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Release of Solubilizate from Micelle upon Core Freezing
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.
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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. Vol. 121, no 45, p. 10353-10363
National Category
Physical Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-220488DOI: 10.1021/acs.jpcb.7b08912ISI: 000416203100007PubMedID: 29050474Scopus ID: 2-s2.0-85034616727OAI: oai:DiVA.org:kth-220488DiVA, id: diva2:1168613
Funder
Swedish Research Council
Note

QC 20171221

Available from: 2017-12-21 Created: 2017-12-21 Last updated: 2018-01-03Bibliographically approved
In thesis
1. 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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Dai, JingAlaei, Zahra

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