-monotonically on s. (iii) Similarly to single-tailed ionic surfactants, the presence of micelles has no direct effect on the dispersion behavior. In combination, these observations also point to an adsorption mechanism that does not involve the formation of micelle-like aggregates on the nanotube surface but rather coverage by individual dispersant molecules.

National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-176438 (URN)
Note

QS 2015

Available from: 2015-11-04 Created: 2015-11-04 Last updated: 2015-11-04Bibliographically approved
7. Strong Spacer Length Effects on The Thermal Behavior and Mesophase Formation By Gemini Surfactants
Open this publication in new window or tab >>Strong Spacer Length Effects on The Thermal Behavior and Mesophase Formation By Gemini Surfactants
Show others...
2015 (English)Manuscript (preprint) (Other academic)
Abstract [en]

The self-aggregation properties in aqueous solution of gemini surfactants of the type alkanediyl-α,ω-bis(dodecyldimethylammonium bromides), 12-s-12, have been extensively reported and are known to be significantly influenced by the number of methylene groups, s, of the covalent spacer. In contrast, the thermal behavior of the anhydrous compounds as a function of varying s has not been investigated in a similarly systematic way. Herein, we present the thermal phase behavior of eight compounds of the 12-s-12 family (with s = 2-6, 8, 10 and 12), resorting to differential scanning calorimetry (DSC), polarized light microscopy (PLM) and X-ray diffraction (XRD). We find that compounds with either the shortest spacer, s = 2, or the longest ones—8, 10 and 12—form several smectic liquid-crystalline phases prior to isotropization to the liquid phase, with appearance of oily streaks, focal conics, mosaic and fan-shaped birefringent textures. In sharp contrast, gemini compounds with intermediate spacers, s = 3-6, decompose and do not form any disordered, fluid mesophases. Both the DSC thermodynamic parameters for the phase transitions and d00l spacings obtained from XRD show non-monotonic trends with spacer variation, indicating that there are significant differences in solid-state packing and melting process. Plausible molecular packing arrangements in the solid-state are presented, consistent with the XRD information and geometric considerations, and their influence on the phase behavior trends critically discussed.

National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-176440 (URN)
Note

QS 2015

Available from: 2015-11-04 Created: 2015-11-04 Last updated: 2015-11-04Bibliographically approved

Open Access in DiVA

Thesis(4631 kB)351 downloads
File information
File name FULLTEXT01.pdfFile size 4631 kBChecksum SHA-512
1c9bf712a7156b8f9f61519764b56aeaac7cdcb5243677ab55c81fc0af38b9fa3ecc1a200065b9125ce445b69b6c09b82970ed3a1770aa3be5e86b6758d20c1f
Type fulltextMimetype application/pdf

Search in DiVA

By author/editor
Fernandes, Ricardo M. Ferreira
By organisation
Applied Physical Chemistry
Physical Chemistry

Search outside of DiVA

GoogleGoogle Scholar
Total: 351 downloads
The number of downloads is the sum of all downloads of full texts. It may include eg previous versions that are now no longer available

isbn
urn-nbn

Altmetric score

isbn
urn-nbn
Total: 906 hits
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf