Identificación y aislamiento de los compuestos presentes en los extractos de corteza extractos de corteza.

86 The CMC, size, shape and interparticle interactions of micelles can be tuned via the molecular design of the photosurfactant and changes in the concentration, photoisomerisation and temperature. Hayashita et al. showed that the CMC of AzoTABs decreases with an increase in the length of the hydrophobic segment and that the hydrophobicity of the surfactant is guided by the length of the alkyl tail.1 It has also been shown that the CMCs of the cis-isomers are systematically lower than the CMCs of the trans-counterparts. For example, Yang et al. reported that the CMC of 4-ethylazobenzene- 4-(oxyethyl)trimethylammonium bromide (C2AzoOC2TAB) increases upon

photoisomerisation of the azobenzene core by 21%.2 As a result, the trans-cis photoisomerisation can lead to complete dissociation of the micellar system to unimers, when the surfactant concentration is above the CMC of the trans-form but below the CMC of the cis-isomer. This difference can be exploited to selectively release components from the micellar system. For example, McCoy et al. reported that carbon nanotubes were solubilised in a micellar solution of 4-butylazobenzene-4-(oxybutyl)trimethylammonium bromide (C4AzoOC4TAB) at 5 mM, but upon trans-cis photoisomerisation, the

photosurfactant loses its ability to solubilise the carbon nanotubes, which flocculate and crash out of the solution.3

SANS has been commonly used to investigate the size, shape and interaction between micelles formed from “regular” surfactants above the critical micelle concentration.4, 5 However, AzoTAB photosurfactants have rarely been studied by SANS, despite the interesting photoisomerisation ability of the azobenzene core to change its size and dipole moment in response to UV light. The first structural study on AzoTABs was performed by Sakai and co-workers, who investigated the controlled release of the oily ethylbenzene molecule in a micellar solution 4-butylazobenzene-4- (oxyethyl)trimethylammonium bromide (C4AzoOC2TAB) at 20 mM.6, 7 They used SANS

to show that the micelles formed were ellipsoidal and that the long radius of the micelles decreased from 38 Å to 32 Å, while the short radius remained constant at 21 Å upon photoisomerisation. They reported that ethylbenzene was encapsulated more efficiently in micelles formed by the trans-isomers compared to micelles formed by cis-isomers.6 The change of the nanoscale organisation of the micelles explains the difference of solubility of small molecules upon photoisomerisation of the azobenzene core. However, the physicochemical modifications induced by the photoisomerisation and the self-assembly of unimers in solution depends on the molecular structure of the photosurfactant. To date, the

87 structure-self-assembly relationship of cationic azobenzene photosurfactants has not been systematically investigated.

Complementarily, cryo-electron microscopy has been used to visualise the shape of the aggregates. Matsumura et al. reported the cryo-transmission electron micrographs of a mixture of 4-butylazobenzene-4-(oxyethyl)trimethylammonium bromide (C4AzoOC2TAB)

and sodium dodecylbenzenesulfonate (SDBS).8 In the trans-form, the images showed the formation of vesicles of around 50 nm in size for the 60%/40% mixture, which disintegrates into micrometer-sized large particles upon trans-cis photoisomerisation for 2 h. The vesicles were reformed after 2 h of irradiation with visible light, which showed the reversibility of the system.

5.1.1 Aims

In Chapter 4, it was shown that the design of the AzoTAB photosurfactant affects the optical properties in dilute solution, below the CMC. In this Chapter, the aims are to further investigate the effect of the length of the hydrophobic segment and the position of the azobenzene core within the surfactants on the CMC, with and without photoisomerisation. Additionally, the size, shape and nanoscale interactions of micelles formed above the CMC, will be studied. SANS will be used to unravel the self-assembly behaviour at the nanoscale level for AzoTABs above their CMCs, as a function of concentration and temperature and upon photoisomerisation. Finally, a representative sample will be studied by cryo-SEM to visualise the aggregates formed, with and without photoisomerisation.

5.2

Experimental

Critical micelle concentration titrations: CMC titrations were performed by sequential dilution of aliquots of a stock solution of AzoTABs (5.0 × 10-3 M) in MilliporeTM water, from 50 × 10-6 M to 5 × 10-3 M. CMC titrations were conducted using surface tensiometry and dynamic light scattering at 20 °C. For surface tensiometry, the presented data points are the average from 5 measurements. For DLS measurements, the MilliporeTM water used for dilution was filtered 3 times with a 250 µm filter prior to utilisation. The intensity of scattered light (kcounts), for a fixed detector distance, was recorded as a function of concentration. The presented data points are the average from 3 runs of 15 measurements.

88 MilliporeTM water (γ = 72.80 mN m-1 at 20 °C) was used as a reference sample for both

techniques.

Solubility experiment: Solubility experiments were performed on trans-C6AzoOC4TAB to

measure the temperature at which the photosurfactant dissolved in water. A solution of trans-C6AzoOC4TAB in water (1 mM, 5 mL) was gently stirred on a hot plate. The

temperature was increased by 0.2 °C every 2 minutes from 25 to 30 °C.

Small-angle neutron scattering: AzoTAB solutions (5 mM, 10 mM and 20 mM) were prepared in D2O to ensure good neutron scattering contrast and placed in quartz cuvettes

(Hellma UK, Type 120, 1 mm pathlength). Heating studies were performed at 20 °C and 60 °C using a Julabo water bath and 10 minutes equilibration time. The cis-isomer samples were irradiated with UV-light (λ_ex = 365 nm) for 3 minutes prior to loading. UV/Vis

absorption spectra were recorded before and after experiments to investigate potential sample damage or reverse isomerisation induced by neutron scattering. SANS profiles were then fitted to Sphere,9 _{oblate Ellipsoid}10 _{or Elliptical Cylinder models}10 _including

polydispersity of 0.1 using nonlinear least-squares optimisation with the SasView program (version 4.0.1).11 Full details of the models can be found in Chapter 3. Validation of the model fits was achieved using the theoretical mass of dry material (φdry) that the fit

returned.

Cryo-scanning electron microscopy: Aliquots of trans- and cis-C8AzoOC2TAB (50 µL, 2

mM) in water were plunged into liquid nitrogen and transferred in-vacuo to the cryo- chamber of the microscope. Images were taken by Dr Clodagh Dooley. The size distribution was calculated using ImageJ software.