Photoresponsive supramolecular soft materials in aqueous media
Chen, Shaoyu
DOI:
10.33612/diss.107818650
IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from it. Please check the document version below.
Document Version
Publisher's PDF, also known as Version of record
Publication date: 2019
Link to publication in University of Groningen/UMCG research database
Citation for published version (APA):
Chen, S. (2019). Photoresponsive supramolecular soft materials in aqueous media. Rijksuniversiteit Groningen. https://doi.org/10.33612/diss.107818650
Copyright
Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).
Take-down policy
If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.
Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum.
Chapter 2
Amphiphilic Azobenzene Doped
Photoresponsive Foams
Published as:
RSC Advances, 2016, 6, 60138
Abstract: The controllability of foam stability is a vital feature that allows for practical applications of foam systems. In contrast to control the foam stability by the addition of chemicals, the use of light as an external stimulus, a non-invasive stimulus with high spatiotemporal precision, offers unique opportunities to reversibly tune the foam stability in an environmentally friendly and sustainable way. In this chapter, the design, synthesis and properties of a series of nonionic azobenzene amphiphiles with different hydrophobic chain lengths (NAACn, n = 4, 8
and 12) are described. NAACn are used as dopants in foaming solutions of sodium
dodecyl sulfate (SDS) to obtain photoresponsive foams. The transformation of structural geometry between trans-NAACn and cis-NAACn allows the formation of
photoresponsive monolayers at the air-water interface, which enables NAACn to
control the foam stability by light. Detailed investigations of reversible photoisomerization, the effects of hydrophobic chain length and concentration of NAACn on photoisomerization, the photochemical fatigue resistance and the
thermal inversion properties of NAACn are determined by UV-vis absorption
spectroscopy. The controllability of foam stability using trans-NAACn and cis-NAACn
is presented. Compared to NAAC8 and NAAC12, NAAC4 is identified as the optimal
structure for preparation of photoresponsive foams in aqueous media, featuring the shortest photoirradiation time for attaining photostationary states and the most effective control of foam stability.
2.1 Introduction
Foams, formed by trapping gas in a liquid, are widely employed in various fields, such as fine chemicals, petroleum chemicals, metallurgical mining, food and textile industries, due to the unique properties including liquidity and large surface area.1–3
As a thermodynamically metastable system, the stability of foams is a crucial factor for practical applications. For example, during foam coloring processes in textile industry, foams with high stability are vital to obtain a uniform color result. In this connection, foam stabilizers, can be surfactants, polymers and solid particles, are commonly used to improve the foam stability by slowing down the foam destabilization processes, i.e., drainage, coarsening and coalescence.4–8 In contrast,
unstable foams with controllable ruptures are required at the end of processes, which allows for an environmentally friendly and simple removal of residual foams. Defoaming agents are usually used to promote the rupture of residual foams. However, both foam stabilizers and defoaming agents result in an irreversible tuning of foam stability, limiting the sustainability of foam-based application systems. From an eco-friendly point of view, significant improvements are urgently needed for these additional chemicals in foam application processes.
The development of responsive surfactants provides a promising solution for reversible tuning of foam stability between the stable state and unstable state by controlling of external stimuli, such as pH, CO2, temperature, ionic strength,
electricity, light, magnetic field, oxidants and enzymes.9–12 Particularly,
photoresponsive surfactants have attracted much attention due to their unique advantages, including tunable wavelength and intensity as well as temporal and spatial controllability.13–16 Photoresponsive surfactants commonly contain a
hydrophilic head group and a hydrophobic tail chain connected via an isomerization moiety, such as azobenzene and stilbene.17,18 Azobenzenes, with a highly reversible
and efficient photoisomerization capability, are promising candidates to develop photoresponsive surfactants.19–21 Chevallier et al.12 reported that a cationic
azobenzene-containing surfactant was employed to control the foam stability. However, the properties of cationic surfactant are easily affected by external environmental factors such as electrolytes, which limits their potential applications. In this connection, the development of nonionic surfactants should be one of the best solutions. Furthermore, it has been reported that dodecanol showed excellent foam stabilization effects.22 Therefore, a series of nonionic azobenzene amphiphiles
(NAACn) with a hydroxy end group and alkyl chain connected via an azobenzene
core are designed (Figure 2.1). We envisioned that NAACn were compatible with the
anionic foaming agent, sodium dodecyl sulfate (SDS), and were able to control the foam stability due to the formation of photoresponsive monolayers by SDS and NAACn at the air-water interface. When NAACn adopt a trans configuration, a
density packed monolayer is formed to provide a foam stabilization effect. After exposure to UV-light, the transformation from trans-NAACn to cis-NAACn results in
a loose packed monolayer, which promotes foam rupture (Figure 2.1). Applying these nonionic azobenzene amphiphiles (NAACn), the foam stability can be
reversibly tuned by light, which show potential application in recycling residual foams to reduce the usage of chemical agents and minimize pollutions.
Figure 2.1 Schematic illustrations of (a) the reversible photoisomerization of nonionic azobenzene amphiphiles (NAACn) and (b) the photoresponsive air-water
interface formed by NAACn and SDS.
2.2 Results and Discussion
2.2.1 Molecular design and synthesis
Photoresponsive nonionic azobenzene amphiphiles with different hydrophobic chain lengths (NAACn, n = 4, 8 and 12) were designed with an azobenzene core, attached
with an alkyl chain to form the hydrophobic half and a hydroxy group to form the hydrophilic half (Figure 2.1). The hydrophobic chain length influences the photoisomerization properties23 and other properties, such as the critical micelle
concentration (CMC) and solubility, which in turns affect the control of foam stability. We envisioned that various alkyl-chain lengths allowed for a systematic modification of the controllability of foam properties. The synthetic pathway is summarized in Scheme 2.1. According to the literature, 4,4′-bis(hydroxy)-azobenzene was prepared by the alkali fusion of p-nitrophenol, KOH and water.24
The nonionic azobenzene amphiphiles (NAACn) were obtained by a Williamson
1-bromohexane or 1-bromododecane), K2CO3 and KI in DMF and the NAACn were
purified by flash column chromatography. The detailed procedures for the synthesis are provided in the section of Experimental Data.
Scheme 2.1 Synthesis of nonionic azobenzene amphiphiles (NAACn). 2.2.2 Isomerization analysis of NAACn
Azobenzene derivatives have been employed in smart materials, e.g., energy storage materials, electronic devices, photopharmacology and photoresponsive foams, due to the unique isomerization process of the azobenzene moiety, such as high reversibility, light/heat response and excellent photochemical fatigue resistance.25–27
Generally, the isomerization process of azobenzene derivatives is determined by UV-vis absorption spectroscopy.28 Freshly prepared ethyl acetate (EtOAc) solutions of
NAACn (0.01 g L-1) were alternately irradiated with UV-light (λ = 365 nm) and
Vis-light (λ = 400 – 780 nm) to investigate the photoisomerization process. A strong absorption (~356 nm) is observed in the UV-vis spectra of trans-NAACn (Figure 2.2a,
2.2c and 2.2e). After irradiating with UV-light for only 1 s, a prominent decrease in the absorbance at ~350-400 nm with a concomitant increase at ~450 nm is observed, indicating the transformation from trans-NAACn to cis-NAACn (Figure 2.2a, 2.2c
and 2.2e). Additionally, no further change is shown in the absorption spectra, even when prolonging the UV-light irradiation time, revealing that a cis-rich photostationary state (PSS) is reached upon exposing to UV-light for 1 s (Figure 2.2a, 2.2c and 2.2e). The results demonstrated that NAACn showed a highly selective
photoisomerization with a short irradiation time for attaining the PSS triggered by UV-light regardless of the hydrophobic chain length. Although the cis-NAACn in a cis-rich solution, obtained by UV-light irradiation, can be switched back to
trans-NAACn by exposure to Vis-light, the Vis-light irradiation time for obtaining a
trans-rich PSS of NAACn shows dependence on the hydrophobic chain length (Figure 2.2b,
irradiation of cis-NAAC12 was required (14 min) to attain a trans-rich PSS. The
results indicate that with the increase of hydrophobic chain length (from C4 to C12),
a longer Vis-light irradiation time for the photoisomerization of NAACn from the
cis-rich PSS to the trans-cis-rich PSS is observed, being consistent with previous reports.23,29,30
Figure 2.2 UV-vis absorption spectra of NAACn (0.01 g L-1) in EtOAc. (a) A EtOAc
solution of trans-NAAC4 was irradiated with UV-light (λ = 365 nm), (b) followed by
exposing to Vis-light (λ = 400 – 780 nm). (c) A EtOAc solution of trans-NAAC8 was
irradiated with UV-light, (d) followed by exposing to Vis-light. (e) A EtOAc solution of trans-NAAC12 was irradiated with UV-light, (f) followed by exposing to Vis-light.
The EtOAc solutions of trans-NAACn were kept in total darkness overnight prior to
measurement to obtain the spectra of all-trans and the EtOAc solutions of trans-NAACn were kept below 0 oC for 24 h and then irradiated with UV-light below 0 oC to
obtain the spectra of all-cis. Except for the spectra of all-cis, the photoirradiations were performed at 25 oC.
To complement the photoisomerization analysis of NAACn, the molar fraction of
PSS was determined on the basis of the UV-vis spectra (Figure 2.2) using the following formula:12
𝐴(𝜆)
𝑙𝐶 = 𝜏𝜀𝑡+ (1 − 𝜏)𝜀𝑐
where A(λ) is the absorbance at the wavelength λ, l is the length of the optical path,
C is the concentration of NAACn, εt and εc are the extinction coefficients of trans-NAACn and cis-NAACn, respectively, and τ is the molar fraction of trans-NAACn.
The EtOAc solutions of trans-NAACn were kept in total darkness overnight prior to
measurement, which were assumed to contain pure trans-NAACn (τ = 1), and the
EtOAc solutions of trans-NAACn were kept below 0 oC for 24 h, followed by
UV-light irradiation below 0 oC to PSS, which were assumed to contain pure cis-NAA Cn
(τ = 0).31,32 On the basis of the absorbances at 352 nm in the spectra of all trans or
all cis (Figure 2.2), the extinction coefficients of trans-NAACn (εt) or cis-NAACn (εc)
can be obtained. With the obtained εt and εc, the molar fraction of cis-NAACn in the cis-rich PSS or the molar fraction of trans-NAACn in the trans-rich PSS can be calculated according to the above formula and the obtained absorbances at 352 nm in the absorption spectra of PSS from Figure 2.2. The molar fraction of cis-NAACn
decreased from 92% (NAAC4) to 86% (NAAC12) in the cis-rich PSS by UV-light (λ
= 365 nm) irradiation, while the molar faction of trans-NAACn showed a reverse
trend, i.e., increased from 66% (NAAC4) to 69% (NAAC12) in the trans-rich PSS by
Vis-light (λ = 400 – 780 nm) irradiation. The results showed that the ratios of trans-NAACn/cis-NAACn in the PSS were dependent on the hydrophobic chain length of
NAACn. Additionally, alternating UV-light and Vis-light irradiation to the EtOAc
solutions of NAACn over 20 cycles show no obvious signs of fatigue (Figure 2.3).
The results clearly demonstrated that the photoirradiation time of NAACn for
attaining the PSS and the ratios of trans-NAACn/cis-NAACn in the PSS could be
controlled systematically by the hydrophobic chain length. Compared to NAAC8 and
NAAC12, NAAC4, functionalized with the shortest hydrophobic chain, showed the
shortest photoirradiation time for reaching the PSS and the highest molar fraction of
cis-NAAC4 in the cis-rich PSS after UV-light irradiation.
Table 2.1 Molar fraction of cis-NAACn in the cis-rich PSS (UV-light irradiation) and
trans-NAACn in the trans-rich PSS (Vis-light irradiation).
Molar fraction in the PSS NAAC4 NAAC8 NAAC12
cis-NAACn 92 % 86 % 86 %
Figure 2.3 Photochemical fatigue resistance of (a) NAAC4, (b) NAAC8 and (c) NAAC12
determined by the variation of absorbance at 352 nm in the absorption spectra of NAACn (0.01 g L-1) in EtOAc solutions after 20 photoirradiation cycles by alternating
UV-light (λ = 365 nm) and Vis-light (λ = 400 – 780 nm).
To investigate the concentration effects of NAACn in EtOAc solution on the
photoirradiation time for attaining the PSS, freshly prepared EtOAc solutions of NAACn (in a concentration range: 0.01 g L-1, 0.02 g L-1 and 0.04 g L-1) were irradiated
with UV-light (λ = 365 nm) to the cis-rich PSS, followed by exposing to Vis-light (λ = 400 – 780 nm) to switch back to the trans-rich PSS. The photoirradiation times for reaching the PSS are plotted as a function of NAACn concentration (Figure 2.4). A
longer photoirradiation time for attaining the PSS was observed upon increasing NAACn concentration reagardless of hydrophobic chain length. For example, the
UV-light irradiation time for attaining the cis-rich PSS of trans-NAAC4 EtOAc
solution increases from 1 s to 2 s when the concentration of NAAC4 increases from
0.01 g L-1 to 0.04 g L-1 (Figure 2.4a). The Vis-light irradiation time for switching
back to the trans-rich PSS of the obtained NAAC4 EtOAc solution in the cis-rich PSS
also increases from 12 min (0.01 g L-1) to 14 min (0.04 g L-1, Figure 2.4a). This is
possibly due to the reduced light penetration at a higher concentration of NAACn,
and consequently a longer irradiation time is required to attain the PSS.17
Figure 2.4 The concentration dependence of (a) NAAC4, (b) NAAC8 and (c) NAAC12 for
attaining the PSS using various UV light (λ = 365 nm) and Vis-light (λ = 400 – 780 nm) irradiation times in EtOAc solutions (concentration: 0.01 g L-1 to 0.04 g L-1).
Compared to the photoisomerization from trans-NAACn to cis-NAACn with UV-light
irradiation, the transformation from cis-NAACn to trans-NAACn required much
longer Vis-light irradiation. As the isomerization from cis-NAACn to trans-NAACn
occurred not only by Vis-light stimulation but also by thermal stimulation, we envisioned that a shorter irradiation time for the isomerization from cis-NAACn to trans-NAACn could be required by exposure to Vis-light at a higher temperature. Freshly prepared EtOAc solutions of NAAC4 (0.01 g L-1) were irradiated with
UV-light (λ = 365 nm) for 1 s to obtain the cis-rich PSS, followed by exposing to Vis-light (λ = 400 – 780 nm) for 15 min at a range of temperature (i.e., 20 oC, 30 oC and
40 oC). The UV-vis absorption spectra were recorded every minute during Vis-light
irradiation. The molar fractions of trans-NAAC4 were calculated on the basis of the
absorbances at 352 nm in the absorption spectra and plotted against the Vis-light irradiation time (Figure 2.5). As expected, as the temperature increased from 20 oC
to 40 oC, the Vis-light irradiation time for reaching the trans-rich PSS decreased from
12 min to 5 min (Figure 2.5). The results indicated that the irradiation time for the isomerization from cis-NAACn to trans-NAACn significantly shortened by exposure
to Vis-light at an elevated temperature.
Figure 2.5 The temperature dependence of NAAC4 (0.01 g L-1) EtOAc solutions in the
cis-rich PSS for attaining the trans-rich PSS upon Vis-light (λ = 400 – 780 nm) irradiation.
2.2.3 Photoresponsive foams of NAACn doped in SDS foaming system
Freshly prepared EtOAc solutions (4.5 mL) of NAACn (in a concentration range from
0 g L-1 to 0.04 g L-1) were irradiated with UV-light (λ = 365 nm) to obtain
cis-rich-NAACn EtOAc solutions or exposed to Vis-light (λ = 400 – 780 nm) to obtain
trans-rich-NAACn EtOAc solutions, followed by doping into the foaming aqueous
solutions of SDS (45.5 mL, 2.0 g L-1). The obtained solutions were stirred by using
a mixer for 2 min to generate foams, and then the times were plotted as a function of the drainage volumes of the foams (Figure 2.6). Foam life refers to the time for reaching the drainage volume of 7 mL, i.e., all foams rupture. The variation of foam
life of foams prepared from the solutions doped with trans-rich-NAACn or
cis-rich-NAACn demonstrated the difference of foam control capability of NAACn. For
instance, foam life of foams prepared from the SDS solution (2.0 g L-1) is 6.7 min,
which increases to 10.4 min after doping with trans-rich-NAAC4 (0.04 g L-1) or
decreases to 5.1 min after doping with cis-rich-NAAC4 (0.04 g L-1). The results
indicated that trans-NAAC4 shows a foam stabilization effect, while cis-NAAC4
accelerates the rupture of the foams (Figure 2.6a). Comparing the variation of foam life of foams prepared from the solutions doped with NAACn, a largest variation was
found when NAAC4 (0.04 g L-1) was doped in the SDS solution, indicating that
NAAC4 showed the most efficient controllability for foam properties (Figure 2.6).
Figure 2.6 Foam controllability of (a) NAAC4, (b) NAAC8 and (c) NAAC12 determined
by the variation of foam life of foams prepared from SDS solutions doped with trans-rich-NAACn (Vis-light irradiation) or cis-rich-NAACn (UV-light irradiation) in a
concentration range (from 0 g L-1 to 0.04 g L-1). Foam life refers to the time for
reaching the drainage volume of 7 mL, i.e., all foams rupture.
To confirm the foam stabilization effect in the presence of trans-NAAC4, the foaming
solutions of SDS (45.5 mL, 2.0 g L-1) doped with cis-rich-NAA
C4 (4.5 mL, 0.04 g L -1) were exposed to Vis-light (λ = 400 – 780 nm) at various time, affording different
molar fraction of trans-NAAC4 in SDS solutions. The foam life times of foams
prepared from the obtained solutions were investigated. As expected, the foam life increases with prolonging Vis-light irradiation (Figure 2.7). Since a higher molar fraction of trans-NAACn can be obtained with a longer Vis-light irradiation time
before reaching the rich PSS, the increase of foam life indicated that trans-NAACn showed a foam stabilization effect.
Figure 2.7 Vis-light irradiation time dependence of the foam life. The EtOAc solutions (4.5 mL) of cis-rich-NAAC4 (0.04 g L-1) were doped into the foaming
aqueous solutions of SDS (45 mL, 2.0 g L-1). The resulting solutions were exposed to
Vis-light (λ = 400 – 780 nm) from 0 min to 12 min, which were followed by the preparation of foams from the solutions and recording the foam life times.
The foam stabilization effect of trans-NAACn could be interpreted by the formation
of order and dense monolayers of trans-NAACn doped in SDS at the air-water
interface, which was possibly due to the π-π stacking forces in a face-to-face arrangement of azobenzene rings.17,33 The strong attractive interaction from the
monolayers of trans-NAACn doped in SDS provided a foam stabilization effect by
hindering the coalescence of the liquid films in foams. In contrast, the destabilization effects of the cis-NAACn might be mainly attributed to the interfacial desorption
properties. The desorption constant of the cis-azobenzene was higher than that of
trans-azobenzene.17 After exposure to UV-light, the trans-NAA
Cn at the air-water
interface converted into cis-NAACn, and rapidly desorbed from the interface to the
bulk solution. The significant desorption resulted in a rapid increase of the surface tension and weakening the thin-liquid films against coalescence.17
Next, the effects of SDS concentration on foam life were also investigated. Freshly prepared EtOAc solutions (4.5 mL) of trans-rich-NAAC4 (0.04 g L-1) or
cis-rich-NAAC4 (0.04 g L-1) were doped into the aqueous solutions of SDS (45.5 mL in a
concentration range from 1 g L-1 to 4 g L-1), respectively, to generate foams. The
foam life was plotted as a function of SDS concentration. The foam life times of cis-rich-NAAC4 doped SDS solutions are about 5 min, while the foam life times of
trans-rich-NAAC4 doped SDS solutions are about 10 min regardless of the SDS
concentration (concentraions: 1.0 g L-1 to 4.0 g L-1, Figure 2.8). As a result, no
significant variation of foam life was observed with the increase of SDS concentration, indicating that the controllability of foam stability is mainly attributed to the transformation between trans-NAAC4 and cis-NAAC4.
Figure 2.8 The SDS concentration dependence (from 1.0 g L-1 to 4.0 g L-1) on foam
life of aqueous solutions of SDS doped with cis-rich-NAAC4 or trans-rich-NAAC4 at a
concentration of 0.04 g L-1.
Furthermore, morphologies of foams prepared from the trans-rich-NAAC4 doped
SDS solution and cis-rich-NAAC4 doped SDS solution were imaged by a USB digital
microscope (Figure 2.9). Compared to foams prepared from either the SDS solution or cis-rich-NAAC4 doped SDS solution, stable foams with uniform sizes are obtained
from the trans-rich-NAAC4 doped SDS solution (Figure 2.9). Before UV-light
irradiation, trans-NAACn adsorbed at the air-water interface orderly and packed
densely, which can stabilize foam film to improve the foam stability. On the contrary, after UV-light irradiation, cis-NAACn desorbed from the air-water interface to the
bulk solution, leading to an elevation of surface tension. Moreover, compared to the monolayer of trans-NAACn doped in SDS at the air-water interface, a looser packing
monolayer of cis-NAACn doped in SDS was expected. The increase of surface
tension and a loose monolayer weakened the liquid foam films against coalescence. The results demonstrated that photoresponsive foams can be prepared by doping the NAACn in the foaming solution of SDS.
Figure 2.9 Foam morphologies and proposed foam control mechanism. The images were taken 5 min after foaming. The blank foams were prepared from a SDS foaming solution (50 mL, 2.0 g L-1). The trans-NAA
C4 and cis-NAAC4 foams were
prepared from foaming solutions of SDS (45.5 mL, 2 g L-1) doped with 4.5 mL of
2.3 Conclusions
A series of nonionic azobenzene amphiphiles (NAACn) with different hydrophobic
chain lengths were synthesized and doped in aqueous foams to control foam stability by UV light (λ = 365 nm) and Vis-light (λ = 400 – 780 nm) irradiation. The hydrophobic chain length affected both photoisomerization properties and controllability for foam stability. The trans-NAACn in an EtOAc solution (0.01 g L -1) converted to cis-NAA
Cn by exposing to UV-light for 1 s and the cis-NAACn was
able to switch back to the trans-NAACn with Vis-light irradiation. Besides, the molar
fractions of cis-NAACn in the cis-rich PSS after UV-light irradiation decreased with
a longer hydrophobic chain length, while the molar fractions of trans-NAACn in the trans-rich PSS after exposing to Vis-light increased. Vis-light irradiation at a higher
temperature could shorten the irradiation time for the isomerization from cis-NAACn
to trans-NAACn. After doping the NAACn into foaming solutions of SDS, the
trans-NAACn showed a foam stabilization effect, while cis-NAACn could promote foam
rupture. In consideration of the photoisomerization properties and the controllability for foam stability, NAAC4 was identified as the optimal structure for obtaining
photoresponsive foams. The photoresponsive foams prepared from the NAACn
doped SDS solutions are promising in the reversible control of foam stability and recycle residual foam in industrial processes, allowing to reduce water consumption and chemical discharge. Further improvements of NAACn solubility will be
discussed in the next chapters.
2.4 Acknowledgements
The authors are grateful for the financial support of the National Natural Science Foundation of China (21174055) and the Priority Academic Program Development of Jiangsu Higher Education Institutions.
2.5 Experimental Data
2.5.1 Materials and Methods
For the general materials and methods, please refer to section A, Materials and Methods, in the Appendix.
Foam preparation: Foams were prepared from foaming solutions of SDS (45.5 mL, 2 g L-1) doped with EtOAc solutions of NAA
Cn (4.5 mL, in a range of concentrations)
by stirring with a mixer (938A, Guangzhou Qihe Electrical Appliance Co., Ltd., Guangdong, China) for 2 min at 25 oC. Morphologies of the foams were imaged by
using a USB digital microscope (AM801, Zhongshan Maisi electronic technology Co., Ltd., Guangdong, China).
2.5.2 Synthesis and Characterization 4,4′-bis(hydroxy)-azobenzene:24
KOH (100 g, 1.78 mol) was dissolved and kept stirring in water (20 mL),
followed by adding p-nitrophenol (20 g, 0.14 mol). The reaction mixture was
heated at 120
oC for 1 h and then slowly heated to 200
oC for 30 min,
subsequently allowing to cool to 25
oC and pour into water (250 mL). The
mixture was acidified by an aqueous solution of HCl (5 M) to pH = 2 and
extracted with diethyl ether (500 mL). The combine organic layers were
washed with brine and dried over Na
2SO
4. The solvent was removed in
vacuum and the residue was subsequently recrystallized from EtOH/water
(v/v = 1/1) to provide 4,4′-bis(hydroxy)-azobenzene as a brown solid (8.6 g,
0.047 mol) in 57% yield. IR, υ/ cm
-1: 3453, 3195, 1310 – 1410.
1H NMR (400
MHz, CD
3OD) δ (ppm) 7.75 – 7.73 (d, J = 8.0 Hz, 4H), 6.90 – 6.88 (d, J =
8.0 Hz, 4H).
Compound 1 (NAAC4):
To a DMF (80 mL) solution of 4,4′-bis(hydroxy)-azobenzene (6.4 g, 0.03 mol), K2CO3 (6.2 g, 0.045 mol) and KI (5.0 g, 0.03 mol) were added at 25 oC, and the
mixture was stirred at 80 oC for 0.5 h. A DMF (40 mL) solution of 1-bromobutane
(4.1 g, 0.03 mol) was then dropwise added to the reaction mixture and the obtained mixture was stirred at 80 oC for 8 h. Subsequently, the reaction mixture was allowed
to cool to 25 oC. After removing the DMF, the mixture was washed with water (200
mL) and CH2Cl2 (200 ml). The organic layers were combined and the solvents were
removed by rotary evaporation. The residue was subjected to column chromatography (Silica gel, petroleum ether/EtOAc, v/v = 7/1) to provide compound 1 (NAAC4) as a yellow solid (5.0 g, 0.018 mol) in 62% yield. 1H NMR (400 MHz,
CDCl3) δ (ppm) 7.84 (dd, J = 12.0, 8.0 Hz, 4H), 6.99 (d, J = 8.0 Hz, 2H), 6.91 (d, J
= 12.0 Hz, 2H), 4.04 (t, J = 6.5 Hz, 2H), 1.85 – 1.75 (m, 2H), 1.57 – 1.45 (m, 2H), 0.99 (t, J = 7.4 Hz, 3H).
Compound 2 (NAAC8):
Using a procedure similar to that for NAAC4, NAAC8 was obtain as a yellow solid
(5.6 g, 0.017 mol, 57% yield) from 4,4′-bis(hydroxy)-azobenzene and 1-bromooctane. 1H NMR (400 MHz, CDCl
3) δ (ppm) 7.84 (dd, J = 12.0, 8.0 Hz, 4H),
6.99 (d, J = 8.0 Hz, 2H), 6.91 (d, J = 8.0 Hz, 2H), 4.04 (t, J = 6.5 Hz, 2H), 1.85 – 1.75 (m, 2H), 1.57 – 1.45 (m, 10H), 0.99 (t, J = 7.4 Hz, 3H).
Compound 3 (NAAC12):
Using a procedure similar to that for NAAC4, NAAC12 was obtain as a yellow solid
(6.9 g, 0.018 mol, 60% yield) from 4,4′-bis(hydroxy)-azobenzene and 1-bromododecane. 1H NMR (400 MHz, CDCl 3) δ (ppm) 7.84 (dd, J = 12.0, 8.0 Hz, 4H), 6.99 (d, J = 12.0 Hz, 2H), 6.92 (d, J = 8.0 Hz, 2H), 4.03 (t, J = 6.6 Hz, 2H), 1.89 – 1.74 (m, 2H), 1.52 – 1.41 (m, 2H), 1.41 – 1.20 (m, 16H), 0.88 (t, J = 6.8 Hz, 3H).
2.6 References
(1) Astarita, A.; Carrino, L.; Franchitti, S.; Langella, A.; Paradiso, V. Manufacturing of Innovative Components Formed Using SPF Processes and Filled with Aluminum Metal Foams. Surf. Interface Anal. 2013, 45 (10), 1638–1642.
(2) Stubenrauch, C.; Shrestha, L. K.; Varade, D.; Johansson, I.; Olanya, G.; Aramaki, K.; Claesson, P. Aqueous Foams Stabilized by n-Dodecyl-β-D-Maltoside, Hexaethyleneglycol Monododecyl Ether, and Their 1: 1 Mixture. Soft Matter 2009,
5 (16), 3070–3080.
(3) Yu, H.; Wang, Y.; Zhong, Y.; Mao, Z.; Tan, S. Foam Properties and Application in Dyeing Cotton Fabrics with Reactive Dyes. Color. Technol. 2014, 130 (4), 266–272. (4) Du, Z.; Bilbao-Montoya, M. P.; Binks, B. P.; Dickinson, E.; Ettelaie, R.; Murray, B. S. Outstanding Stability of Particle-Stabilized Bubbles. Langmuir 2003, 19 (8), 3106–3108.
(5) Shrestha, L. K.; Saito, E.; Shrestha, R. G.; Kato, H.; Takase, Y.; Aramaki, K. Foam Stabilized by Dispersed Surfactant Solid and Lamellar Liquid Crystal in Aqueous Systems of Diglycerol Fatty Acid Esters. Colloid. Surf. A 2007, 293 (1–3), 262–271. (6) Song, X. W.; Zhang, L.; Wang, X. C.; Zhang, L.; Zhao, S.; Yu, J. Y. Study on Foaming Properties of Polyoxyethylene Alkyl Ether Carboxylic Salts with Different Structures. J. Dispers. Sci. Technol. 2011, 32 (2), 247–253.
(7) Dickinson, E.; Ettelaie, R.; Kostakis, T.; Murray, B. S. Factors Controlling the Formation and Stability of Air Bubbles Stabilized by Partially Hydrophobic Silica Nanoparticles. Langmuir 2004, 20 (20), 8517–8525.
(8) Georgieva, D.; Cagna, A.; Langevin, D. Link between Surface Elasticity and Foam Stability. Soft Matter 2009, 5 (10), 2063–2071.
(9) Fameau, A. L.; Carl, A.; Saint-Jalmes, A.; Von Klitzing, R. Responsive Aqueous Foams. ChemPhysChem 2015, 16 (1), 66–75.
(10) Chu, Z.; Feng, Y. pH-Switchable Wormlike Micelles. Chem. Commun. 2010, 46 (47), 9028–9030.
(11) Brown, P.; Butts, C. P.; Eastoe, J. Stimuli-Responsive Surfactants. Soft Matter 2013,
9 (8), 2365–2374.
(12) Chevallier, E.; Monteux, C.; Lequeux, F.; Tribet, C. Photofoams: Remote Control of Foam Destabilization by Exposure to Light Using an Azobenzene Surfactant.
Langmuir 2012, 28 (5), 2308–2312.
(13) Peng, S.; Guo, Q.; Hughes, T. C.; Hartley, P. G. Reversible Photorheological Lyotropic Liquid Crystals. Langmuir 2014, 30 (3), 866–872.
(14) Zhao, Y. Light-Responsive Block Copolymer Micelles. Macromolecules 2012, 45 (9), 3647–3657.
(15) Cicciarelli, B. A.; Hatton, T. A.; Smith, K. A. Dynamic Surface Tension Behavior in a Photoresponsive Surfactant System. Langmuir 2007, 23 (9), 4753–4764.
(16) Schumers, J. M.; Fustin, C. A.; Gohy, J. F. Light-Responsive Block Copolymers.
Macromol. Rapid Commun. 2010, 31 (18), 1588–1607.
(17) a) Chevallier, E.; Mamane, A.; Stone, H. A.; Tribet, C.; Lequeux, F.; Monteux, C. Pumping-out Photo-Surfactants from an Air-Water Interface Using Light. Soft
Matter 2011, 7 (17), 7866–7874. b) for earlier works see chapter 1: ref 58-61.
(18) Eastoe, J.; Dominguez, M. S.; Wyatt, P.; Beeby, A.; Heenan, R. K. Properties of a Stilbene-Containing Gemini Photosurfactant: Light-Triggered Changes in Surface Tension and Aggregation. Langmuir 2002, 18 (21), 7837–7844.
(19) Feng, C.; Qu, G.; Song, Y.; Jiang, L.; Zhu, D. Surface Arrangement of Azobenzene Moieties in Two Different Azobenzene-Derived Langmiur-Blodgett Films. Surf.
Interface Anal. 2006, 38 (10), 1343–1347.
(20) Housni, A.; Zhao, Y.; Zhao, Y. Using Polymers to Photoswitch the Aggregation State of Gold Nanoparticles in Aqueous Solution. Langmuir 2010, 26 (14), 12366–12370. (21) Zhang, X.; Feng, Y.; Lv, P.; Shen, Y.; Feng, W. Enhanced Reversible Photoswitching of Azobenzene-Functionalized Graphene Oxide Hybrids. Langmuir 2010, 26 (23), 18508–18511.
(22) Wang, J.; Nguyen, A. V.; Farrokhpay, S. Effects of Surface Rheology and Surface Potential on Foam Stability. Colloid. Surf. A 2016, 488, 70–81.
(23) Hayashita, T.; Kurosawa, T.; Miyata, T.; Tanaka, K.; Igawa, M. Effect of Structural Variation within Cationic Azo-Surfactant upon Photoresponsive Function in Aqueous Solution. Colloid Polym. Sci. 1994, 272 (12), 1611–1619.
(24) Surampudi, S. K.; Patel, H. R.; Nagarjuna, G.; Venkataraman, D. Mechano-Isomerization of Azobenzene. Chem. Commun. 2013, 49 (68), 7519–7521.
(25) Abellán, G.; García, H.; Gómez-García, C. J.; Ribera, A. Photochemical Behavior in Azobenzene Having Acidic Groups. Preparation of Magnetic Photoresponsive Gels.
J. Photochem. Photobiol. A 2011, 217 (1), 157–163.
(26) Ferri, V.; Elbing, M.; Pace, G.; Dickey, M. D.; Zharnikov, M.; Samor, P.; Mayor, M.; Rampi, M. A. Light-Powered Electrical Switch Based on Cargo-Lifting Azobenzene Monolayers. Angew. Chem. Int. Ed. 2008, 47 (18), 3407–3409.
(27) Liu, J.; Nie, J.; Zhao, Y.; He, Y. Preparation and Properties of Different Photoresponsive Hydrogels Modulated with UV and Visible Light Irradiation. J.
(28) Feng, C.; Jin, J.; Zhang, Y.; Song, Y.; Xie, L.; Qu, G.; Xu, Y.; Jiang, L. Reversible Light-Induced Wettability of Fluorine-Containing Azobenzene-Derived Langmuir-Blodgett Films. Surf. Interface Anal. 2001, 32 (1), 121–124.
(29) Bandara, H. M. D.; Burdette, S. C. Photoisomerization in Different Classes of Azobenzene. Chem. Soc. Rev. 2012, 41 (5), 1809–1825.
(30) Crecca, C. R.; Roitberg, A. E. Theoretical Study of the Isomerization Mechanism of Azobenzene and Disubstituted Azobenzene Derivatives. J. Phys. Chem. A 2006, 110 (26), 8188–8203.
(31) Rau, H.; Greiner, G.; Gauglitz, G.; Meier, H. Photochemical Quantum Yields in the A (+hv) to B (+hv, ∆) System When Only the Spectrum of A Is Known. J. Phys.
Chem. 1990, 94, 6523–6524.
(32) Price, A. D.; Ignés-Mullol, J.; Àngels Vallvé, M.; Furtak, T. E.; Lo, Y. A.; Malone, S. M.; Schwartz, D. K. Liquid Crystal Anchoring Transformations Induced by Phase Transitions of a Photoisomerizable Surfactant at the Nematic / Aqueous Interface.
Soft Matter 2009, 5 (11), 2252–2260.
(33) Yim, K. S.; Fuller, G. G. Influence of Phase Transition and Photoisomerization on Interfacial Rheology. Phys. Rev. E. 2003, 67 (4), 1-10.
