Synthesis of photo-responsive azobenzene molecules with different hydrophobic chain length for controlling foam stability

Article abstract of DOI:10.1039/c6ra06459k

To obtain an aqueous foam photo-switch, azobenzene molecules [4-hydroxy-4′-oxoalkyl azobenzene (HC_nAzo) n = 4, 8, and 12] were synthesized. The hydrophobic chain length affected both photo-responsive properties and foam stability controllability. trans → cis isomerization of HC_nAzo occurred by exposing to UV light for 1 s and the cis → trans process for HC₄Azo, HC₈Azo and HC₁₂Azo was carried out by visible light irradiation for 12 min, 13 min and 14 min, respectively. The reversible isomerization was repeatable, maintaining high sensitivity. Due to the small steric effect and isomerization barrier, photo-isomerization of HC₄Azo was more rapid than HC₈Azo and HC₁₂Azo. With the increase of the hydrophobic chain length, the decrease of thermal isomerization barrier resulted in less cis isomer and more trans isomer in photo-stationary state via UV and visible light irradiation, respectively. The combination of visible light and heat can accelerate the cis → trans isomerization speed. trans HC₄Azo, HC₈Azo and HC₁₂Azo increased foam life from 6.67 min to 10.38, 9.91 and 7.74 min, respectively, resulting from the absorption on the air-water interface and a high affinity. cis HC₄Azo, HC₈Azo and HC₁₂Azo decreased foam life from 6.67 min to 5.12, 5.49 and 6.02 min, respectively, attributed to the interfacial desorption and increase of surface tension. HC₄Azo with sensitive photo-isomerization and effective foam stability controllability was the best choice for an aqueous foam photo-switch.

Full text of DOI:10.1039/c6ra06459k

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PAPER
Synthesis of photo-responsive azobenzene
molecules with different hydrophobic chain length
for controlling foam stability†
Cite this: RSC Adv., 2016, 6, 60138
*
Shaoyu Chen, Chaoxia Wang, Yunjie Yin and Kunlin Chen
To obtain an aqueous foam photo-switch, azobenzene molecules [4-hydroxy-40-oxoalkyl azobenzene
(HCnAzo) n ¼ 4, 8, and 12] were synthesized. The hydrophobic chain length affected both photo-
responsive properties and foam stability controllability. trans / cis isomerization of HCnAzo occurred by
exposing to UV light for 1 s and the cis / trans process for HC4Azo, HC8Azo and HC12Azo was carried
out by visible light irradiation for 12 min, 13 min and 14 min, respectively. The reversible isomerization
was repeatable, maintaining high sensitivity. Due to the small steric effect and isomerization barrier,
photo-isomerization of HC4Azo was more rapid than HC8Azo and HC12Azo. With the increase of the
hydrophobic chain length, the decrease of thermal isomerization barrier resulted in less cis isomer and
more trans isomer in photo-stationary state via UV and visible light irradiation, respectively. The
combination of visible light and heat can accelerate the cis / trans isomerization speed. trans HC4Azo,
HC8Azo and HC12Azo increased foam life from 6.67 min to 10.38, 9.91 and 7.74 min, respectively,
resulting from the absorption on the air–water interface and a high affinity. cis HC4Azo, HC8Azo and
HC12Azo decreased foam life from 6.67 min to 5.12, 5.49 and 6.02 min, respectively, attributed to the
interfacial desorption and increase of surface tension. HC4Azo with sensitive photo-isomerization and
effective foam stability controllability was the best choice for an aqueous foam photo-switch.
Received 10th March 2016
Accepted 3rd June 2016
DOI: 10.1039/c6ra06459k
www.rsc.org/advances
light has attracted much attention to control the photo-
responsive systems owing to its tunable parameters, including
wavelength, intensity and duration.12,13 In addition, a light
trigger can be accurately localized, enabling both temporal and
spatial control and is easily achieved using optical lenses,
lters, and masks.14,15
Introduction
Foam, composed of gas bubbles and separated by thin liquid
lms, is a thermodynamically metastable system, which is
widely used in the ne chemicals, petroleum chemicals,
metallurgical mining, and food and textile industries.1–3 The
stability of foam is a crucial factor in its application. Surfac-
tants, polymers and solid particles are usually used as foam
stabilizers.4–6 The role of the foam stabilizer is to slow down
foam destabilization mechanisms, i.e., drainage, coarsening
and coalescence, by improving foam lm strength or increasing
viscosity.7,8 Due to the large space occupation of residual foam,
it is expected to be destroyed in a rapid and controlled way.
Defoaming agents are usually used to break foam, but they are
not a good choice for the reason that they prevent further re-
foaming. Switchable foam is a new strategy to rapidly destroy
residual foam and re-foam in a controllable way.9
Photosensitive chemicals contain a suitable chromophore
whose surface activity changes upon UV-light irradiation. The
chromophore can locate in the head group or tail chain of
surfactant molecule, or even the connecting base of geminal
surfactants. Typical chromophores are isomerization groups
such as azobenzene and stilbene.16 Azobenzene is very prom-
ising moiety due to its unique reversible photo-isomerization
capability, high sensitivity and electrochemical activity.17–19
Azobenzene derivatives have been proposed to utilize their
trans–cis isomerization in energy storage, electronic devices and
medical elds.20–22 E. Chevallier et al.23 reported a cationic
azobenzene-containing surfactant. This surfactant can control
stability and breakage of aqueous foam. However, the proper-
ties of cationic surfactants are easily affected by environmental
factors such as electrolyte, which limits their eld of applica-
tion. For example, adhesive, as an indispensable agent in
pigment dyeing and printing, is usually anionic. Therefore, the
cationic azobenzene-containing surfactant cannot be applied in
such a system. Compared to a cationic surfactant, a neutral
molecule is more applicable. It can be compounded with all
Switchable foam is the ability of foam to be tuned by external
stimuli such as pH,10 CO2, temperature, ionic strength, elec-
tricity, light, magnetic eld, oxidants and enzymes.11 Recently,
Key Laboratory of Eco-Textile, Ministry of Education, School of Textile & Clothing,
Jiangnan University, 1800 Lihu Road, Wuxi, 214122, People's Republic of China.
E-mail: wangchaoxia@sohu.com
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra06459k
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potassium carbonate (K2CO3, 99%), bromododecane (C12H25Br,
98%), bromooctane (C8H17Br, 98%), bromobutane (C4H9Br,
98%), N,N-dimethylformamide (DMF, 99%), dichloromethane
ꢀ
(CH2Cl2, 99%), petroleum ether (60–90 C, 99%), ethyl acetate
(99%), silica gel (FCP) and lauryl sodium sulfate (SDS, 99%)
were purchased from Sinopharm Chemical Reagent Co., Ltd.
Potassium iodide (KI, 99%) was obtained from Aladdin Indus-
trial Inc. The water was deionized water.
Fig. 1 Structure of azobenzene nonionic surfactants.
types of surfactants. However, the hydrophobic chain length of
the neutral molecule inuences its photo-response properties24
and other properties, such as CMC value and solubility, which
in turn affect the interface adsorption properties.
Synthesis of HCnAzo
HCnAzo was synthesized via substituting the hydrogen atom of
the OH group in DHAzo (homemade) with the alkyl group of the
bromoalkane by the substitution reaction, as shown in
Scheme 1.
In this study, photo-responsive azobenzene molecules
(HCnAzo) were synthesized (as shown in Fig. 1) to optimize the
hydrophobic chain length. They were synthesized with bro-
moalkane and 4,40-dihydroxy azobenzene (DHAzo). The design
of HCnAzo was inspired from dodecanol, which possesses
a superior foam stabilization capability.25 HCnAzo is expected to
realize the quick switch between foam stabilizer and defoamer
by alternating UV light and visible light irradiation, respectively.
HCnAzo characterization is performed with FT-IR, LC-MS and
H-NMR analysis. The effects of raw materials, molar ratio and
reaction time on yield are investigated for optimizing the
synthesis process. Photo-isomerization speed and extent over
a wide range of concentrations are investigated by UV absorp-
tion spectra. Inuences of trans and cis isomers on foam
stability and foamability are also investigated. HCnAzo can
tailor foam stability by light, which will show potential use in
recycling residual foam to save chemical agents and decrease
pollution.
Intermediate DHAzo. 100 g of KOH was dissolved and kept
stirred in 20 mL of deionized water. 20 g of p-nitrophenol was
introduced into the mixture of KOH and water. The reaction
mixture was le to melt at 120 ꢀC for 1 h and slowly heated to
200 ꢀC. Then the reaction mixture was dissolved in water and
adjusted to pH 3 with concentrated HCl solution. The product
precipitated as a brown-yellow solid, which was extracted with
diethyl ether aer ltering. Aer evaporation, the precipitate
was recrystallized in 50 : 50 v/v solution of ethanol/water to
obtain a pure brownish-red crystalline product and nally
dried under vacuum overnight. IR, n/cmꢁ1: 3453, 3195, 1310–
1
1410. H NMR (400 MHz, CD3OD) d 6.92–6.94 (d, 4H), 7.77–
7.79 (d, 4H).
Photo-responsive HCnAzo. 6.35 g of DHAzo, 6.14 g of K2CO3
(1.5 eq.) andꢀ0.5 g of KI were placed in 80 mL of DMF and
stirred at 80 C for 0.5 h. Then, the solution of bromoalkane
and DMF was added dropwise to the reaction mixture and
stirred at 80 ꢀC for several hours. Aer reaction, the crude
product was evaporated to remove DMF and then extracted
with CH2Cl2. Aer removing CH2Cl2, the product was further
Experimental
Materials
Potassium hydroxide (KOH, 99%), p-nitrophenol (99%), chlor- puried with column chromatography (silica gel, petroleum
ohydric acid (HCI, 35%), diethyl ether (99%), ethanol (95%), ether/ethyl acetate, 7 : 1).
Scheme 1 Synthesis of HCnAzo.
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containing either a cis-rich isomer or a trans-rich one. Foam was
prepared from the dispersion containing HCnAzo solutions and 2
g Lꢁ1 sodium dodecyl sulfate solution by stirring with a mixer
(938A, Guangzhou Qihe Electrical Appliance Co., Ltd., Guang-
dong, China) for 2 min at room temperature. Foam stability was
evaluated by foam life and drainage speed. Foam microscopy
photographs were also used to evaluate foam properties, which
were taken with a 10ꢂ USB digital microscope (AM801, Zhong-
shan Maisi Electronic Technology Co., Ltd., Guangdong, China).
Characterization
Infrared spectra were obtained for solid samples on a Thermo
Finnigan, Nicolet iS10, (USA) FT-IR spectrometer in KBr medium
with the mass ratio of 1 : 100 to ensure uniform dispersion at
ambient temperature. The mixed powders of the composites were
pressed as clean discs. Liquid chromatography-mass spectrom-
etry was recorded on a Thermo Finnigan, MALDI SYNAPT MS LC-
MS (USA) spectrometer. The samples were dissolved in CDCl3
and H-NMR spectroscopy was performed on a Bruker, Advance III
400 MHz spectrometer (Switzerland). The results are presented in
the ESI.†
Results and discussion
Isomerization analysis
Isomerization monitoring
In general, absorption spectroscopy can offer information about
the structural characterization of the azobenzene derivatives
owing to the transition moments of the azo group.28 Fig. 2A–F
Isomerization of HCnAzo was monitored with ultraviolet (UV)
absorption spectra taken by a Cary 50 ultraviolet spectropho-
tometer. The ethyl acetate solutions of HCnAzo (0.01 g Lꢁ1, 0.02
g Lꢁ1 and 0.04 g Lꢁ1) were exposed to UV light (l ¼ 360 nm, 175
mW cmꢁ2, Intelli-ray 600) and the change of their absorption
spectra was monitored until the absorption maxima peak at 356
nm was constant, which meant reaching a cis-rich photo-
stationary state. cis / trans reversible isomerization was
monitored by irradiating with visible light (YG(B)982X, Wenz-
hou Darong Textile Instrument Co. Ltd., Zhejiang, China) and
obtained absorption spectra until the disappeared absorption
maxima peak re-emerged. A trans-rich photo-stationary state
was obtained when the absorption spectra remained
unchanged aer prolonged visible light irradiation.
shows the UV-Vis absorption spectra of HCnAzo (0.01 g Lꢁ1
)
revealing the photo-isomerization properties. Fig. 2A, C, and E
show a strong p / p* absorption peak at 356 nm and a weak n
/ p* absorption peak at about 450 nm. Irradiation with UV
light for only 1 s resulted in sufficient trans-to-cis photo-
isomerization and
a cis-rich photo-stationary state was
reached, as evidenced by a prominent decrease in the absor-
bance at around 350–400 nm and a weak emergence in the n /
p* transition at 450 nm. Therefore, HCnAzo were highly sensi-
tive to UV light. On the other hand, Fig. 2B, D, and F show the
opposite phenomenon indicating the reversible photo-
isomerization (cis-to-trans) by visible light irradiation. In addi-
tion, as shown in Fig. 3A–C, the HCnAzo photo-isomerization is
reversible and it maintains the high sensitivity aer repeating
the photo-isomerization 20 times.
Calculation of isomeric compositions in photo-stationary
states
The isomeric compositions in photo-stationary states represent
the photo-isomerization extents, which are determined by
spectrophotometry and calculated by eqn (1):23
AðlÞ
¼ s3t þ ð1 ꢁ sÞ3c
(1)
lC
where A(l) is the absorbance at the wavelength l, l is the length
of the optical path, C is the concentration of HCnAzo, 3t and 3c
are the extinction coefficients of the trans and cis forms,
respectively, and s is the molar fraction of trans isomer. From
absorbance of the all-trans and all-cis spectra, the extinction
coefficients, 3t and 3c, were determined at the wavelength of 356
nm by eqn (1) with s ¼ 1 and 0, respectively. The isomeric
compositions of trans and cis in any other spectrum can thus be
calculated by eqn (1) with the value of 3t and 3c. The samples of
all-trans were kept in total darkness overnight prior to
ꢀ
measurement and the all-cis samples were kept below 0 C for
24 h before measurement so that the thermal reaction is frozen-
out and one can assume that 3c is temperature independent.26,27
Foam control properties analysis
Unless otherwise stated, solutions of HCnAzo (0.01 g Lꢁ1, 0.02 g
Lꢁ1, and 0.04 g Lꢁ1, in ethyl acetate/water (v/v) ¼ 1 : 10) were
exposed under either UV or visible light before use. Under these
Fig. 2 Photo-isomerization of HCnAzo at concentration of 0.01 g Lꢁ1
in UV light and visible light: (A) and (B) HC4Azo, (C) and (D) HC8Azo and
conditions, the solutions reach
a photo-stationary state (E) and (F) HC12Azo.
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generated by trans isomers excitation. S1 ) S0 excitation occurs
along the rotation pathway. Following S2 ) S0 excitation, the S2
state rapidly relaxes to the S1 state along the rotation pathway as
the primary isomerization mechanism and then the S1 state
relaxes to the S0 state.29,30
As displayed in Fig. 2, trans / cis photo-isomerizations are
rapid by exposing to UV light regardless of hydrophobic chain
length. However, the cis / trans photo-isomerization speed
slowed down upon increasing the hydrophobic chain length,
which was consistent with the results reported by Hayashita.24
The reason was that with the increase of the hydrophobic chain
length, greater steric hindrance caused the inversion transition
state to be higher in energy. On the other hand, increasing the
hydrophobic chain length enhanced the electron-donating
effect. This raised the inversion barrier height, making it
harder to isomerize.30 In addition to the photo-isomerization
speed of HCnAzo, the molar fractions of cis and trans isomer,
representing the isomerization extent, in the photo-stationary
state were calculated with the absorbance values in Fig. 2 and
given in Table 1.
According to the results presented above, the cis isomer
fraction decreased from 91.7% to 86.0% with the increase of
hydrophobic chain length in the cis-rich photo-stationary state
by UV light irradiation, while the trans isomer fraction showed
a reverse trend in the trans-rich photo-stationary state by visible
light irradiation. The increase of hydrophobic chain length
enhanced the electron donating effect, which resulted in the
decrease of thermal isomerization barrier in the cis to trans
isomerization process. The cis isomer was easier to transform
into the trans isomer by thermal isomerization, leading to the
decrease of cis isomer fraction.
Fig. 5A–C present the comparison of photo-isomerization
speed between different HCnAzo concentrations. It shows that
the photo-isomerization speeds slow down upon increasing
HCnAzo concentration regardless of hydrophobic chain length.
This is because the photo-isomerization speed is decided by the
photo-conversion of the bulk solution. With the increase of
concentration, the solutions possessed higher absorbance in
which the photo-conversion speed was very slow.31 In addition,
in a dilute HCnAzo solution, the surfactant molecules exist in
a monomer molecular state, whereas further increase of the
surfactant concentration caused the formation of aggregations
and micelles.24 The photo-isomerization process occurred much
more easily in the monomer molecular state than in the
aggregated state.
Fig. 3 Repeat times of HCnAzo photo-isomerization at a concentra-
tion of 0.01 g Lꢁ1. (A) Absorbance of HC4Azo after exposing to UV light
for 1 s (low absorbance) and visible light for 12 min (high absorbance).
(B) Absorbance of HC8Azo after exposing to UV light for 1 s (low
absorbance) and visible light for 13 min (high absorbance). (C) Absor-
bance of HC12Azo after exposing to UV light for 1 s (low absorbance)
and visible light for 14 min (high absorbance).
Because HCnAzo is an azobenzene derivative (Abs), its photo-
isomerization mechanism can be learned from azobenzene (AB).
It is well known that AB exists in two different congurations
(trans and cis) in S0 (the electronic ground) state. According to
some experimental results,29 t-AB isomerization always occurs by
UV light, which is in the S1 (n–p*) state by an inversion process
regardless of the initial excitation. Rotation contributes signi-
cantly in non-viscous media following S1 ) S0 excitation.
Furthermore, c-AB undergoes photo-isomerization by rotation
and thermal isomerization by inversion (shown in Fig. 4);
therefore, it can occur not only by visible light irradiation, but
also thermally in the dark.
However, substituents affect Abs isomerization through
steric and electronic effects.29 As for HCnAzo, the electron-
donating substituents (i.e. hydroxyl and alkoxy) cause
a modest decrease in the thermal isomerization barrier by
increasing the electron density in the p* orbital; therefore, it
has higher rates of cis / trans thermal isomerization than AB.
In the trans / cis isomerization process, S1 and S2 states are
cis to trans photo-isomerization was a little slow due to its
higher excitation energies compared to trans isomers. It was
more dependent on the structure and concentration of HCn-
Azo.30 Since cis / trans photo-isomerization can occur not only
Table 1 trans and cis isomer molar fraction of HCnAzo
Molar fraction
HC4Azo
HC8Azo
HC12Azo
cis isomer (UV light)
trans isomer (Vis light)
91.7%
66.3%
86.2%
68.6%
86.0%
68.8%
Fig. 4 Isomerization of AB.
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Foam control properties
As illustrated in Fig. 7, each of HC4Azo, HC8Azo and HC12Azo
can modulate foam stability over a wide range of concentra-
tions. For instance, foam life increased from 6.67 min to 10.38
min in trans-rich HC4Azo, while it decreased from 6.67 min to
5.12 min in cis-rich HC4Azo at the concentration of 0.04 g Lꢁ1
.
Therefore, it seemed that trans isomers showed a foam stabili-
zation effect, and cis isomers accelerated rupture of the foam. In
order to further conrm the stabilization effects of trans
isomers, the life of foam which was prepared from the bulk and
exposed to visible light for various times was measured. The
trans isomer fraction gradually increased with prolonging
visible light irradiation. As shown in Fig. 8, the foam life
increases with a longer visible light irradiation. The trans
isomers can thus be proven to have the capability to stabilize
foam.
The interfacial adsorption of trans isomers can explain the
stabilization of the foam. The trans isomer exhibited a high
affinity, having p stacking forces in a face-to-face arrangement
of azobenzene rings, which led to the formation of a nematic
phase.31,32 The strong attractive interaction caused the mole-
cules of trans isomers reorient parallel to maintain planarity,
resulting in the improvement of foam stability. In contrast, the
destabilization of the cis isomers was mainly attributed to the
interfacial desorption properties. The desorption constant of
the cis isomers was higher than that of the trans isomers.23 The
trans isomers converted into cis isomers and rapidly desorbed
from the interface when the trans-rich interface was exposed to
UV light. The signicant desorption resulted in a rapid increase
of the surface tension and a weakening of the protection of the
thin liquid lms against coalescence.31 In addition, non-
homogeneous light irradiation intensities led to a gradient of
Fig.
5 Effects of HCnAzo concentration on photo-isomerization
speed: (A) HC4Azo, (B) HC8Azo and (C) HC12Azo.
by visible light irradiation but also by thermal stimulation, it is
predictable that cis / trans photo-isomerization speed can
accelerate by supplementing thermal stimulation. Therefore,
the cis / trans photo-isomerization processes of HC4Azo were
monitored at varied temperatures by UV-Vis absorption spectra,
followed by calculating the trans isomer fractions. As shown in
Fig. 6, it canꢀbe observeꢀd that as the temperature increases from
ꢀ
20 C to 30 C and 40 C, the cis / trans photo-isomerization
time decreases from 12 min to 6 min and then 5 min, respec-
tively. The results indicated that the cis to trans isomerization
speed is signicantly increased by a combination of visible light
irradiation and thermal stimulation in an appropriate temper-
ature range (30 ꢀC for 0.01 g Lꢁ1 of HC4Azo in ethyl acetate
solution). Over the temperature range, there was no obvious
acceleration for cis to trans isomerization by further increasing
the temperature. The cis to trans isomerization time is enough
for recycling residual foam in common dyeing and nishing
processes as most of these processes generally take at least
30 min.
Fig. 7 Drainage volume dependence on time of foams stabilized at
Fig. 6 Effects of temperature on cis / trans photo-isomerization different concentrations of HCnAzo: (A) HC4Azo, (B) HC8Azo and (C)
speed (0.01 g Lꢁ1 of HC4Azo).
HC12Azo.
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Fig. 8 Effects of visible light irradiation time on foam life (0.04 g Lꢁ1 of
HC4Azo and 2 g Lꢁ1 of SDS).
Fig. 10 Comparison of foam microscopy images and foam control
mechanism (the images were taken after foaming for 5 min in 0.04 g
L
surface tension, resulting in Marangoni ows and accelerated
liquid drainage. The irregular and random structure of the cis
isomer without specic molecular interactions occupied
a higher surface area, resulting in a rapid decrease of the
surface excess. Interfacial adsorption/desorption properties of
trans and cis HC12Azo made it possible to modulate foam
stability.
ꢁ1 of HC4Azo).
on HCnAzo. The results are displayed in Fig. 9. It was obvious
that the foam life showed no signicant change with the
increase of SDS concentration, which indicated that the foam
control properties only resulted from the transformation of
HC4Azo between trans and cis isomers. Because the ionic
strength was determined by SDS concentration in this investi-
gation, the results demonstrated that the ionic strength in
solution presented no inuence on the isomerization process.
Foam morphology was further investigated. As shown in
Fig. 10, the comparison of foam morphology showed that foam
with trans-rich HC4Azo was more uniform and dense aer
foaming for 5 min. Before photo-isomerization, trans HCnAzo
adsorbed on foam air/water interface both in an orderly manner
and densely, which can dramatically enhance the strength of
the foam lm. On the contrary, the cis isomer of HCnAzo des-
orbed from the air/water interface, leading to an increase in the
surface tension and the decrease of surface excess. Moreover,
the increase of the steric hindrance resulted in more loose
packing of HCnAzo in the foam lm. Therefore, the cis isomer of
HCnAzo made the foam ruptures quickly. It was realizable to
prepare switchable foam by alternating the illumination with
UV and visible light.
Fig. 7A–C also shows effects of hydrophobic chain length of
HCnAzo on controllability of foam stability, which were depen-
dent on its concentration. When the concentration was less
than 0.01 g Lꢁ1, HC12Azo showed the best foam control prop-
erties among HC4Azo, HC8Azo and HC12Azo. The reason was
that the trans isomer fraction of HC12Azo was higher than
HC4Azo and HC8Azo in photo-stationary state. In addition, the
longer hydrophobic chain length increased the affinity between
trans isomers and the affinity of trans isomer and SDS (foaming
agent). The foam lm strength was increased, thereby
increasing foam stability. However, HC4Azo showed much more
superior foam stability and controllability by further increase of
the surfactant concentration. This was attributed to several
factors such as CMC value and solubility. According to the
report,24 the CMC value decreased as the hydrophobic chain
length increased. This meant that HCnAzo, with longer hydro-
phobic chain length, was easier to aggregate at the same
concentration and thus the number of monomer molecules
adsorbed on the air/liquid interface was decreased. In addition,
the solubility also decreased as the hydrophobic chain length
increased. Due to the limit of solubility, the total number of
soluble HC12Azo molecules in the bulk solution was not
increased with increasing concentration. Moreover, the insol-
uble HC12Azo particles covered in the air/liquid interface or
dispersed in the foam lm, aggravating foam instability.
The effects of SDS concentration on foam life were investi-
gated to prove that the foam control properties solely depended
Conclusions
Azobenzene molecules HCnAzo were synthesized and applied in
aqueous foam to control foam stability. From the UV-Vis
absorption spectra, HCnAzo showed highly sensitive photo-
response in UV light (1 s for trans / cis isomerization)
regardless of the hydrophobic chain length, and in visible light,
the cis / trans photo-isomerization time was increased upon
increasing the hydrophobic chain length (i.e. 12, 13, 14 min for
cis / trans isomerization of HC4Azo, HC8Azo and HC12Azo,
respectively). The increase of the hydrophobic chain length
enhanced the electron-donating and steric effects, which
increase the isomerization barrier, making the isomerization
process slow. In addition, the enhanced electron-donating
effect decreases the thermal isomerization barrier, resulting
in less cis isomer and more trans isomer in photo-stationary
state via UV and visible light irradiation, respectively. The cis
/ trans isomerization speed was accelerated by the combina-
tion of visible light and heat. Due to the adsorption properties,
Fig. 9 Effects of SDS concentration on foam life (0.04 g Lꢁ1 of
HC4Azo).
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trans HC4Azo, HC8Azo and HC12Azo increased foam life from 12 S. Peng, Q. Guo, T. C. Hughes and P. G. Hartley, Langmuir,
6.67 min to 10.38, 9.91 and 7.74 min, respectively. In contrast, 2013, 30, 866–872.
cis HC4Azo, HC8Azo and HC12Azo decreased foam life from 6.67 13 Y. Zhao, Macromolecules, 2012, 45, 3647–3657.
min to 5.12, 5.49 and 6.02 min, respectively. In consideration of 14 B. A. Cicciarelli, T. A. Hatton and K. A. Smith, Langmuir,
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Acknowledgements
The authors are grateful for the nancial support of the
National Natural Science Foundation of China (21174055) and
the Priority Academic Program Development of Jiangsu Higher
Education Institutions.
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