Adsorption, micellization and antimicrobial activity of formyl-containing cationic surfactant in diluted aqueous solutions

Article abstract of DOI:10.1016/j.molliq.2020.115168

Surfactants based on dynamic covalent bonds have attracted significant interest over the past few decades due to the coexistence of covalent and non-covalent bonds. However, studies on the fundamental properties of their precursors are rare. Herein, we synthesized a series of formyl-containing cationic surfactants (N-alkyl-N,N-dimethyl-N-(2-(4-formyl-phenoxy)ethyl) ammonium bromides) bearing different hydrophobic chains, which can form dynamic imine bonds. The adsorption, micellization and antimicrobial activity were systematically investigated using surface tension, fluorescence, electrical conductivity, and ITC techniques. The results showed that the insertion of the 2-(4-formyl-phenoxy)ethyl group (equivalent to ~7 methylene groups) endows the surfactant with an asymmetrical double-chain structure, enhancing the hydrophobic interaction. As a result, the critical micelle concentration (cmc) and contribution per methylene group to the standard Gibbs free energy of micellization (?G_m^o) were significantly decreased, compared to conventional single surfactant. Upon increasing the hydrophobic chain length, the cmc, the negative logarithm of the surfactant concentration in the bulk phase required to produce a 20 mN·m^?1 reduction in the surface tension of the solvent (pC₂₀) and maximum surface excess amount of the adsorbed surfactant (Γ_max) all decreased accompanied by an increase in the minimum molecular occupation area (A_min), while the ?G_m^o and standard Gibbs free energy of adsorption (?G_ads^o) become more negative and thus more favorable for the formation of micelles and the adsorption film. Moreover, micellization changed from being entropy-driven to enthalpy-driven and is more sensitive toward NaCl than urea. In addition, the current formyl-containing cationic surfactants exhibit excellent antimicrobial activity against both Gram-positive and Gram-negative bacteria and fungi, especially the surfactant with a hydrophobic chain containing ~20 atoms. Such a study will be favorable for the design of new dynamic covalent surfactants and an understanding of the change in their aggregation.

Full text of DOI:10.1016/j.molliq.2020.115168

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Journal of Molecular Liquids 325 (2021) 115168
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Adsorption, micellization and antimicrobial activity of formyl-containing
cationic surfactant in diluted aqueous solutions
Pingping Lu a, Shuai Heb, Yue Zhou a, Yongmin Zhanga,
⁎
a
Key Laboratory of Synthetic and Biological Colloids, Ministry of Education, School of Chemical & Materials Engineering, Jiangnan University, No. 1800 Lihu Avenue, Wuxi 214122, PR China
College of Chemistry and Environmental Protection Engineering, Southwest Minzu University, Chengdu 610041, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 8 October 2020
Received in revised form 8 December 2020
Accepted 19 December 2020
Available online 25 December 2020
Surfactants based on dynamic covalent bonds have attracted significant interest over the past few decades due to
the coexistence of covalent and non-covalent bonds. However, studies on the fundamental properties of their
precursors are rare. Herein, we synthesized a series of formyl-containing cationic surfactants (N-alkyl-N,N-di-
methyl-N-(2-(4-formyl-phenoxy)ethyl) ammonium bromides) bearing different hydrophobic chains, which
can form dynamic imine bonds. The adsorption, micellization and antimicrobial activity were systematically in-
vestigated using surface tension, fluorescence, electrical conductivity, and ITC techniques. The results showed
that the insertion of the 2-(4-formyl-phenoxy)ethyl group (equivalent to ~7 methylene groups) endows the sur-
factant with an asymmetrical double-chain structure, enhancing the hydrophobic interaction. As a result, the crit-
ical micelle concentration (cmc) and contribution per methylene group to the standard Gibbs free energy of
micellization (ΔGom) were significantly decreased, compared to conventional single surfactant. Upon increasing
the hydrophobic chain length, the cmc, the negative logarithm of the surfactant concentration in the bulk
phase required to produce a 20 mN·m−1 reduction in the surface tension of the solvent (pC20) and maximum
surface excess amount of the adsorbed surfactant (Γmax) all decreased accompanied by an increase in the mini-
mum molecular occupation area (Amin), while the ΔGom and standard Gibbs free energy of adsorption (ΔGaods) be-
come more negative and thus more favorable for the formation of micelles and the adsorption film. Moreover,
micellization changed from being entropy-driven to enthalpy-driven and is more sensitive toward NaCl than
urea. In addition, the current formyl-containing cationic surfactants exhibit excellent antimicrobial activity
against both Gram-positive and Gram-negative bacteria and fungi, especially the surfactant with a hydrophobic
chain containing ~20 atoms. Such a study will be favorable for the design of new dynamic covalent surfactants
and an understanding of the change in their aggregation.
Keywords:
Formyl-containing surfactant
Micellization
Adsorption
Thermodynamic
Antimicrobial activity
© 2020 Elsevier B.V. All rights reserved.
1. Introduction
molecules, in which stimuli-responsive surfactants are an important
representative [9,10]. Thus, stimuli-responsive surfactants that can
Surfactants are typical amphiphiles composed of a hydrophilic
headgroup and hydrophobic tail. Due to their unique amphiphilic char-
acteristics, surfactants prefer to adsorb onto the air/solution interface at
low concentration, and self-assemble into various aggregates (including
micelles, vesicle, liquid crystal, and so on) above the critical micellar
concentration (cmc), giving them an important role in many industrial
processes (cleaning, cosmetics, oil recovery, agriculture, dying and
painting) and fundamental research [1–3].
Because of their special application performance and smart control-
lability, numerous responsive colloidal systems (wormlike micelles, liq-
uid crystals, emulsions/microemulsions, foams, etc.) have continued to
emerge over the past 30 years [4–8]. Whereas, the design and fabrica-
tion of smart colloidal systems cannot do without basic functional
change their surface activity upon responding to an external stimuli
such as pH [11], temperature [12,13], light irradiation [14–16], CO2/N2
[17–20], redox [21–23] and magnetic field [24,25] have emerged
over time.
Recently, dynamical combinational chemistry has been gradually
applied toward the design of stimuli-responsive surfactants, i.e.,
dynamic covalent surfactants, which can switch their surface activity
via the stimuli-induced formation and breakage of dynamic covalent
bonds [26]. To date, dynamic covalent surfactants are mainly produced
by mixing a formyl-containing moiety and amine-containing moiety
to form an imine bond [27–29]. For instance, Takakura et al. [29]
developed a micro-sized giant vesicle formed by a dynamic covalent
surfactant, which was obtained via the in situ mixing of N-dodecyl-N,
N-dimethyl-N-(2-(4-formyl-phenoxy)ethyl) ammonium bromide and
dodecylamine. As a kind of formyl-containing precursor, N-alkyl-N,
N-dimethyl-N-(2-(4-formyl-phenoxy)ethyl) ammonium bromide, has
⁎
Corresponding author.
E-mail address: zhangym@jiangnan.edu.cn (Y. Zhang).
https://doi.org/10.1016/j.molliq.2020.115168
0167-7322/© 2020 Elsevier B.V. All rights reserved.

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P. Lu, S. He, Y. Zhou et al.
Journal of Molecular Liquids 325 (2021) 115168
also been used to fabricate other dynamic covalent surfactants with dif-
ferent molecular structures or stimuli-responsiveness, such as Gemini
structured, pH/redox and pH/light dual responsive surfactants. To better
understand the aggregation in solution and at the interface, studies on
the fundamental properties of formyl-containing precursors, including
their adsorption and micellization properties, are highly desirable.
It is well known that the incorporation of an additive, as well as its
molecular structure will affect the physicochemical properties, such as
the degree of ionization, reaction rates and clouding/phase separation
[32,33]. Thus, the effects of organic or inorganic additives on the funda-
mental properties is also crucial and cannot be ignored.
Herein, we have synthesized three formyl-containing surfactants
bearing different hydrophobic tails, N-alkyl-N,N-dimethyl-N-(2-(4-
formyl-phenoxy)ethyl) ammonium bromide (DOBAB, TOBAB and
HOBAB corresponding to the C12-, C14- and C16-hydrophobic tail,
Scheme 1), which can be used as the precursors of dynamic imine
bond surfactants. Their molecular structures were characterized
using 1H NMR spectroscopy and ESI-MS, and the fundamental prop-
erties of their dilute solutions, including the adsorption at air/solu-
tion interface, micellization in solution, and antimicrobial activity,
were investigated. This study may help to design novel dynamic co-
valent surfactants and understand aggregation before and after the
formation of dynamic covalent bonds.
2.2.2. Synthesis of N-alkyl-N,N-dimethyl-N-(2-(4-formyl-phenoxy)ethyl)
ammonium bromide
In a 250 mL flask, 4-(2-bromoethoxy)benzaldehyde (9.16 g,
0.04 mol) and N,N-dimethylhexadecylamine (12.94 g, 0.048 mol)
were mixed in 100 mL EtOH and refluxed for 24 h under a nitrogen
atmosphere. The solvent was removed under reduced pressure and
then the crude product was recrystallized from acetone to yield N,
N-dimethyl-N-hexadecyl-N-p-oxybenzylethylammonium bromide
(HOBAB) as a white solid (8.1 g). DOBAB and TOBAB were obtained
through the same route.
The molecular structure was characterized by 1H NMR (Bruker Ad-
vance III 400 MHz spectrometer, Bruker, Switzerland) and ESI-MS (Wa-
ters MALDI SYNAPT MS, USA).
1H NMR of DOBAB [400 MHz, (CDCl3), Fig. S1]: δ/ppm = 0.87–0.90
(t, 3H, J = 6 Hz), 1.26–1.38 (m, 20H), 1.85 (s, 2H), 3.53(s, 6H),
3.62–3.66 (t, 2H, J= 8 Hz), 4.32–4.34 (d, 2H), 4.67–4.68 (d, 2H),
7.08–7.10 (d, 2H), 7.86–7.88 (d, 2H), 9.91 (s, 1H). MS of DOBAB
(Fig. S2): Calcd: 362.0 (M – Br−). Found: m/z = 362.3.
1H NMR of TOBAB [400 MHz, (CDCl3), Fig. S3]: δ/ppm = 0.87–0.91 (t,
3H, J= 8 Hz), 1.26–1.39 (m, 22H), 1.81–1.87 (m, 2H), 3.55 (s, 6H),
3.65–3.69 (m, 2H), 4.34–4.37 (t, 2H, J = 6 Hz), 4.67–4.69 (t, 2H, J =
6 Hz), 7.08–7.10 (d, 2H), 7.86–7.88 (d, 2H), 9.91 (s, 1H). MS of DOBAB
(Fig. S4): Calcd: 391.2 (M – Br−). Found: m/z = 391.2.
1H NMR of HOBAB [400 MHz, (CDCl3), Fig. S5]: δ/ppm = 0.87–0.90
(t, 3H, J = 6 Hz), 1.27–1.39 (m, 20H), 1.84 (s, 2H), 3.56(s, 6H),
3.65–3.69 (m, 2H), 4.36–4.38 (t, 2H, J = 8 Hz), 4.67–4.69 (m, 2H, J =
6 Hz), 7.08–7.10 (d, 2H), 7.87–7.89 (d, 2H), 9.92 (s, 1H). MS of HOBAB
(Fig. S6): Calcd: 418.37 (M – Br−). Found: m/z = 418.3.
2. Experimental section
2.1. Materials
1,2-Dibromoethane (>99.0%), 4-hydroxybenzaldehyde (>99.0%), N,
N-dimethylhexadecylamine (>98.0%), N,N-dimethyltetradecylamine
(>90.0%), N,N-dimethyldodecylamine (>97.0%), Nile red (>99.0%),
and all the other organic solvents used in this study were analytical-
grade products from Admas-beta. These reagents were directly used
without further purification. S. aureus (ATCC6538), E. coli (ATCC25922)
and C. albicans (ATCC10231) were obtained from BeNa culture collection
(China). Water was triply distilled by a quartz water purification system
(R = 18 MΩ cm).
2.3. Surface tension
The surface tension (γ) was measured with a Krüss K100 tensi-
ometer by the automatic du Noüy ring model at 25
cover was used to minimize the evaporation of water. To obtain equi-
librium γ, measurements were made every 3 min until the change
was below 0.03 mN·m−1. Prior to measurement, the tensiometer
0.1 °C. A
was calibrated with triple-distilled water at 25
0.1 °C (reference
value 71.97 mN·m−1).
2.2. Synthesis and characterizations of surfactant
2.4. Conductivity
The synthesized route of N-alkyl-N,N-dimethyl-N-(2-(4-formyl-
phenoxy)ethyl) ammonium bromide was exhibited in Scheme 1.
A FE30 conductometer (Mettler Toledo, USA) was used to measure the
electrical conductivities of the surfactant solutions. The temperature was
controlled by a thermostatic water-circulator bath (accuracy 0.05 °C).
Every sample was measured for three times, and the conductivity was
the average value. Before measurement, the instrument was calibrated
by inserting the electrode into a solution of KCl (0.01 mol L−1) and
adjusting the constant to 1.57.
2.2.1. Synthesis of 4-(2-bromoethoxy)benzaldehyde
In a 500 mL round-bottom flask, 1,2-dibromoethane (28.5 mL), ace-
tonitrile (200 mL) and K2CO3 (8.2 g) were mixed and refluxed by
heating. 4-Hydroxybenzaldehyde (4.1 g) was dissolved in acetonitrile
(50 mL) and then the solution was dropwise added. The mixture was
continuously stirred for 24 h under reflux and progress was controlled
by TLC. After cooling to room temperature the mixture was extracted
using water (150 mL) and diethyl ether (100 mL). The organic layers
were washed with brine and solvents were removed under reduced
pressure. The crude product was further purified by column chromatog-
raphy (eluent: ethyl acetate/petroleum ether 1:5 v/v) to yield white
crystals.
2.5. Fluorescence experiment
Fluorescence of Nile red in surfactant solution was measured using a
Varian Cary Eclipse spectrometer (Varian Inc. USA) equipped with a
thermostatic water-circulator bath. The excitation wavelength was set
at 550 nm, and the excitation and emission slit widths were both set
at 5 nm. 10 μL solution of 2 mmol·L−1 Nile red in THF was added to
Scheme 1. Synthesized route of N-alkyl-N,N-dimethyl-N-(2-(4-formyl-phenoxy)ethyl) ammonium bromide.
2

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P. Lu, S. He, Y. Zhou et al.
Journal of Molecular Liquids 325 (2021) 115168
5 mL aqueous solution of surfactant, and then the sample was
ultra-sonicated for 30 min [19,34]. Prior to measurement, all the sam-
ples were kept at 25 0.1 °C no less than 12 h.
2.6. Isothermal titration calorimetry (ITC)
An isothermal titration calorimeter (MicroCal VP-ITC, Malvern, U.K.)
was used to measure the enthalpies of micellization at 25
0.1 °C.
Twenty-eight 10 μ L aliquots of surfactant stock solution were injected
sequentially into a 1400 μ L titration cell initially loaded with Milli-Q
water. Each injection lasted 30 s, with an interval of 120 s between suc-
cessive injections. The solution in the titration cell was stirred at a speed
of 242 rpm throughout experiments. Before experiment, the instrument
was calibrated by injecting Milli-Q water [35]. The enthalpy change of
the micelle formation was obtained from the calorimetric curves.
2.7. Antimicrobial activity
The antimicrobial performance of the formyl-containing surfactants
was evaluated using the Minimum Inhibitory Concentration (MIC)
assay. The MIC is defined as the minimal concentration for antimicrobial
agent that inhibited the visible growth after 24 h of incubation. S. aureus
(ATCC6538), E. coli (ATCC25922) and C. albicans (ATCC10231) were cho-
sen as the representatives of typical Gram-positive and Gram-negative
bacteria and fungi, respectively. The MIC was determined using 96-
well microtiter plates by a 2-fold serial dilution method.
Fig. 1. Plots of surface tension as a function of the logarithm of surfactant concentration at
25 °C.
The samples were added to a 96-well microtiter plate and were di-
luted with sterile water to a series of concentrations. Each well contains
100 μL sample solution and 100 μL prepared bacterial suspension
(106 CFU/mL). In addition, 100 μL sterile water plus 100 μL liquid me-
dium was used as a negative control and 100 μL sterile water plus
100 μL bacterial suspension was used as a positive control. After that,
the 96-well plates were placed in an incubator at 37 °C or 28 °C for
24 h and measured the OD (optical density) values at 600 nm. The
rise in turbidity as well as OD values reflected the increase in both
mass and bacteria number. The minimum concentration corresponding
to the well that does not produce turbidity as well as the turning point
of OD values is the MIC of the sample against the bacteria. All items
used in the experiments were sterilized by high temperature and high
pressure, and all the steps were proceeded in a sterile environment.
in the form of logcmc = A − Bn, where A and B are both positive constants
relating to the molecular structure of the surfactant. A represents the ef-
fect of hydrophilic head on micellization, while B signifies the mean con-
tribution to micellization per methene group. Generally, the larger the A
value is, the weaker the micellization ability of the surfactant. For the cur-
rent formyl-containing surfactants, A = 1.64, which is lower than those of
alkyltrimethylammonium bromides (2.0) and alkylpyridinium bromides
(1.7) [1], reflecting its stronger micellization ability. In fact, the lower
cmc of the current formyl-containing surfactant compared to their corre-
sponding alkyltrimethylammonium bromides and alkylpyridinium bro-
mides [1], also confirm its strong micellization ability. This means that
the insertion of the 2-(4-formyl-phenoxy)ethyl group enhances the hy-
drophobic interactions between the molecules, i.e., favoring micellization.
Thus, if 2-(4-formyl-phenoxy)ethyl group is extended by forming an
imine bond, the dialkyl-chain surfactant will be formed and the micelliza-
tion ability will be further enhanced and become controllable. These will
be discussed in another work.
3. Results and discussion
3.1. Surface tension
Upon comparison, it can be found that the cmc of DOBAB was be-
tween those of N-dodecyl-N-octyl-N,N-dimethylammonium bromide
(1.50 mmol·L−1) and N-dodecyl-N-hexyl-N,N-dimethylammonium
bromide (3.29 mmol·L−1) [36], implying that the 2-(4-formyl-
phenoxy)ethyl group is approximately equivalent to 7 methylene
groups. Therefore, the contribution of the 2-(4-formyl-phenoxy)ethyl
group to the hydrophobicity was relatively large. In other words, the
current formyl-containing surfactant can be regarded as an asymmetri-
cal double-chain surfactant.
Due to their unique molecular structures, surfactants are generally
adsorbed onto the air/liquid interface in an oriented fashion, and thereby
decrease the interfacial tension (air solution). As shown in Fig. 1, the equi-
librium surface tension (γ) of the formyl-containing cationic surfactants
first underwent a rapid decrease with increasing the logarithm of the sur-
factant concentration and then tended toward a constant beyond a critical
value, corresponding to the cmc. There is no minimum in either of the sur-
face tension curves, ruling out the presence of any surface-active impuri-
ties. Although the chemical compositions of hydrophobic tails are almost
identical, the minimum γ above the cmc (γcmc) showed a slight decrease
with shortening the hydrophobic tail. This may be related to the maxi-
mum surface excess amount of the adsorbed surfactant (Γmax). A larger
Γmax generally leads to a more compact air/liquid interfacial film
possessing stronger effectiveness in reducing the surface tension [1].
Many properties of the surfactant solution undergo a dramatic change
above the cmc. Thus, the cmc plays an important role in the practical ap-
plications. Similar to most homologous straight-chain ionic surfactants,
the cmc of the formyl-containing surfactants decreased by about one
order of magnitude upon increasing the hydrophobic tail by two –CH2–
groups. Upon plotting the logarithm of the cmc as a function of the num-
ber of carbon atoms (n) in the hydrophobic chain, a relation can be found
For the adsorption of surfactant at the air/water interface, the Γmax
and minimum molecular occupation area (Amin) can be calculated by
applying Gibbs adsorption eqs. (1) and (2) [1], respectively,
ꢀ
ꢁ
1
∂γ
∂logC
Γmax ¼ −
ð1Þ
2:303nRT
1
Amin
¼
ð2Þ
NΓmax
where R is the universal gas constant (8.314 J⋅mol−1⋅K−1), T is the abso-
lute temperature (K), C is the surfactant concentration (mol⋅L−1), and N
is the Avogadro constant. The value of n, which stands for the effective
3

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Journal of Molecular Liquids 325 (2021) 115168
Table 1
Surface activity parameters for synthesized formyl-containing cationic surfactants at
25 °C.
Surfactant
cmc (mN·m−1
DOBAB
TOBAB
HOBAB
40.48
γ
)
37.04
2.11
2.25
1.89
0.077 38.60
0.041a 0.590
0.097
0.042
0.00082a
0.0022b
0.017
0.013
0.018
cmc (mmol·L−1
)
0.023a 0.0802
0.076b 0.0873
0.10b
0.097
0.045
0.014
0.646
1.51
1.08
4.05
10−6 Γmax (mol·m−2
)
0.085
0.038
0.023
0.082
1.45
1.15
4.76
31.49
Amin (nm2 per molecule) 0.88
pC20
3.32
34.93
Π
cmc (mN·m−1
)
0.041 33.44
0.042
a
Obtained from surface tension method.
Obtained from fluorescence method using Nile red as probe.
b
number of species present at the interface, was taken to be 2 for a 1–1
type ionic surfactant [1]. The data listed in Table 1 showed that the Γmax
decreased from (1.89
0.097) × 10−6 to (1.45
0.017) × 10−-
6 mol·m−2 with a hydrophobic chain from C16 to C12, accompanied
by an increase in the Amin from 0.88 0.045 to 1.15 0.013 nm2 per
molecule. This indicates that the interfacial film formed by DOBAB was
the tightest, and that formed by HOBAB was the loosest. Therefore, the
γcmc of DOBAB was the lowest, and the γcmc of HOBAB was the highest.
Due to the insertion of the 2-(4-formyl-phenoxy)ethyl group, the cur-
rent formyl-containing surfactants have larger Amin than conventional
cationic surfactants, such as N-tetradecyl-N,N,N-trimethylammo-
nium bromide (TTAB, 0.61 nm2 per molecule at 30 °C) [1], N-cetyl-
N-methylpyrrolidinium bromide (C16MPB, 0.45 nm2 per molecule at
25 °C) [37], selenium-hybride C16MPB (0.43–0.59 nm2 per molecule
at 25 °C) [21,23]. Notwithstanding, their Amin values are very close to
those of N-alkyl-N,N,N-tripropylammonium bromide [1], implying a
positive contribution from the 2-(4-formyl-phenoxy)ethyl group to
the hydrophilic headgroup.
Fig. 2. Variations in the intensity of the Nile red fluorescence with surfactant concentration
at 25 °C.
which is usually interpreted to be the solubilization of Nile red in a hy-
drophobic microenvironment. That is to say, micelles are formed. There-
fore, the critical concentration corresponds to the cmc. As listed in
Table 1, the cmc obtained from the variation in the intensity of the
Nile red fluorescence were 0.0873
2.25
0.0022, 0.646
0.076 and
0.10 mmol·L−1 for HOBAB, TOBAB and DOBAB, respectively.
The effectiveness in the γ reduction (Πcmc) can be obtained using
eq. (3):
These values are close to those obtained using the surface tension
method.
Πcmc ¼ γ0−γcmc
ð3Þ
where γ0 is the γ of pure water. As listed in Table 1, the Πcmc of DOBAB,
3.3. Conductivity
TOBAB, and HOBAB were relatively similar as a result of their similar
γ
cmc. Nevertheless, small differences still exist. Due to the same solvent,
The conductivity (κ) of the formyl-containing surfactant solutions
was measured at 25, 35 and 45 °C, respectively. From the results exhib-
ited in Fig. 3, it can be seen that κ showed a linear increase at both low
and high concentrations, but the change in κ was not monotonous. At
low concentrations, the change in κ was more appreciable than that at
high concentrations because of the transition of the conducting species
[1,21,38]. Namely, the κ-C curve can be divided into two linear segments
by one breakpoint, corresponding to the cmc. The cmc of DOBAB, TOBAB
and HOBAB at 25 °C obtained via conductivity measurements were
2.45 0.070, 0.62 0.021 and 0.089 0.006 mmol·L−1, respectively
(Table 2), which are very close to those obtained by the surface tension
measurements or fluorescence method (Table 1).
Generally, heating accelerates the migration of conducting ions in a
solution, and thus leads to higher conductivity. Thus, with increasing
the temperature, the slopes of the two linear segments both increased
to some extent, and the breakpoints shifted to higher concentration,
reflecting a larger cmc. As listed in Table 2, the cmc values increased
the Πcmc shows an opposite change with that of the γcmc. A larger Πcmc
denotes a stronger ability to decrease the γ. Therefore, DOBAB is more
effective in reducing the γ than HOBAB. This is agreement with the con-
clusion deduced from the γcmc and Γmax
.
In addition, the adsorption efficiency of a surfactant is usually evalu-
ated using the negative logarithm of the surfactant concentration in the
bulk phase required to produce a 20 mN·m−1 reduction in the γ of the
solvent (pC20) [1]. The larger the pC20, the higher the adsorption effi-
ciency. As listed in Table 1, the pC20 of the formyl-containing surfactants
linearly increased with the number of carbon atoms in the hydrophobic
tail, which was consistent with the results obtained for conventional
surfactants [1]. Obviously, the longer the hydrophobic chain, the larger
the driving force for migration to the interface and the higher the ad-
sorption efficiency.
3.2. Nile red fluorescence
from 2.45
0.070, 0.62
0.021 and 0.089
0.006 mmol·L−1 to
Nile red is an oil-soluble fluorescent probe [19], which exhibits little
fluorescence in water. However, when micelles are formed, Nile red can
be solubilized in the hydrophobic interior and then fluoresce. Based on
this property, Nile red is usually used as probe to determine the cmc.
Fig. 2 showed the variation in the intensity of the Nile red fluorescence
signal at 625 nm with the surfactant concentration. On the whole, below
a critical concentration, the fluorescence intensity remains unchanged
(~0), suggesting that no Nile red is solubilized. However, the fluores-
cence intensity starts to quickly increase above a critical concentration,
3.11 0.052, 0.72 0.023 and0.16 0.0042 mmol·L−1, for DOBAB,
TOBAB and HOBAB, respectively. This tendency is often observed in con-
ventional ionic surfactants, such as C16MPB [37], selenium-hybride
C16MPB [21]. Note that upon increasing the temperature from 25 to
45 °C, the cmc of DOBAB has an appreciable increase of 0.66 mmol·L−1
,
but that of HOBAB only increased by 0.071 mmol·L−1, approximately
1/10 of the amplification of DOBAB. In other words, the influence of tem-
perature on the cmc is more prominent for short-chain surfactants than
that of long-chain surfactants [37,39].
4