| 1. |
- Dai, Jing, et al.
(författare)
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Release of Solubilizate from Micelle upon Core Freezing
- 2017
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Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 121:45, s. 10353-10363
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 2. |
- Plazzotta, Beatrice, et al.
(författare)
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Core Freezing and Size Segregation in Surfactant Core-Shell Micelles
- 2015
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Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 119:33, s. 10798-10806
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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