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Studying the Kinetic Pathways for Micellar Solubilisation Using Small Angle Scattering Techniques: Interaction Between an Anionic Surfactant and Polymer Micelles

Myhre, Synne
Master thesis
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Studying-the-Ki ... t-and-Polymer-Micelles.pdf (11.71Mb)
Year
2019
Permanent link
http://urn.nb.no/URN:NBN:no-71920

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  • Kjemisk institutt [845]
Abstract
Polymer micelles can be used in pharmaceuticals, detergents, cosmetics and have a potential for drug delivery systems. In many applications of polymeric micelles, there are also surfactant molecules present. Still, it is an open question of how these affect the polymer micelles, and the present master thesis focuses on the interaction between polymeric micellar systems and surfactants. For example, the kinetic processes involved in mixtures of surfactants and block copolymer micelles are not well understood. It is commonly known that surfactants exhibit rather fast equilibration kinetics, in the order of micro- to milliseconds, while polymers are much slower, in the order of minutes to months. This master thesis is a study of the stability and solubilisation kinetics of block copolymers micelles upon addition of surfactant sodium dodecyl sulphate (SDS) using small-angle X-ray scattering (SAXS) and time-resolved small-angle neutron scattering (TR-SANS). The ability of the surfactant to dissolve polymer micelles or form mixed micelles has been investigated by using two types of amphiphilic polymers, poly(ethylene propylene)-poly(ethylene oxide) (PEP-PEO) and alkyl-functionalized PEO (C28-PEO5 and C21PEO5 ). The exchange kinetics of C21PEO5 micelles occurs over a few seconds, while for C28PEO5 micelles the chains exchange on time scales in the order of hours. Finally, PEP1-PEO20 micelles are known to be frozen on any practical time scales. In this work, we show that the addition of SDS to PEP1-PEO20 shows virtually no change, even after an extended period. However, using time-resolved SAXS, we observe micellar dissolution and formation of mixed micelles within hours, when adding SDS to C28PEO5. Noteworthy, upon the addition of SDS to C21PEO5, these processes occur within seconds. In addition, we found that the kinetics of formation of mixed micelles is accelerated with the amount of added surfactant for both C28PEO5 and C21PEO5. The measured scattering curves have been analyzed with a newly developed three-shell model consisting of an alkane core, an SDS head group shell, and a PEO corona, which was tested for the first time on this system. The polymer micelles are found to break down by two processes, one fast fragmentation reaction and one slow re-organisation step by unimer exchange, and, thus, is highly dependent on the length of the hydrophobic block.
 
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