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Self-assembly in mixtures of a double-chain surfactant and a dimeric Gemini surfactant
KTH, School of Chemical Science and Engineering (CHE).
2014 (English)Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesisAlternative title
Självassociation i blandningar av en dubbelkedjig tensid och en tvillingtensid (Swedish)
Abstract [en]

Gemini surfactants consist of two conventional surfactants connected by a spacer unit near the hydrophilic head groups. Compared to corresponding conventional surfactants, gemini surfactants are more surface active and have a lower critical micelle concentration (cmc). Mixtures of two different surfactants are used in a wide variety of applications since their properties often are better than those of pure surfactants. In this project, the self-assembled aggregates in binary mixtures of ester-containing cationic gemini surfactants and didodecyl dimethylammonium bromide (DDAB) were studied using dynamic light scattering (DLS) and cryogenic transmission electron microscopy (cryo-TEM). Three different gemini surfactants were examined, one with dodecanoyl chains and a three-carbon spacer (Q311), one with decanoyl chains and a three-carbon spacer (Q39) and one with decanoyl chains and a six-carbon spacer (Q69). Samples of different total surfactant concentrations and different surfactant compositions were investigated. The surfactant composition in the aggregates (                         ) was calculated for all samples according to the non-ideal Poisson-Boltzmann mean field theory. Since the gemini surfactants form globular or elongated micelles as pure surfactants while DDAB forms vesicles, it was expected that a micelle-to-bilayer transition would occur as the surfactant composition was varied.

It was found that a micelle-to-bilayer transition occurred for all systems as the mole fraction of DDAB in the samples was increased. The transition seemed to be controlled by the surfactant composition in the aggregates rather than the overall surfactant composition for the systems DDAB/Q39 and DDAB/Q69. For DDAB/Q311 the correlation between the transition and surfactant composition was less evident. The micelle-to-bilayer transition occurred at a higher DDAB content for DDAB/Q69 ( ) than for DDAB/Q311 ( ) and DDAB/Q39 ( ), indicating that the spacer length has a considerable impact on the transition.

At low DDAB content, the cryo-TEM micrographs showed that DDAB/Q69 formed globular micelles and DDAB/Q311 and DDAB/Q39 formed elongated micelles. The elongated micelles formed by DDAB/Q311 were significantly longer than the ones formed by DDAB/Q39. As  was increased, DDAB/Q311 and DDAB/Q39 first formed small bilayer disks, and later vesicles. For DDAB/Q69, small disks started forming at the same  as vesicles. The small bilayer disks seemed to be more abundant in DDAB/Q39 and DDAB/Q311 than in DDAB/Q69. This could be elucidated in terms of bending rigidity, since the lower bending rigidities of Q39 and Q311 make them more prone to form heterogeneously curved aggregates, such as disks. In addition, a low bending rigidity promotes formation of small vesicles, since it reduces the free energy penalty of forming structures with a large difference in curvature. In the cryo-TEM micrographs it could be observed that the average size of the vesicles formed by DDAB/Q311 and DDAB/Q39 were smaller than for DDAB/Q69.

 

Place, publisher, year, edition, pages
2014.
Keyword [en]
Mixed micelles, Micelle-to-bilayer transition, Dynamic light scattering, Cryo-transmission electron microscopy, Esterquat Gemini
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:kth:diva-158872OAI: oai:DiVA.org:kth-158872DiVA: diva2:779115
Available from: 2015-01-12 Created: 2015-01-12 Last updated: 2017-08-30Bibliographically approved

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