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Adsorption, aggregation and phase separation in colloidal systems
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.ORCID iD: 0000-0002-9570-4187
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
Keyword [en]
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: urn:nbn:se:kth:diva-220669ISBN: 978-91-7729-647-8 (print)OAI: oai:DiVA.org:kth-220669DiVA, id: diva2:1169739
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
List of papers
1. Core Freezing and Size Segregation in Surfactant Core-Shell Micelles
Open this publication in new window or tab >>Core Freezing and Size Segregation in Surfactant Core-Shell Micelles
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2015 (English)In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 119, no 33, p. 10798-10806Article in journal (Refereed) Published
Abstract [en]

Nonionic surfactants containing poly(ethylene oxide) are chemically simple and biocompatible and form core-shell micelles at a wide range of conditions. For those reasons, they and their aggregates have been widely investigated. Recently, irregularities that were observed in the low-temperature behavior of surfactants of the kind [CH3(CH2)(n)O-(CH2CH2O)(m)H], (abbreviated CnEm) were assigned to a freezing-melting phase transition in the micellar core. In this work we expand the focus from the case of single component systems to binary surfactant systems at temperatures between 1 and 15 degrees C. By applying small-angle X-ray scattering (SAXS), differential scanning calorimetry (DSC), nuclear magnetic resonance (NMR), and density measurements in pure C18E20 and C18E100 solutions and their mixtures, we show that core freezing/melting is also present in mixtures. Additionally, comparing SAXS data obtained from the mixture with those from the single components, it was possible to demonstrate that the phase transition leads to a reversible segregation of the surfactants from mixed micelles to distinct kinds of micelles of the two components.

Keyword
BLOCK-COPOLYMER MICELLES; ANGLE X-RAY; NEUTRON-SCATTERING; TEMPERATURE; DIFFUSION; SYSTEMS; MICROEMULSION; AGGREGATION; MIXTURES; DELIVERY
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-173435 (URN)10.1021/acs.jpcb.5b06041 (DOI)000360026400041 ()26226298 (PubMedID)2-s2.0-84939825963 (Scopus ID)
Note

QC 20150915

Available from: 2015-09-15 Created: 2015-09-11 Last updated: 2018-01-03Bibliographically approved
2. Release of Solubilizate from Micelle upon Core Freezing
Open this publication in new window or tab >>Release of Solubilizate from Micelle upon Core Freezing
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2017 (English)In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 121, no 45, p. 10353-10363Article in journal (Refereed) Published
Abstract [en]

By combining NMR (yielding H-1 chemical shift, spin relaxation, and self-diffusion data) and small-angle X-ray scattering experiments, we investigate the complex temperature dependence of the molecular and aggregate states in aqueous solutions of the surfactant [CH3(CH2)(17)(OCH2CH2)(20)OH], abbreviated as C18E20, and.hexamethyldisiloxane, HMDSO. The latter molecule serves as a model for hydrophobic solubilizates. Previously, the pure micellar solution was demonstrated to exhibit core freezing at approximately 7-8 degrees C. At room temperature, we find that HMDSO solubilizes at a volume fraction of approximately 10% in the core of the C18E20 micelles, which consists of molten and thereby highly mobile alkyl chains. Upon lowering the temperature, core freezing is found, just like in pure micelles, but at a temperature shifted significantly to 3 degrees C. The frozen cores contain immobile alkyl chains and exhibit a higher density but are essentially devoid (volume fraction below 1%) of the solubilizate. The latter molecules are released, first gradually and then rather steeply, from the core in the temperature range that is roughly delimited by the two core freezing temperatures, one for pure micelles and one for micelles with solubilizates. The release behavior of systems with different initial HMDSO loading follows the same master curve. This feature is rationalized in terms of loading capacity being strongly temperature dependent: upon lowering the temperature, release commences once the loading capacity descends below the actual solubilizate content. The sharp release curves and the actual release mechanism with its molecular features shown in rich detail have some bearing on a diverse class of possible applications.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2017
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-220488 (URN)10.1021/acs.jpcb.7b08912 (DOI)000416203100007 ()29050474 (PubMedID)2-s2.0-85034616727 (Scopus ID)
Funder
Swedish Research Council
Note

QC 20171221

Available from: 2017-12-21 Created: 2017-12-21 Last updated: 2018-01-03Bibliographically approved
3. Propofol adsorption at the air/water interface: a combined vibrational sum frequency spectroscopy, nuclear magnetic resonance and neutron reflectometry study
Open this publication in new window or tab >>Propofol adsorption at the air/water interface: a combined vibrational sum frequency spectroscopy, nuclear magnetic resonance and neutron reflectometry study
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Propofol interaction with cell membrane has remarkable influence on neuronal function, yet data regarding its behaviour at an interface is still scarce. Here we present the results of propofol adsorption at the air/water interface studied by means of vibrational sum frequency spectroscopy (VSFS), neutron reflectometry (NR), and surface tensiometry. VSFS was utilized to elucidate the orientation change of propofol at the surface as a function of concentration, and the water of hydration was studied by probing the OH vibrational region of the spectrum in two different polarisation combinations. Data show that propofol adsorbs at the air/water interface in an ordered fashion showing strong interactions with the water of hydration, as well as weak interactions with water in the proximity of the hydrocarbon parts of the molecule. In the concentration range studied (0 – 0.89 mM) there is almost no change in the orientation adopted at the interface. NR shows that propofol forms a dense monolayer with a thickness of 4.8 Å, and this result is consistent with a limiting area per molecule equivalent to a close-packed monolayer as demonstrated by surface tensiometry. The possibility that islands or multilayers of propofol form at the air/water interface is therefore excluded. Additionally, the ability of propofol to form associations/multimeric structures in water was studied using nuclear magnetic resonance (NMR). The 1H NMR chemical shifts recorded indicate that propofol does not form dimers or multimers in bulk water (D2O).

National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-220694 (URN)
Note

QC 20180103

Available from: 2017-12-30 Created: 2017-12-30 Last updated: 2018-01-03Bibliographically approved
4. The dispersion process of carbon nanotubes sonicated in aqueous solutions of a dispersant
Open this publication in new window or tab >>The dispersion process of carbon nanotubes sonicated in aqueous solutions of a dispersant
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Single-walled carbon nanotube (SWNT) dispersions are created by sonicating pristine SWNT powders added to aqueous solutions of the dispersant block copolymer Pluronic F127. In those dispersions, the amount of the dispersed SWNT is determined by the combination of TGA and UV-Vis methods, while the dispersant concentration is estimated by 1H NMR spectroscopy. In addition, the amount of dispersant adsorbed at the SWNT surface is obtained by 1H NMR diffusion experiments. A part of the dispersant is taken up by non-dispersed and precipitated particles. Dispersion curves recording the amount of the dispersed SWNT as a function of either the initial dispersant concentration or the final dispersant concentration are obtained at different initial SWNT loadings and sonication times. The results show in detail the way the original SWNT particles are divided into smaller and smaller sizes thereby increasing the available SWNT surface to be covered by dispersant. Centrifugation sets the size-threshold above which SWNT particles are retained in the dispersion which determined the SWNT content as a function of sonication time.

National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-220693 (URN)
Note

QC 20180103

Available from: 2017-12-30 Created: 2017-12-30 Last updated: 2018-01-03Bibliographically approved
5. Block copolymers adsorbed on single-walled carbon nanotubes. Block polydispersity and the modes of surface attachment
Open this publication in new window or tab >>Block copolymers adsorbed on single-walled carbon nanotubes. Block polydispersity and the modes of surface attachment
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

1H NMR peak intensities, 1H NMR diffusion measurements and TGA experiments were used to clarify the fate of the dispersing molecules, block copolymer Pluronic F127, during preparation of single-walled carbon nanotube (CNT) dispersions and their state in the created dispersions. In the dispersions, a fraction of the F127 molecules is adsorbed to the CNT surface. The mode of adsorption is the attachment and significant immobilization of the hydrophobic polypropylene oxide (PPO) block to the CNT surface and, as a result, the 1H NMR signal from the attached PPO blocks is lost. On the other hand, the hydrophilic polyethylene oxide (PEO) blocks remain highly mobile and thereby detectable by NMR. The F127 is revealed to exhibit significant block polydispersity. Molecules with large PPO blocks become enriched upon the surface of that fraction of the initial CNT powder that does not become dispersed. The molecular motions involved in creating the observed NMR features are clarified.

National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-220692 (URN)
Note

QC 20180103

Available from: 2017-12-30 Created: 2017-12-30 Last updated: 2018-01-03Bibliographically approved
6. 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
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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. 

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
7. Polymer nanocomposites: insights on rheology, percolation, jamming and molecular mobility
Open this publication in new window or tab >>Polymer nanocomposites: insights on rheology, percolation, jamming and molecular mobility
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Carbon nanotubes (CNTs) loading in a polymer matrix strongly affect the rheological behavior and in turn hamper the overall performance of the composite. The majority of the research in this topic has focused on bulk rheological properties, while here we employ NMR diffusion experiments to explore the mobility (diffusivity) of epoxy molecules when loaded with CNTs. Rheology and light microscopy indicate percolation, or jamming events of CNT aggregates, caging a substantial amount of epoxy molecules while small angle X-ray scattering indicates rearrangements of epoxy molecules in the vicinity of the nanotubes. NMR diffusion experiments distinguish between the diffusion of the caged molecules and that of the free ones, and relate the fraction of the former to the system viscosity. These findings demonstrate the utility of NMR diffusion experiments as an additional method applied to the rheological behavior of polymer mixtures.

National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-220695 (URN)
Note

QC 20180103

Available from: 2017-12-30 Created: 2017-12-30 Last updated: 2018-01-03Bibliographically approved

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