Synthesis and surface properties of a pH-regulated and pH-reversible anionic Gemini surfactant

Article abstract of DOI:10.1021/la5016669

A new series of N,N′-dialkyl-N,N′-diacetate ethylenediamine, differing by the length of the carbon tails (8, 10, and 12), was synthesized in two steps. Their surface properties and aggregation behavior were studied in aqueous solution using pH titration,

Full text of DOI:10.1021/la5016669

Article
pubs.acs.org/Langmuir
Synthesis and Surface Properties of a pH-Regulated and pH-
Reversible Anionic Gemini Surfactant
Jing Lv,† Weihong Qiao,*,† and Chongqiao Xiong‡
†State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian, 116024, P. R.
China
‡College of Letters & Science, University of California, Berkeley, California 94720, United States
S
* Supporting Information
ABSTRACT: A new series of N,N′-dialkyl-N,N′-diacetate
ethylenediamine, differing by the length of the carbon tails
(8, 10, and 12), was synthesized in two steps. Their surface
properties and aggregation behavior were studied in aqueous
solution using pH titration, surface tension, zeta potential,
dynamic light scattering (DLS), transmission electron
microscopy (TEM), and fluorescence measurements. On the
basis of the pKa values obtained, surface tension was measured,
as well as key surface property parameters. Combined with the
zeta potential and DLS results, the experiments produced
vesicles and reflected their pH-controllability through
subsequent TEM and fluorescence measurements. pH-switch-
ability was found to be reversible by light transmittance.
Emulsion stability of dodecane-in-water in different pH showed that emulsion type was reversed between “on” for the O/W
emulsion type and “off” for the W/O.
concentration (CMC), high surface activity, low Krafft
temperature, unusual rheological properties, multifarious
aggregate structures, and better wettability.3,4 Owing to these
unique properties, gemini surfactants have attracted great
attention in both academics and industry.
1. INTRODUCTION
Surfactants are widely used in many areas of the commercial
and industrial world, which makes it seemingly unattractive for
researchers to further investigate their structures.1 The
discovery of gemini (dimeric) surfactants reverses this point
of view. Gemini surfactants are made up of two surfactant-like
moieties connected at the level of the head groups or on the
alkyl chains in close vicinity to the head groups by a spacer
group of varying nature and length (see Scheme 1).2 Due to
their special structure, gemini surfactants are superior to the
corresponding conventional monomeric surfactants in certain
characteristics, such as remarkably low critical micellar
Similar to conventional surfactants, gemini surfactants are
also classified into anionic,5 cationic,6 nonionic,7 and
zwitterionic gemini surfactants,8 among which anionic gemini
surfactants have an important place. The negative polar groups
can be carboxylate,9 sulfonate,10 sulfate,11 or phosphate;12 and
the spacer can be polar13 or nonpolar, rigid or flexible.14 Many
types of anionic gemini surfactants have been synthesized and
the investigation of their properties of cmc, surface tension,
foaming, and wettability has been reported.15,16 However, data
are unable to show a sufficient relationship between the
structures and properties of anionic gemini surfactants.
Moreover, investigating properties such as aggregation number
and size of anionic gemini surfactant micelles in solution is also
critical.17,18
Scheme 1. Schematic Representation of a Gemini Surfactant
with the Spacer Group Connecting (A) the Two Head
Groups (Often the Case for Cationic Surfactants) and (B) he
Alkyl Chains at a Location Very Close to the Head Groups
(Case of Nonionic and Anionic Gemini Surfactants)
Surfactants are commonly known for forming micelles in
solution. All the properties of the surfactant are built on the
micelle. Micelles are in dynamic equilibrium, constantly
disintegrating and reforming. As for the gemini surfactant, the
spacer as an important factor affects the micelle behavior and
Received: October 9, 2013
Revised: June 20, 2014
Published: June 27, 2014
© 2014 American Chemical Society
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thus cannot be ignored. The kinetics of exchange, size, and
shape in micellar systems of cationic gemini surfactant of the
alkanediyl-α,ω-bis(dodecyldimethylammonium bromide) type
were studied,19 with the alkanediyl being 1,2-ethylene, 1,3-
propylene, and 1,4-butylene. Evidently, the results indicate that
to obtain the truly unique properties of C12 cationic gemini
surfactants, an ethylene spacer should be used. Vesicles are
observed above the cmc for gemini surfactants. Wang et al.20
extensively studied the aggregation behaviors of a series of
microscope, light transmittance, and fluorescence measure-
ments, are conducted at different pH intervals.
2. EXPERIMENTAL SECTION
2.1. Materials. For the synthesis of Ace(n)-2-Ace(n), ethylenedi-
amine (Kermel Laboratory Equipment Co., Ltd.), 1-bromooctane, 1-
bromodecane, 1-bromododecane (J&K Chemical, Ltd.), and bromoa-
cetate acid (Sinopharm Chemical Reagent Co, Ltd) were used. Other
reagents were methanol, 1-propanol, acetone, diethyl ether, and
sodium hydroxide. The alkane used in the emulsion measurements was
n-dodecane (Kermel Laboratory Equipment Co., Ltd.). The chemicals
listed above were of analytical grade and used without further
purification.
‑
anionic sulfonate gemini surfactants [(CnH2n+1)(C3H6SO3 )-
NCsN(C3H6SO3 )(CnH2n+1)]2Na+ and their corresponding
‑
monomeric surfactants. The gemini surfactants
C12CpxC12(SO3)2 and C12C3C12(SO3)2 exhibit a stronger
aggregation ability in comparison with the monomeric
surfactant C12NSO3. Zhu et al.21 synthesized four sulfonate-
containing gemini surfactants Cy-Mx-Cy. All the spacers of these
gemini surfactants contained two rigid benzene rings and two
rigid carbonyl groups. The aggregation behavior of C7-Mx-C7 in
water was observed using DLS and typical TEM. The analyses
proved that Cy-Mx-Cy with a long, fully rigid spacer preferred to
form vesicles.
2.2. Synthesis. The final surfactants, Ace(n)-2-Ace(n) (n = 8, 10,
and 12), were synthesized via the following two steps (see also Scheme
2). The products obtained in each step were characterized with mass
spectrometry (High Performance Liquid Chromatography/Mass
selective Detector, HP America, model HP1100LC/MSD), Accu-
1
rate-Mass TOF LC/MS (Aglient 6224, Aglient Technology Inc.), H
NMR and 13C NMR (Bruker, France, Model AVANCEII, 400 MHz),
and FT-IR (Nicolet 6700 Flex FT-IR Spectrometer, Thermo Fisher
Scientific). The synthesis details and structural characterization are
described in the Supporting Information.
A number of studies9,22−24 have focused on the synthesis of
gemini surfactants with ethylenediamine as the spacer and
carboxylate as the anionic group in the same molecule. A new
type of gemini carboxylate surfactant based on EDTA was
2.3. Measurements. The aqueous surfactant solutions were
prepared via direct dissolution of the solid samples at the desired
concentration. The surfactant solutions used in all the measurements
(except emulsification) were all in a homogeneous phase. All the
measurements were performed at 25 °C. Except for the acidic titration,
surface tension, and emulsion, the sample concentration for all the
other measurements was fixed at 10−3 mol/L.
2.3.1. Acidic Titration. Acidic titration of the anionic gemini
surfactants was performed to determine the different species present in
solution as a function of the varying pH. The surfactants were
dissolved in excess of aqueous NaOH, the concentration was fixed at
0.01 M and the volume was 20 mL. The aqueous HCl concentration
was 0.1 M. During the acidic titration, the changes of pH and
conductivity were followed with a pH meter model (PHS-3C,
Shanghai Hong Yi Instrumentation Co., Ltd) and with a conductivity
meter (DDSJ-308A conductivity meter, Shanghai Hong Yi Instrument
Co., Ltd.), respectively. The values of pKa were fundamental for all the
measurements. The following measurements were all conducted at
different pH intervals in accordance with the pKa.
synthesized by Laurent Wattebled and Andre
́
Laschewsky.25
The existence of N−CH2COO‑ groups implied that this kind of
gemini surfactant existed in different forms in solution as a
function of the pH, which were expressed as pKa1 and pKa2.
This indicated that the properties of this structure could be
influenced by pH. In consideration of the nitrogen atom and
carboxylate both able to combine with H+, it can be predicted
that the properties of such surfactants are controlled by pH.
However, the surface adsorption and micellization character-
istics in this field have seldom been reported.
Herein we focus on the synthesis and adsorption/
micellization behavior of anionic gemini surfactants synthesized
from ethylenediamine, Ace(n)-2-Ace(n) (N,N′-dialkyl-N,N′-
diacetate ethylenediamine, where n is the hydrocarbon chain
length with size 8, 10, and 12). The synthesis route and
structure are shown in Scheme 2. This paper is focused on
exploring the physicochemical properties of Ace(n)-2-Ace(n) at
the air/water solution interface and in aqueous solution. The
pKa values are determined by acidic titration and all the
property measurements, including surface tension, zeta
potential, dynamic light scattering, transmission electron
2.3.2. Surface Tension. The surface tension was measured with
Kruss K100C tensiometer. The cmc and the surface tension at the cmc
̈
were determined from the break point of the surface tension and the
logarithm of the concentration curve. Repeated measurements were
performed until the difference between two values was less than 0.2
mN/m.
2.3.3. Zeta Potential. The zeta potential was measured using
Nanoparticle Zeta Potential Measurement (ZETASIZER nano series
Nano ZS-90, Malvern, UK). The zeta potential is determined by the
surface charge of aggregates formed by surfactant molecules in
solution.26 The pH values of all the samples corresponded to the
samples in the test of surface tension. Each Ace(n)-2-Ace(n) at each
pH was measured three times at 298 K.
a
Scheme 2. Synthetic Route of Ace(n)-2-Ace(n)
2.3.4. Dynamic Light Scattering. All static light scattering
measurements were performed on a Malvern autosizer (ZETASIZER
nano series Nano ZS-90, Malvern, UK) at 90° scattering angle. All the
samples were dispersed with ultrasound for 1 h. The obtained
scattering data were fitted using an intensity-weighted cumulative
analysis to estimate the diffusion coefficient of the aggregates in
aqueous solution.27 The average hydrodynamic diameter Dho and the
polydispersion index (PDI) of the solution were obtained.
2.3.5. Transmission Electron Microscopy. TEM micrographs were
obtained with JEM-2000EX. Negative staining samples were prepared
with phosphotungstic acid acetate solution (2%) as the staining agent.
One drop of the solution was placed onto a carbon-coated copper
TEM grid (300 mesh).28 Filter paper was employed to suck away the
a
n = 8, 10, and 12.
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Figure 1. Acidic titration curves of Ace(n)-2-Ace(n): (a) Ace(8)-2-Ace(8); (b) Ace(10)-2-Ace(10); (c) Ace(12)-2-Ace(12).
Scheme 3. Transitions Occurring during the Titration of Gemini Surfactants with HCl
excess liquid. Then, one drop of the staining agent was placed onto the
copper grid. Excess liquid was also sucked away by filter paper.
2.3.6. Fluorescence Measurements. Steady-state fluorescence
spectra were recorded on HITACHI F-7000 Fluorescence Spec-
trophotometer. Pyrene was chosen as the fluorescent agent. The
pyrene−surfactant binary solution was dispersed with ultrasound and
kept overnight. The emission spectra wavelength ranged from 350 to
550 nm, and the excitation wavelength was focused at 335 nm.29
Intensities of peaks labeled as I1 and I3 were taken from the emission
intensities at 374 and 383 nm, respectively. Excitation and emission
slits were fixed at 2.5 and 5 nm, respectively. The pyrene concentration
was kept at 3 × 10−6 mol/L in the measurements to avoid excimer
formation.
2.3.7. Light Transmittance. Light transmittance change with pH
was measured on a Mettler ToledoT90 instrument. During the
measurement, the pH and transmittance data were collected
simultaneously. The original sample was dissolved in excess of
NaOH and the pH was fixed at nearly 10. In one cycle test, the pH was
adjusted from alkalinity to acidity (0.1 mol/L HCl was used) and then
from acidity back to alkalinity (0.1 mol/L NaOH was used). This
process was repeated at least three times in order to explore whether it
could be reversible.
Each acid titration curve exhibits two turning points
corresponding to two abrupt transition points of conductivity
change.
With the addition of HCl in acidic titration, the
concentrations of H+ and Cl− were increased resulting in
conductivity increase up around the whole concentration. The
appearance of these two breaking points demonstrated that
there was a protonation between H+ and the tertiary amine
group in the surfactant molecule. The first turning point at pH
around 8.00 corresponds to the addition of one proton to the
anionic dimer. The second turning point at pH near 6.50 is
attributed to the protonation of the second amine of the spacer
group. The different equilibria involved in titration are
schematized in Scheme 3. The change of the molecular
structure of Ace(n)-2-Ace(n) with pH has been justified with
FT-IR (Figure S4 in the Supporting Information, take Ace(12)-
2-Ace(12) for example). From titration data, the protonation
constants pKa1 and pKa2 were summarized in Table 1.
Table 1. Protonation Constants pKa1 and pKa2 of Ace(n)-2-
Ace(n)
2.3.8. Emulsion Stability Measurements. Classical bottle tests were
run to characterize the oil in water emulsion stability.30 Ten milliliter
surfactant solution and 10 mL alkane (dodecane) were added to a 100
mL beaker. Dispersion machine SFJ-400 was used to complete the
emulsion process and the condition was 800 rpm for 10 min. Emulsion
stability was determined by observing the emulsion that was held for
24 h. The whole process was completed in a water bath at 25 °C.
Gemini surfactants
pKa1
pKa2
Ace(8)-2-Ace(8)
Ace(10)-2-Ace(10)
Ace(12)-2-Ace(12)
7.59
7.83
8.41
7.02
6.59
6.55
3. RESULTS AND DISCUSSION
As shown from the data in Table 1, three gemini surfactants
have different pKa1 and pKa2 values. However, an obvious law
can be inferred: as the hydrocarbon chain length increases, the
value of pKa1 becomes higher and the value of pKa2 becomes
lower, which means the range between pKa1 and pKa2 broadens.
The impact this law would bring to the performance of Ace(n)-
2-Ace(n) will be discussed in a later section.
3.1. Values of pKa. The surfactant was dissolved in excess
of a known NaOH solution, pH of 11−12. During the acidic
titration, pH and conductivity data were collected at the same
time. Titration curves obtained by addition of HCl (0.1 M) to
surfactant solution as well as conductivity change are shown in
Figure 1a,b,c. The dotted lines indicate a visual gradual change
of the solution: transparent, turbid, and suspensions (corre-
sponding to the section A, B, and C, respectively, in the figure).
3.2. Adsorption to the Air/Water Solution Surface.
Figure 2a,b,c depicts the surface tension versus log concen-
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Figure 2. Six curves on surface tension versus log concentration and three curves on I1/I3 versus surfactant concentration: (a) and (d) Ace(8)-2-
Ace(8); (b) and (e) Ace(10)-2-Ace(10); (c) and (f) Ace(12)-2-Ace(12); comparison between surface tension of aqueous Ace(n)-2-Ace(n) solution
at different pH conditions: (g) pHa, (h) pHb, (i) pHc.
tration plots for Ace(n)-2-Ace(n) (n = 8, 10, and 12) at three
different pH intervals in accordance with the pKa: between pKa1
and pKa2, approximate pKa1 (pH is slightly higher than pKa1),
and far above pKa1 (for the sake of simplicity, use pHa, pHb,
pHc to represent these three pH intervals, respectively, similarly
hereinafter). Table 2 lists the surface properties of Ace(n)-2-
Ace(n) and some other common surfactants at 298 K. By
contrast, the cmc of Ace(n)-2-Ace(n) is lower by more than
one order of magnitude. Similarly, a significantly lower γcmc
value is obtained as compared with that for the conventional
surfactants. This can be ascribed to the characteristics in their
molecule structures: two hydrophilic/hydrophobic groups are
connected together by the chemical band in gemini surfactant,
which overcomes the repulsive force between ions and the
interaction force between alkyl chains to some extent. In
addition, this effect brings adjacent hydrophilic groups closer,
and simultaneously results in the distance between hydrophobic
groups becoming smaller than in traditional surfactants.
Therefore, there would be more alkyl hydrophobic chains
arranged closely at the air/water interface of the gemini
surfactant solution. Moreover, with the help of spacer, the alkyl
chains tend to be presented in erected direction, leaving more
methyl exposed to air. Because the surface energy of methyl is
less than that of methylene, this kind of arrangement is
supposed to confer higher surface activity to gemini surfactants.
That is why Ace(n)-2-Ace(n) possesses a greater ability to
reduce the surface tension of water. Besides, the adsorption and
micellization of surfactant can also be expressed by the pC20
parameter. pC20 is the value of the logarithm of reciprocal C20,
which is concentration of surfactants leading to a decrease in
the surface tension of water by 20 mN/m. The larger the pC20
value is, the higher the surface activity of surfactant is. The data
showed that pC20 of Ace(n)-2-Ace(n) is always larger than that
of traditional surfactants. As explained above, the compactness
of both the adsorption on the interface and the micelles in the
solution formed by gemini surfactant is greater due to the two
alkyl chains. So, the double-chained Ace(n)-2-Ace(n) has
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g
Table 2. Surface Properties of Ace(n)-2-Ace(n) and Some Other Common Surfactants at 298 K
cmc/mol/L
γcmc
Γcmc
Ao
ΔGm°ic
ΔGa°ds
ST
FL
mN/m
μmol/m2
R2
Å2
pC20
kJ/mol
kJ/mol
a
Ace(8)-2-Ace(8)
4.10 × 10−4
3.53 × 10−4
3.54 × 10−4
2.77 × 10−4
2.63 × 10−4
1.31 × 10−4
2.06 × 10−4
4.93 × 10−5
1.68 × 10−5
1.78 × 10−3
2.0 × 10−2
8.2 × 10−3
1.80 × 10−2
1.80 × 10−3
5.00 × 10−4
1.04 × 10−4
1.00 × 10−4
3.26 × 10−4
5.00 × 10−5
8.12 × 10−5
5.54 × 10−4
5.30 × 10−5
2.28 × 10−5
30.0
29.6
29.6
29.1
28.2
27.9
36.0
31.5
28.8
35.1
37.5
32.5
39.7
36.5
3.42
3.13
5.70
3.64
3.39
8.82
2.60
2.57
7.05
3.26
2.3
0.613
0.994
0.975
0.937
0.941
0.962
0.954
0.997
0.468
48.6
53.1
29.2
45.6
49.0
18.8
63.9
64.6
23.6
51.0
69.0
53.0
40.0
46.5
4.29
4.77
4.17
4.52
4.71
4.10
4.66
5.92
5.24
3.72
2.43
2.5
−10.5
−10.7
−10.7
−11.0
−11.1
−11.9
−11.4
−13.1
−14.5
−22.8
−24.4
−18.1
−22.8
−24.0
−16.9
−25.2
−28.9
−20.6
a
a
Ace(10)-2-Ace(10)
Ace(12)-2-Ace(12)
b
Ace(8)-2-Ace(8)
b
b
Ace(10)-2-Ace(10)
Ace(12)-2-Ace(12)
c
Ace(8)-2-Ace(8)
c
c
Ace(10)-2-Ace(10)
Ace(12)-2-Ace(12)
d
SDBS
e
C11H23COONa
e
SDS
3.16
4.51
3.57
f
f
C10H21N(CH3)2CH2COO
C12H25N(CH3)2CH2COO
2.59
3.56
a
b
c
d
e
f
g
pHa. pHb. pHc. Ref 21. Ref 24; C11H23COONa was measured at pH = 10. Ref 29. cmc: critical micelles concentration in mol/L. γcmc: surface
tension. Γcmc surface excess concentration. “ST” refers to surface tension measurement. Ao: area/surfactant molecule at the interface. pC20: logarithm
of reciprocal C20 (concentration of surfactants to lead to a decrease of in surface tension of water by 20 mN/m). ΔG°mic: standard free energy of
micellization. ΔGa°ds: standard free energy of adsorption. “FL” refers to fluorescence measurement. “R2” means the correlation coefficient of the
surface tension versus log concentration line.
higher effectiveness than single-chained surfactants on the
adsorption and micellization.
According to the surface tension curves, surface excess
concentration at the air/solution interface Γ can be calculated
from the Gibbs equation
To confirm the cmc of Ace(n)-2-Ace(n), the micellization of
Ace(n)-2-Ace(n) at 298 K was investigated by steady-state
fluorescence spectra. The fluorescence intensity ratio of the first
to the third band I1/I3 for pyrene can be taken as a measure of
the polarity of the microenvironment. When the micelles are
formed, pyrene solubilized in water will be preferentially
incorporated into the inner hydrophobic region of micelles,
resulting in an abrupt change of the I1/I3 ratio. Figure 2d,e,f
displays the I1/I3 ratio variation with the concentration of
Ace(n)-2-Ace(n). The cmc values calculated by the inflection
point of I1/I3 versus c curves are in agreement with those
determined by surface tension measurement (Table 2).
Combining Figure 2 and data about cmc and pC20 in Table 2,
we reached three conclusions. First, for each Ace(n)-2-Ace(n),
the cmc value increases when the pH decreases; the cmc of
Ace(12)-2-Ace(12) shows obviously bigger fluctuation than
that of Ace(10)-2-Ace(10) (Figure 2b,c) and Ace(8)-2-
Ace(8)’s cmc doesn’t distinctly change (Figure 2a), reflected
as its order of magnitude stays at 10−4 throughout the whole
pH range. This phenomenon can be explained as the decrease
of pH (or the adding of H+) worsens the Ace(n)-2-Ace(n)’s
solubility in water and results in the increase in cmc values. So,
it can be perceived that Ace(8)-2-Ace(8) owns the strongest
ability to adapt the change of pH, with Ace(10)-2-Ace(10)
being second, and finally Ace(12)-2-Ace(12). This prediction
can also be confirmed in Figure S6 (in the Supporting
Information). Second, from the perspective of comparing
results among Ace(n)-2-Ace(n)s: for each pH interval, it shows
the expected increase in surface activity--lower cmc and lower
γcmc values--with the increase in alkyl chain length. Third, pC20
value of Ace(n)-2-Ace(n) decreases as pH decreases, which
shows a lower surface activity trend with decreasing pH. Similar
to the cmc values, the change of pC20 values becomes greater
with alkyl chain lengthening. This trend can be thought of as
the screening effect caused by the excess of NaCl. So, both salt
and pH effects are more obvious for longer chains.
⎛
⎜
⎞
⎟
⎠
1
d γ
Γ
= −
T, P
cmc
2.303nRT d log C
⎝
(1)
where R is the perfect gas constant, T is the absolute
temperature, and C is concentration of surfactant solution.
The solution was prepared with excess of NaOH, so n was
taken as 1.24 The surface area per surfactant molecule Ao can be
calculated using the following equation
1
A0 =
N Γ
(2)
A
where NA is Avogadro’s number. A greater value of Γ or a
smaller value of Ao means a denser arrangement of surfactant
molecules at the air/water interface.
The standard free energy of micellization (ΔG°mic) for ionic
gemini surfactants can calculated by the equation31
⎛
⎜
⎞
⎛
⎜
⎞
⎟
1
2
RT
⎟
°
ΔGmic = RT
+ β lnXcmc
−
ln 2
⎝
⎠
⎝
⎠
(3)
2
where β is the apparent degree of counterion binding to the
micelle/solution interface calculated from β = 1 − α. Here α is
taken as 1 for the present gemini surfactants because the
surfactants dissolve into surfactant ions and counterions in
sodium hydroxide solution. The standard free energy of
adsorption (ΔG°ads) at the air/water interface is calculated by
the equation32
°
°
ΔGads = ΔGmic − (Πcmc/Γ)
(4)
where Πcmc = γ0 − γcmc, with γ0 = surface tension of water. The
values of ΔGm°ic and ΔGa°ds are listed in Table 2.
Recently, the calculation of area per molecule of surfactant
through Gibbs adsorption equation has been questioned.
Menger et al.33−35 and Laven et al.36 have argued about
whether the Gibbs equation for analyzing surface tension data
to obtain the surface excess is appropriate. In response to this
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Figure 3. TEM micrographs of Ace(n)-2-Ace(n) at 10−3 mol/L. Bars in the photographs represent 100 nm: (a) Ace(8)-2-Ace(8)a, (b) Ace(10)-2-
Ace(10)a, (c)Ace(12)-2-Ace(12)a, (d) Ace(8)-2-Ace(8)c, (e) Ace(10)-2-Ace(10)c, and (f) Ace(12)-2-Ace(12)c.
issue, two newest articles of Thomas37,38 have provided the
ensuing discussion about the application of the Gibbs equation
to the air/water interface. It was pointed out in ref 38 that (i)
the Gibbs equation cannot in principle be applied to an
aqueous solution of ionic surfactant in the absence of
electrolyte, (ii) impurities (mainly referring to divalent ion,
i.e., Ca2+, Mg2+) present special difficulties for surface tension. If
adsorption reached a plateau before the cmc, the analysis is
straightforward and can be reasonably accurate. According to
Thomas’s research results, both measurements at higher
concentrations and measurements using added electrolyte
indicate that the adsorption of anionic surfactants has generally
not reached a packing limit at the cmc. Several scholars39−42
having analyzed the effect of a range of concentration of added
NaCl on the surface tension of sodium dodecyl sulfate also
supported Thomas’s point. In this paper, all the aqueous
systems in measurements contained excess NaCl which was
brought in during sample preparation. So, the error caused by
salts or impurities was uniform for three compounds.
Furthermore, the Γcmc calculated from the constant slope of
the surface tension vs the logarithm of the concentration curve
was just used for qualitative analysis, not for further accurate
calculation. The prediction obtained from the changing trend of
Γcmc needs support from other data sources.
The values of Γcmc and γcmc under different pH conditions are
also presented in Table 2. It is worth mentioning that the
change of γcmc shows a related trend with Γcmc: the bigger Γcmc
is, the lower γcmc is. It is reasonable that the bigger Γcmc means
lower Ao and more tightly packed surfactant molecules arranged
at the interface. It is the air/water interface becoming more
nonpolar at higher Γcmc values that results in lower γcmc. More
interestingly, Γcmc is the highest at pHb and the lowest at pHc
(except for n = 12). As for Ace(12)-2-Ace(12), its Γcmc still
shows the highest value at pHb, but lowest at pHa. This
tendency is caused by the decreasing solubility at lower pH
condition. After all, the pH effect is more obvious for longer
chains. Even so, the Γcmc values of Ace(12)-2-Ace(12) are still
distinctly bigger than those of the other two Ace(n)-2-Ace(n)s,
which once again demonstrates that longer chains have better
adsorption at the air/water interface. Superficially, the reason
for this result is the change of pH conditions, but in fact, it is
caused by the change of intramolecular and intermolecular
interaction due to the increased positive charge from the
nitrogen atoms, and the intermolecular interaction can also be
affected by alkyl chain. This is exactly what is meant by
“structure determines performance”. Although the solubility of
Ace(n)-2-Ace(n) decreases with decreasing pH, which leads to
higher cmc and lower pC20 values, the Γcmc values do not show
consistent decrease with surface activity, but increase at the two
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Figure 4. Transmittance of Ace(n)-2-Ace(n) solution as a function of time of treatment with HCl followed by NaOH solution: (a) Ace(8)-2-Ace(8),
(b) Ace(10)-2-Ace(10), and (c) Ace(12)-2-Ace(12).
lower pH conditions. It shows a kind of balance between the
solubility and adsorbability. Moreover, the effect of excess of
NaCl cannot be ignored. It is these three impacts--alkyl chain
length, pH, salt--functioning together that result in this
changing law.
Figure S5 and subsequent description (in the Supporting
Information) explain in detail the effect on the molecular
structure brought about by change of pH conditions. To
demonstrate this effect’s reasonability, zeta potential was
introduced to measure surface charge distribution of the
aggregates formed by surfactant molecules. The results are
shown in Table S1 of the Supporting Information.
The solution showing a bluish tint suggests the existence of
aggregate formed in solution. The aggregate can be micelle,
vesicle, wormlike, or lamellae, so dynamic light scattering was
further employed to study the aggregate size distributions of
surfactant at 10−3 mol/L. The experimental results are
presented in Figure S7 and Table S3 in the Supporting
Information. As shown in Figure S7, only one distribution is
detected in all of the investigated surfactant solutions at the
neutral environment and two distinct distributions are observed
in alkaline environment. Besides, PDI values in neutral
environment are all smaller than those in the alkaline
environment, which indicates that aggregates are better
distributed in lower pH environment. Correspondingly, TEM
measurements were carried out on the same surfactant
solutions (the same concentration and the same sample
preparation method). In Figure 3, spherical vesicles are found
for all the surfactants and comparable aggregate numbers are
shown. Similarly, the aggregate number in neutral environment
is more than that at alkaline condition, except for Ace(10)-2-
Ace(10), whose aggregate numbers in two pH environments
show little difference. The DLS and TEM results can
preliminarily prove that not only a better distribution but also
a higher quantity of aggregate can be generated in Ace(n)-2-
Ace(n) solutions under neutral pH environment.
To further demonstrate the change of aggregate number in
Ace(n)-2-Ace(n) solutions, steady-state fluorescence spectra of
pyrene−surfactant solution was measured. When the vesicles
materialized, pyrene solubilized in water can be preferentially
incorporated into the inner region of aggregates. The larger the
aggregate quantity is, the greater the intensity of the
fluorescence spectra. As shown in the fluorescence spectra of
pyrene with varied surfactants and pure water (Figure S8 in the
Supporting Information), the fluorescence intensity follows
such a trend: Ace(12)-2-Ace(12)a > Ace(10)-2-Ace(10)a >
Ace(8)-2-Ace(8)a > Ace(8)-2-Ace(8)c > Ace(10)-2-Ace(10)c >
Ace(12)-2-Ace(12)c, which suggests the aggregate number is
much bigger. The fluorescence intensity of pyrene-Ace(n)-2-
Ace(n) solution at neutral pH is much stronger, which supports
the TEM micrograph results.
Table S2 (in the Supporting Information) displayed a
comparison Ace(n)-2-Ace(n)c with some other reported gemini
surfactants. The data shows that Ace(n)-2-Ace(n) is not inferior
to any other gemini surfactant, and further proves that gemini
surfactants truly possess a great effect on surface activity.
Among those reported gemini surfactants, some owned similar
structure to Ace(n)-2-Ace(n). As mentioned in the Introduc-
tion, the existence of N−CH2COO− groups indicated that the
properties of this structure could be influenced by pH, but the
surface adsorption and micellization characteristics in this field
have seldom been reported. The above results show that pH
indeed has a great influence on the performance of the Ace(n)-
2-Ace(n) series. So, the following measurements for Ace(n)-2-
Ace(n) were focused on alkaline (∼12.00) and neutral (∼7.50)
properties in two distinct pH environments.
3.3. Aggregation Behavior in the Solution. During the
zeta potential measurements, the prepared samples (concen-
tration is 10−3 mol/L, at neutral pH) appeared bluish.
Photograph of Ace(n)-2-Ace(n) in different pH environments
is presented in Figure S6 (in the Supporting Information). As
displayed in the photograph, with pH decreasing, the solution
first appeared bluish and then precipitated as a white solid.
From the point of view of appearance, Ace(12)-2-Ace(12) is
most influenced by pH since the bluish just showed up at pH =
6.50 and surfactant completely precipitated at pH = 4.00; while
under the same condition, the pH range for the bluish
appearance is relatively wider for Ace(10)-2-Ace(10) and
Ace(8)-2-Ace(8), especially for Ace(10)-2-Ace(10) (the bluish
is still obvious at pH = 4.00). The disappearance of the bluish
phenomenon and appearance of precipitation happened with
the longest chain (n = 12), but not for shorter chains. This is
confirmation of the pH effect on the alkyl chain, just as stated
in section 3.2.
3.4. pH-Responsive Reversibility Measurements. The
results of the above measurements indicate that Ace(n)-2-
Ace(n) are pH-responsive. During preparation of the samples
for these measurements, the change in appearance of the
solution with various pH values was observed: from totally
transparent, bluish, and milky to finally a suspension. This
change in appearance can be considered consistent with
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aggregate behavior in solution. Considering that this trans-
formation is controlled by pH, we think the pH reversibility
merits additional study. The reversibility and repeatability of
the process were confirmed by monitoring the transmittance of
the solution of Ace(n)-2-Ace(n) while HCl/NaOH solution
was added into the solution over at least three cycles (Figure
4). In the first cycle, transmittance diminished when HCl was
titrated and reached a plateau region, then it rose upon NaOH
addition. Based on the first cycle, the same process was
repeated for two more cycles (four for Ace(8)-2-Ace(8)).
There are some differences among the reversible measurement
results of three Ace(n)-2-Ace(n): (1) for n = 8, the
transmittance cannot recover to the original state in the
repeated cycles (the data decayed from 84.5% to 63.0%), and
the maximum transmittance increases in the repeated cycle as
the cycle number grows (63.0%, 65.7%, 68.5%, 70.1%,
respectively); (2) for n = 10, far from reduction, the
transmittance maximum increases all along (from 75.3%,
80.5%, 86.8%, to 90.6%); (3) for n = 12, the transmittance
maximum also appears attenuated but remains comparatively
stable; the degree of transmittance maximum attenuation is
smaller than that of Ace(8)-2-Ace(8) (from the original value
98.2% to 94.3%, 94.5%, 94.7%, respectively).
From pH-responsive reversibility results, the difference
among Ace(n)-2-Ace(n) should be attributed to a different
sensitivity to salinity. The added volume of HCl or NaOH
solution during each cycle was merely 1 mL, so the change in
surfactant concentration can be ignored for the measured 40
mL solution. However, considering that the excess NaOH
existed in the original solution, the concentration of NaCl after
the first round of HCl addition became a factor that could not
be neglected, not to mention that it had gradually increased in
the following cycles. From the discussion in the previous
paragraph, it seems that NaCl indeed plays an important role in
affecting the aggregation behavior of Ace(n)-2-Ace(n). The
increase in transmittance for surfactants with shorter chains (n
= 8, 10) after repeated HCl/NaOH cycles means a
disaggregation effect due to the increased concentration of
Na+ and Cl−. The long-chained Ace(12)-2-Ace(12) are
protected against the disaggregation due to the strong
hydrophobic interaction of dodecyl.
3.5. Emulsion-Type Conversion. As we previously
discussed, aqueous Ace(n)-2-Ace(n) solution presented differ-
ent aspects at three different pH environments (transparent,
semitransparent, and milky), but the solution is always
homogeneous. Knowing that surface adsorption of Ace(n)-2-
Ace(n) is pH-dependent, we then tested the stability of the
dodecane-in-water emulsion prepared at different pH values.
One thing we need to emphasize is that all the samples used
were from the same batch as those used for surface tension
measurements. This proves a strong correspondence to the
previous results of surface tension.
Figure 5. Photographs of 1:1 (v/v) dodecane/water mixtures
containing 10−2 mol/L Ace(n)-2-Ace(n) at different pH values after
10 minutes of stirring followed by a waiting period of 24 h. From left
to right, pH values are as follows: (a) Ace(8)-2-Ace(8), 11.47, 8.10,
7.50; (b) Ace(10)-2-Ace(10), 11.76, 8.42, 7.26; (c) Ace(12)-2-
Ace(12), 11.60, 9.34, 8.15.
Ace(n)-2-Ace(n) is not a good emulsifier, for demulsification
happened within 24 h. So, further work in this area will include
improvement of compounding Ace(n)-2-Ace(n) and other
additives. The target is to develop a perfect temporary
emulsifier which is obtained when switching “on” the O/W
emulsion type of Ace(n)-2-Ace(n) and can be broken by
switching it “off” to the W/O type. The additives can be other
kinds of surfactants, nanoparticles, or polymers.
4. CONCLUSIONS
In this work, gemini surfactants Ace(n)-2-Ace(n) (n = 8, 10,
12) were synthesized and their characterization has been
1
confirmed by Accurate-Mass Spectrum, H NMR, and 13C
NMR. Ace(n)-2-Ace(n) existed in different forms in solution as
a function of pH, expressed as pKa1and pKa2. Surface properties
were systematically investigated in these three different pKa
intervals (far above pKa1, near pKa1 and between pKa1 and pKa2)
at 298 K by several combined techniques. A series of useful
parameters were obtained to evaluate their surface activity. The
results indicate that Ace(n)-2-Ace(n) has far superior surface
activity than traditional surfactants depending on much lower
cmc/γcmc and higher pC20 values. The pH, alkyl chain, and
excess of NaCl have great impact on the properties of Ace(n)-
2-Ace(n). By analyzing the results of measurement, conclusions
can be drawn about the changing law on the performance of the
Ace(n)-2-Ace(n) series as follows. (1) The surface activity of
Figure 5 is the photographs of dodecane-in-water prepared
with Ace(n)-2-Ace(n) and stabilized after 24 h. The trans-
formation of the emulsion type was patently exhibited. O/W
emulsion was generated in the first two different pH
environments, namely, pHc and pHb, while W/O emulsion
was formed for pHa. The double phase inversion is also
supported by comparing the emulsion conductivities (as given
in Table S4 in the Supporting Information). This characteristic
provides a kind of pH-controllable surfactant, which not only
controls the on-and-off aspect of the emulsion, but also reveals
an environmentally friendly feature. Unfortunately, single
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Ace(n)-2-Ace(n) increases with the increase in alkyl chain
length. The longer the alkyl chain is, the more conspicuous the
effects from pH and NaCl will be, reflected as the surface
tension showing obvious fluctuation at n = 12 and not at n = 10
and 8; for each Ace(n)-2-Ace(n), cmc increases and pC20
decreases with decreasing pH, and the change is also more
obvious for longer chains; the bluish phenomenon is obviously
shorter lived for Ace(12)-2-Ace(12) than the other two Ace(n)-
2-Ace(n)s. (2) pH can control the adsorption of Ace(n)-2-
Ace(n) at the air/water interface and aggregation behavior in
bulk solution: the Γcmc reaches a maximum at pH near pKa1;
aggregation becomes much greater at pH between pKa1 and
pKa2, supported by TEM and fluorescence intensity. Neutral
pH makes much more and better distributed aggregates
generated in Ace(n)-2-Ace(n) solution. (3) pH-controllability
is reversible, verified by cycled transmittance measurements.
Finally, from the standpoint of practical application, pH-
responsive emulsion type conversion was described. With pH
changing from alkaline to neutral, the emulsion type reverses
from O/W to W/O. Considering the transmittance’s
reversibility, the emulsion’s conversion shall also be repeatable.
Many industrial applications relying on emulsions would
benefit from this efficient, rapid method to produce switchable
surfactants based on pH-responsivity.
REFERENCES
■
(1) Menger, F.M.; Keiper, J.S. Gemini Surfactants. Angew. Chem., Int.
Ed. 2000, 39, 1906−1920.
(2) In, M.; Zana, R. Phase Behavior of Gemini Surfactants. J.
Dispersion Sci. Technol. 2007, 28, 143−154.
(3) Han, Y.H.; Wang, Y.L. Aggregation Behavior of Gemini
Surfactants and Their Interaction with Macromolecules in Aqueous
Solution. Phys. Chem. Chem. Phys. 2011, 13, 1939−1956.
(4) Sakai, K.; Sakai, H.; Abe, M. Recent Advances in Gemini
Surfactants: Loeic Acid-based Gemini Surfactants and Polymerizable
Gemini Surfactants. J. Oleo Sci. 2011, 60 (4), 159−163.
(5) Shukla, D.; Tyagi, V.K. Anionic Gemini Surfactants: A Distinct
Class of Surfactants. J. Oleo Sci. 2006, 55 (5), 215−226.
(6) Zana, R.; Benrraou, M.; Rueff, R. Alkanediyl-α, ω-bis-
(dimethylalky1ammonium bromide) Surfactants. 1. Effect of the
Spacer Chain Length on the Critical Micelle Concentration and
Micelle Ionization Degree. Langmuir 1991, 7, 1072−1075.
(7) Yoshimura, T.; Ishihara, K.; Esumi, K. Sugar-based Gemini
Surfactants with Peptide Bonds-synthesis, Adsorption, Micellization,
and Biodegradability. Langmuir 2005, 21, 10409−10415.
(8) Xie, Z.F; Feng, Y.J. Synthesis and Properties of Alkylbetain
Zwitterionic Gemini Surfactants. J. Surfactants Deterg. 2010, 13 (1),
51−57.
(9) Xu, J.; Liu, X.J.; Wang, B.; Qiao, Z.J. Synthesis and
Characterization of Sodium N,N′-dioctyl-ethylenediamine Dicayl
Propionate. Int. J. Chem. (Toronto, ON, Can.) 2010, 2, 194−197.
(10) Sakai, K.; Kaji, M.; Takamatusu, Y.; Tsuchiya, K.; Torigoe, K.;
Tsubone, K.; Yoshimura, T.; Esumi, K.; Sakai, H.; Abe, M.
Fluorocarbon−Hydrocarbon Gemini Surfactant Mixtures in Aqueous
Solution. Colloids Surf., A 2009, 333, 26−31.
ASSOCIATED CONTENT
■
S
* Supporting Information
Synthesis and chemical characterization, schematic diagram on
effect on molecular structure brought about by the change of
pH, zeta potential of aqueous Ace(n)-2-Ace(n) solution at 10−3
mol/L, comparison of Ace(n)-2-Ace(n) with other reported
gemini surfactants, photograph of Ace(n)-2-Ace(n) solution at
10−3 mol/L, DLS results of Ace(n)-2-Ace(n) at 10−3 mol/L,
fluorescence spectra of pyrene−surfactant solution, conductiv-
ity (κ) of 1:1 (v/v) emulsion of dodecane and 10−2 mol/L
Ace(n)-2-Ace(n) at different pH values. This material is
available free of charge via the Internet at http://pubs.acs.org.
(11) Zhu, Y.P.; Masuyama, A.; Okahara, M. Preparation and Surface
Active Properties of Amphipathic Compounds with Two Sulfate
Groups and Two Lipophilic Alkyl Chains. J. Am. Oil Chem. Soc. 1990,
67, 459−461.
(12) Duivenvoorde, F.L.; Feiters, M.C.; Van der Gaast, S.J.; Engberts,
J. B. F. N. Synthesis and Properties of Di-n-dodecyl-α, ω-Alkyl
Bisphosphate Surfactants. Langmuir 1997, 13, 3737−3743.
(13) Sommerdijk, N. A. J. M.; Hoeks, T. H. L.; Synak, M.; Feiters,
M.C.; Nolte, R. J. M.; Zwanenburg, B. Stereodependent Fusion and
Fission of Vesicles: Calcium Binding of Synthetic Gemini
Phospholipids Containing Two Phosphate Groups. J. Am. Chem. Soc.
1997, 119, 4338−4344.
(14) Song, L.D.; Rosen, M.J. Surface Properties, Micellization, and
Premicellar Aggregation of Gemini Surfactants with Rigid and Flexible
spacers. Langmuir 1996, 12, 1149−1153.
(15) You, Y.; Wu, X.N.; Zhao, J.X.; Ye, Y.Z.; Zou, W.S. Effect of Alkyl
Tail Length of Quaternary Ammonium Gemini Surfactants on
Foaming Properties. Colloids Surf., A 2011, 384, 164−171.
(16) Kuo, C.J.; Lin, L.H.; Dong, M.Y.; Chang, W.S.; Chen, K.M.
Preparation and Properties of New Ester-Linked Cleavable Gemini
Surfactants. J. Surfactants Deterg. 2011, 14, 195−201.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: qiaoweihong@dlut.edu.cn; fax: +86-0411-84986232.
Author Contributions
The manuscript was written through contributions of all
authors. Initial conception, design, provision of resources,
collection of data, analysis, interpretation of data and
manuscript writing were completed by Jing Lv; Prof. Weihong
Qiao provided theoretical guidance; Chongqiao Xiong made
substantial contributions to writing and revising paper. All
authors have given approval to the final version of the
manuscript.
(17) Feng, J.; Liu, X.P.; Zhang, L.; Zhao, S.; Yu, J.Y. Preparation and
Properties of New Ester-Linked Cleavable Gemini Surfactants.
Langmuir 2010, 26 (14), 11907−11914.
(18) Zana, R. Dimeric and Oligomeric Surfactants. Behavior at
Interfaces and in Aqueous Solution: a Review. Adv. Colloid Interface Sci.
2002, 97, 205−253.
Notes
(19) Groth, C.; Nyden, M.; Holmberg, K.; Kanicky, J.R.; Shah, D.O.
Kinetics of the Self-Assembly of Gemini Surfactants. J. Surfactants
Deterg. 2004, 7 (3), 247−255.
The authors declare no competing financial interest.
(20) Wang, Y.X.; Han, Y.C.; Huang, X.; Cao, M.W.; Wang, Y.L.
Aggregation Behaviors of a Series of Anionic Sulfonate Gemini
Surfactants and Their Corresponding Monomeric Surfactant. J. Colloid
Interface Sci. 2008, 319, 534−541.
(21) Zhu, D.Y.; Chen, F.; Chen, Y.; Jiang, S.C. Preparation,
Characterization and Properties of Anionic Gemini Surfactants with
Long Rigid or Semi-rigid Spacers. Colloids Surf., A 2012, 397, 1−7.
ACKNOWLEDGMENTS
■
This work was financially supported by Petro China Daqing
Oilfield Co. Ltd. (DQYT-0508003-2011-JS-362). Dalian
University of Technology Chemistry Analysis & Research
Center is acknowledged for providing measurement instru-
ments.
8266
dx.doi.org/10.1021/la5016669 | Langmuir 2014, 30, 8258−8267

Langmuir
Article
(22) Frost, A.E., Jr.; Martell, E. The Preparation and Properties of
Some N,N′-Dialkylethylenediaminediacetic Acids. J. Am. Chem. Soc.
1950, 76, 3743−3746.
(23) Sakai, K.; Umezawa, S.; Tamura, M.; Takamatusu, Y.; Tsuchiya,
K.; Torigoe, K.; Tsubone, K.; Ohkubo, T.; Yoshimura, T.; Esumi, K.;
Sakai, H.; Abe, M.; Sakai, K. Adsorption and Micellization Behavior of
Novel Gluconamide-type Gemini Surfactants. J. Colloid Interface Sci.
2008, 318, 440−448.
(41) Hall, D. G.; Pethica, B. A.; Shinoda, K. The Interpretation of
Surface Tension Data for Solutions of Ionic Surfactants In the
Presence of Electrolyte. Bull. Chem. Soc. Jpn. 1975, 48, 324−326.
(42) Gurkov, T. D.; Dimitrova, D. T.; Marinova, K. G.; Bilke-Crause,
C.; Gerber, C.; Ivanov, I. B. Ionic Surfactants on Fluid Interfaces:
Determination of the Adsorption; Role of the Salt and the Type of the
Hydrophobic Phase. Colloids Surf., A 2005, 261, 29−38.
(24) Yoshimura, T.; Esumi, K. Synthesis and Surface Properties of
Anionic Gemini Surfactants with Amide Groups. J. Colloid Interface Sci.
2004, 276, 231−238.
(25) Wattebled, L.; Laschewsky, A. New Anionic Gemini Surfactant
Based on EDTA Accessible by Convenient Synthesis. Colloid Polym.
Sci. 2007, 285, 1387−1393.
(26) Leonard, I.A.; Gerard, R. Micellization of pH-stimulable Poly(2-
vinylpyridine)-b-poly(ethylene oxide) Copolymers and Their Com-
plexation with Anionic Surfactants. J. Colloid Interface Sci. 2013, 395,
190−197.
(27) Ngo, T. H. V.; Damas, C.; Naejus, R.; Coudert, R. Aggregation
of New Bipolar Glucosyled and Cationic Fluoroamphiphiles in
Aqueous Media. J. Colloid Interface Sci. 2013, 389, 157−163.
(28) Lin, Y.Y.; Han, X.; Cheng, X.H.; Huang, J.B.; Liang, D.H.; Yu, C.
pH-Regulated Molecular Self-Assemblies in a Cationic-Anionic
Surfactant System: From a “1-2” Surfactant Pair to a “1-1” Surfactant
Pair. Langmuir 2008, 24, 13918−13924.
(29) Wang, X.Q.; Liu, J.; Yu, Li; Jiao, J.J.; Wang, R.; Sun, L.M.
Surface Adsorption and Micelle Formation of Imidazolium-based
Zwitterionic Surface Active Ionic Liquids in Aqueous Solution. J.
Colloid Interface Sci. 2013, 391, 103−110.
(30) Argillier, J.F.; Zeilinger, S.; Roche, P. Enhancement of Aqueous
Emulsion and Foam Stability with Oppositely Charged Surfactant/
Polyelectrolyte Mixed Systems. Oil & Gas Science and Technology
2009, 64 (5), 597−605.
(31) Zana, R. Critical Micellization Concentration of Surfactants in
Aqueous Solution and Free Energy of Micellization. Langmuir 1996,
12, 1208−1211.
(32) Rosen, M.J.; Aronson, S. Standard Free Energies of Adsorption
of Surfactants at the Aqueous/solution/air Interface From Surface
Tension Data in Vicinity of the Critical Micelle Concentration.
Colloids Surf. 1981, 3 (3), 201−208.
(33) Menger, F. M.; Shi, L.; Rizvi, S. A. A. Re-evaluating the Gibbs
Analysis of Surface Tension at the Air/Water Interface. J. Am. Chem.
Soc. 2009, 131, 10380−10381.
(34) Menger, F. M.; Rizvi, S. A. A.; Shi, L. Reply to “Should the
Gibbs Analysis Be Revised? Langmuir 2011, 27, 7963−7965.
(35) Menger, F. M.; Shi, L.; Rizvi, S. A. A. Additional Support for a
Revised Gibbs Analysis. Langmuir 2010, 26 (3), 1588−1589.
(36) Laven, J.; De.With, G. Should the Gibbs Analysis Be Revised?
Langmuir 2011, 27, 7958−79622.
(37) Li, P. X.; Li, Z. X.; Shen, H.; Thomas, R. K.; Penfold, J.; Lu, J. R.
Application of the Gibbs Equation to the Adsorption of Nonionic
Surfactants and Polymers at the Air-Water Interface: Comparison with
Surface Excesses Determined Directly using Neutron Reflectivity.
Langmuir 2013, 29, 9324−9334.
(38) Xu, H.; Li, P. X.; Ma, K.; Thomas, R. K.; Penfold, J.; Lu, J. R.
Limitations in the Application of the Gibbs Equation to Anionic
Surfactants at the Air/Water Surface: Sodium Dodecylsulfate and
Sodium Dodecylmonooxyethylenesulfate Above and Below the CMC.
Langmuir 2013, 29, 9335−9351.
(39) Matuura, R.; Kimizuka, H.; Yatsunami, K. The Study of the
Adsorption of Detergents at a Solution-air Interface by Radiotracer
Method. III. The Effect of Inorganic Electrolytes on the Adsorption of
Sodium Dodecyl Sulfate. Bull. Chem. Soc. Jpn. 1959, 32, 646−650.
(40) Tajima, K. Radiotracer Studies on Adsorption of Surface Active
Substance at Aqueous surface. II. The Effects of Excess Salt on the
Adsorption of Sodium Dodecylsulfate. Bull. Chem. Soc. Jpn. 1970, 43,
3063−3066.
8267
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