Positively-charged AzoTAB photosurfactants possess a higher solubility in water than neutral azobenzene and anionic photosurfactants, as first reported by Hayashita et al.74 They showed that the self-assembly of AzoTABs was affected the by molecular structure. More specifically, the CMC and conductivity were lowered by the elongation of the carbon tail. They demonstrated the greater importance of the alkyl chain (R1) compared to the spacer (R2) in determining the properties of the photosurfactant (e.g. the CMC of C2AzoOC4TAB was higher than the CMC of C4AzoOC2TAB). However, this study was
limited to moderate carbon tail length structures (i.e. R1 + R2 < 10) and few structure- property studies on the nanostructural organisation of AzoTABs have been reported since.49
It was reported that the photoisomerisation of the azobenzene core affects the surface excess of azobenzene photosurfactants.75-78 Yang et al. studied the effect of UV light exposure (
λ
ex = 360 nm) on the surface excess and hydrophobicity of 4-ethylazobenzene 4'-(oxyethyl) trimethylammonium bromide (C2AzoOC2TAB).79 They
showed that the cis-isomer does not participate in surface adsorption at the CMC of the trans-isomer. They calculated that the trans- and cis-azobenzene cores had the equivalent hydrophobicity of an alkyl chain that contain 8.6 atoms of carbon and 5.7 atoms of carbon, respectively, which might explain the difference of surface excess.
AzoTABs have received great attention in the design of light-responsive gels,80 proteins,81 modified graphene oxide,82photomanipulation of droplets,51 templating,83 foam stabilisation,84 nanocarriers85 and photoluminescent complexes,86 using the positive- negative electrostatic interactions (with polyelectrolytes, DNA, proteins, anionic complexes). For example, Chevallier et al. reported foams that were stable for more than 15 minutes, consisting of 66 wt% of trans-4-butylazobenzene 4'-(oxypropyl) trimethylammonium bromide (C4AzoOC3TAB), obtained by flushing air for 5 seconds in
solution.84 Upon photoisomerisation at
λ
ex = 365 nm, the foam was destabilised andcontained a mixture 16% of trans- and 84% of cis-isomers (Figure 1.14a). They showed that the foam stability can be achieved by carefully controlling the trans-cis isomer ratio.
18 Figure 1.14. Some examples of applications of AzoTAB photosurfactants taken from the literature. (a) Foam destabilisation induced by UV light exposure. The trans-cis isomer ratio can be controlled to achieve foam stabilisation. Reprinted (adapted) with permission from E. Chevallier, C. Monteux, F. Lequeux and C. Tribet, Langmuir, 2012, 28, 2308-2312. Copyright (2012) American Chemical Society. (b) Light-induced disruption of vesicles, formed by a mixture of AzoTAB photosurfactants and cholesterol sulfate, leading to the control release of the interior. Reprinted (adapted) with permission from Z. K. Cui, T. Phoeung, P. A. Rousseau, G. Rydzek, Q. Zhang, C. G. Bazuin and M. Lafleur, Langmuir, 2014, 30, 10818-10825. Copyright (2014) American Chemical Society. (c) Carbon nanotube colloidal suspension destabilised by UV light exposure and recovered by exposition at λex = 450 nm for 10 minutes. Reproduced from Ref. 82
with permission from The Royal Society of Chemistry. (d) Photomanipulation of droplets using the surface tension gradient of the trans-cis isomers. Reprinted (adapted) with permission from A. Diguet, R. M. Guillermic, N. Magome, A. Saint-Jalmes, Y. Chen, K. Yoshikawa and D. Baigl, Angew. Chem., 2009, 121, 9445-9448. Copyright (2009) WILEY-VCH Verlag.
In another study, Cui et al. reported the formation of non-phospholipid photoresponsive liposomes that contained a mixture of cholesterol sulfate and an azobenzene 4'-(oxydecyl) triethylammonium bromide (C0AzoOC12TAB) photosurfactant
in a 25/75 molar ratio. The authors showed that leakage from the liposomes was achieved upon photoisomerisation at
λ
ex = 350 nm (Figure 1.14b). The permeability of theliposomes was fully restored by cis-trans photoisomerisation at
λ
ex = 450 nm, proving thatthe system can be used as a nanocontainer to release on demand the contents of the liposome. Recently, McCoy et al. investigated the light-controllable dispersion and recovery of carbon nanotubes in water promoted by 4-butylazobenzene 4'-(oxybutyl) trimethylammonium bromide (C4AzoOC4TAB).82 They showed that complete
solubilisation of the carbon nanotubes was achieved in water by the trans-isomer at concentrations above the CMC (> 1 mM, Figure 1.14c). Upon irradiation at
λ
ex = 365 nmfor 1 hour, the carbon nanotubes crashed out in the solution and formed a distinct phase to (b) Ionic interaction
(d) Photomanipulation of droplets (c) Carbon nanotube suspension
(a) Foam destabilisation
Unstable foams Stable foams
19 the AzoTAB. The process was entirely reversible by illumination at
λ
ex = 450 nm for 10minutes to re-disperse the carbon nanotubes in solution.
Finally, Diguet et al. showed that oil droplets of oleic acid can be photomanipulated by using the difference of surface tension of 4-ethyloxyazobenzene 4'- (oxyethyl) trimethylammonium bromide (C2OAzoOC2TAB) in water (Figure 1.14d). The
surface tension increases by 8 mN m-1 upon photoisomerisation at
λ
ex = 365 nm.51 This iscalled the chromocapillary effect and the authors showed that the droplets were repelled from UV irradiated areas, due to the light-induced interfacial tension gradient. The oily droplets could be guided by simple illumination and formed tuneable motifs at the surface of the solution.
Studies have shown that the behaviour of light-responsive amphiphiles can be controlled by careful adjustment of external parameters such as salt concentration,87 or pH88 but little has been done on the design of photosurfactants and the structure-self- assembly relationship. Zakrevskyy et al. investigated the interaction of C4AzoOR2TAB (R2
= 6, 8, 10, 12) with charged-particles (DNA, polyelectrolytes) and the effect that the length of the spacer had on these interactions.87, 89 They reported that an elongated spacer (R2 = 10 and 12) yielded more hydrophobic AzoTABs which ease the DNA compaction/decompaction ratio by hydrophobic effects20-23 but lead to a less efficient gel contraction due to steric effects.80, 88 Similarly, Diguet et al. reported that for C4OAzoOR2TAB (R2 = 2, 5, 8), the best reversible control of gel contraction required a
compromise in the photosurfactant design, between a short spacer length that enabled good permeability and a compaction efficiency induced by long spacer length.90 The comparison between these two studies highlights that a higher surface tension and hydrophobicity difference are obtained for AzoTABs bearing only one oxy-group. All the studies described above use the positively charged AzoTABs as a means to investigate the interaction with negatively charge-entities (DNA, polyelectrolytes and proteins).
The description of the aforementioned studies takes advantage of the photoresponsive properties associated with the hydrophobic and electrostatic forces, but AzoTABs play a supporting role and are not studied as the main entity. Only a few studies have investigated AzoTABs for specific applications such as foam stability10, 84, 91, 92 or
small molecule release,8, 93, 94 using small angle scattering techniques to probe the nanoscale organisation of micelles.15