Dynamic covalent surfactants

Dynamic covalent surfactants

Minkenberg, C.B.

Van Esch, J.H. (promotor)

Applied Sciences

DelftChemTech

2012-07-03

In this thesis the development of surfactant aggregates with fast exchange dynamics between the aggregated and non-aggregated state is described. Dynamic surfactant exchange plays an important role in natural systems, for instance in cell signaling, cell division, and uptake and release of cargo. Remarkably, dynamic surfactant exchange in synthetic systems has received relatively little attention. In this thesis we have shown that surfactants and surfactant aggregates with diverse morphologies with fast exchange dynamics could be obtained by making use of dynamic covalent surfactants. These dynamic covalent surfactants were formed in water by mixing of two isotropic solutions of a surfactant precursor. As surfactant precursors we used hydrophilic aldehyde functionalized building blocks and hydrophobic, but water soluble, amine building blocks. By mixing both building blocks in water, surfactants were formed in situ by the formation of reversible imine bonds between the two precursor building blocks. These surfactants form surfactant aggregates above their critical aggregation concentration (CAC). By following this approach, we have shown that spherical micelle formation can be switched on and off by reversible displacement of the equilibrium between the nonamphiphilic building blocks and their single tailed surfactant counterparts. Also double tailed surfactants could be obtained by this approach, from aqueous solutions of a bisaldehyde functionalized head group and an amine functionalized apolar tail through the in-situ formation of dynamic covalent imine bond between the precursors. The reversible nature of the dynamic covalent imine bond made the bilayer of the resulting vesicle assemblies highly dynamic, which is quite unusual for vesicles composed of double tailed surfactants. Due to the fast formation and dissociation kinetics of the imine bond, these dynamic covalent vesicles are highly responsive to changes in their environment, resulting for instance in complete dissociation upon dilution or changes in pH. Since dynamic covalent surfactants are capable of reversible formation due to the presence of reversible dynamic covalent bonds in the molecular structure, it was assumed that surfactant mixtures would be able to change composition in response to a change in their environment, on account of their continuous building block exchange. To test this hypothesis, we chose a micelle forming dynamic covalent surfactant system and a vesicle forming dynamic covalent surfactant system as a starting point. Next, we combined these systems, in such a way that there is building block competition between a micelle forming hydrophobic tail and a vesicle forming hydrophobic tail for binding with a hydrophilic head group or for a micelle forming head group and a vesicle forming head group for a single hydrophobic tail . Here, we show that vesicle forming surfactants and vesicles self-select from a mixture of aldehyde and amine functionalized non-surface active surfactant precursors, which are capable of forming multiple aggregate types and surfactants. We were able to describe the experimental data with a thermodynamic model, which strongly suggests that the selection of bilayer-forming surfactants and bilayer morphologies is thermodynamically controlled. We believe that these results bear some significance for the mechanisms behind the formation of bilayer compartments in prebiotic systems. We have shown that also dynamic covalent wormlike micellar morphologies can be obtained by imine bond formation upon mixing of simple water soluble gemini-surfactant precursors. Analogous to the previously obtained micelles and vesicles, these dynamic covalent wormlike micelles could be associated and dissociated upon changing the pH or temperature, using the fast association and dissociation of the imine bond. Also, exchange of the mono-functional amine tail for a diamine derivative in the vesicle-forming system led to the formation of crosslinked networks of dynamic vesicles, gelling the surrounding solvent. These vesicle gels are highly responsive to changes in their environment, but also partly self-heal after removal of any perturbation. The work in this thesis provides a new approach for the development of responsive surfactant assemblies that are easily formed by simple mixing of isotropic aqueous precursor solutions. The resulting surfactant assemblies show fast dissociation kinetics and switchable surfactant morphologies. In the future, this work might find application in the development of dynamic encapsulation and release materials.

surfactants
imine
dynamic covalent
responsive
micelle
vesicle

http://resolver.tudelft.nl/uuid:077edab8-1475-436a-95c4-3d0b9f8a1bc1

2012-07-01

Institutional Repository

doctoral thesis

(c) 2012 Minkenberg, C.B.