Effects of pH, temperature and ionic strength on aggregation behavior of dodecyldiethoxylamine oxide

Article abstract of DOI:10.1007/s11743-013-1508-7

The isoelectric point of dodecyldiethoxylamine oxide (DDEAO) containing hydroxyethyl groups was determined with the method of acid-base titration. The isoelectric point value of DDEAO is 7.8. The effects of pH, temperature and ionic strength on the aggregation behavior of DDEAO were investigated by surface tension measurements. The results show a good surface activity over a large pH range, especially under weakly acidic conditions. The surface activity of DDEAO increases trend increasing temperature. The thermodynamic parameters were also evaluated and the results show that the micelle formation is entropy-driven within the temperature range investigated. CMC values decrease with increases in ionic strength. The packing parameters (P) were evaluated and ensure the formation of vesicles for DDEAO at various pH values, which is expected to have potential applications in some fields, such as in nanomaterials synthesis and the application in drug delivery.

Full text of DOI:10.1007/s11743-013-1508-7

J Surfact Deterg
DOI 10.1007/s11743-013-1508-7
ORIGINAL ARTICLE
Effects of pH, Temperature and Ionic Strength on Aggregation
Behavior of Dodecyldiethoxylamine Oxide
•
•
•
Ruitao Wang Yunling Li Yongbo Song
Lifei Zhi
Received: 16 April 2013 / Accepted: 5 July 2013
Ó AOCS 2013
Abstract The isoelectric point of dodecyldiethoxylamine
oxide (DDEAO) containing hydroxyethyl groups was
determined with the method of acid–base titration. The
isoelectric point value of DDEAO is 7.8. The effects of pH,
temperature and ionic strength on the aggregation behavior
of DDEAO were investigated by surface tension mea-
surements. The results show a good surface activity over a
large pH range, especially under weakly acidic conditions.
The surface activity of DDEAO increases trend increasing
temperature. The thermodynamic parameters were also
evaluated and the results show that the micelle formation is
entropy-driven within the temperature range investigated.
CMC values decrease with increases in ionic strength. The
packing parameters (P) were evaluated and ensure the
formation of vesicles for DDEAO at various pH values,
which is expected to have potential applications in some
fields, such as in nanomaterials synthesis and the applica-
tion in drug delivery.
and their composition depends on the pH of the aqueous
solution [3]. These surfactants produce less irritation to the
skin, have high foaming properties and an excellent thick-
ening function [4–6]. Among amine oxides, dodecyldim-
ethylamine oxide (DDMAO) is the most widely used
surfactant and has been extensively researched [7–9].
Mixtures of DDMAO and sodium dodecylsulfate (SDS) or
dodecyltrimethylammonnium bromide (DTAB) have been
widely studied. Results reported have shown good syner-
gistic effects of mixed solutions [10–12]. DDMAO has been
used in detergents, shampoos, cosmetics and textile auxil-
iaries based on their characteristics and advantages [5, 6].
Many applications such as sewage treatment [13],
preparation of nanostructured materials [14] and the
application in drug delivery [15] depend on the aggregation
behavior of surfactants. Therefore, there is a lot of research
into the aggregation behavior of surfactants in the last few
years. The aggregation behavior of amine oxide surfactants
have been extensively researched employing surface ten-
sion, rheological measurements, small-angle neutron scat-
tering, and static light scattering [10, 16–22]. The results
indicate that the aggregation behavior of amine oxide
surfactants can be controlled by altering the length of the
hydrocarbon chain, the pH of the aqueous solution, ionic
type, the media of solution, temperature and ionic strength.
In spite of much research into the aggregation behavior of
amine oxide surfactants, amine oxide surfactants contain-
ing hydroxyethyl groups have been studied less.
Keywords Amine oxide ꢀ Aggregation behavior ꢀ
Surface activity
Introduction
Amine oxide surfactants show nonionic characteristics in
neutral or alkaline solutions, and exhibit a cationic behavior
in acid solutions [1, 2]. They are a kind of mixture made up
of protonated and nonprotonated species in aqueous media,
In this paper, dodecyldiethoxylamine oxide (DDEAO)
containing hydroxyethyl groups was synthesized according
to the literature. The synthesis route and chemical structure
are shown in Scheme 1. The isoelectric point of DDEAO
was measured. The effects of pH, temperature and ionic
strength on the aggregation behavior of DDEAO were
investigated. The various physicochemical properties and
R. Wang ꢀ Y. Li (&) ꢀ Y. Song ꢀ L. Zhi
China Research Institute of Daily Chemical Industry,
#34 Wenyuan Str., Taiyuan 030001, Shanxi,
People’s Republic of China
e-mail: lyunl0068@sina.com
123

J Surfact Deterg
CH2CH2OH
pH is greater than 7.8. Moreover, the pH of DDEAO is 6.0
in aqueous media. So DDEAO behaves similarly to cat-
ionic surfactants in aqueous solutions. It is expected to
form micelles containing both protonated and nonproto-
nated species.
CH2CH2OH
CH2CH2OH
H2O2
2EO
C12H25NH2
C12H25
N
C12H25
N
O
CH2CH2OH
Scheme 1 Synthesis route of dodecyldiethoxylamine oxide
The surface activity of DDEAO were investigated by
surface tension measurements with different pH values at
298 K. The critical micelle concentration (CMC) values
and the surface tensions at CMC (cCMC) of DDEAO at
various pH values were determined from the plots of sur-
face tension (c) versus surfactant concentration in Fig. 2.
The results are presented in Table 1. CMC values versus
pH plots are depicted in Fig. 3. As can be seen, CMC and
thermodynamic parameters were evaluated. The results
were also compared with those of DDMAO.
Materials and Methods
Materials
cCMC values of DDEAO decrease with increasing pH down
to a minimum and then increase upwards in the range of pH
2–10. The minimum of CMC is approximately at pH = 6
(below the pI value of DDEAO). This is probably due to
the synergism between protonated and nonprotonated
species in aqueous media. Since the amount of nonproto-
nated species increases as the pH value (pH \ 6) increases
in solution, the electrostatic repulsion between the hydro-
philic groups is weakened, resulting in a decrease in CMC.
However, CMC values have a slight increase in pH [ 6.
The result is caused by a weak interaction between
hydrocarbon chains with a rise in pH. cCMC values show a
similar trend to CMC. This can be explained by the fact
that DDEAO shows different characteristic of surfactants at
various pH values. It is noteworthy that DDEAO has lower
CMC value than DDMAO (1.7 mM) in aqueous solution
[10]. Moreover, in the case of DDMAO, CMC values of
nonprotonated species are approximately 1 mM [9].
However, CMC values of DDEAO at high pH in this work
are lower than those of DDMAO. It is well established that
hydroxyethyl group of DDEAO is a dominant factor in the
determination of CMC values. This result indicates that the
Dodecyldiethoxylamine oxide (DDEAO) was prepared by
the reaction of dodecyldiethoxylamine with hydrogen
peroxide in water as the solvent in the laboratory based on
the literature [23]. Water was removed by vacuum, and
then the crude material was recrystallized three times from
acetone to obtain purity. The pH value of DDEAO in
aqueous solution was adjusted by NaOH or HCl aqueous
solutions. NaCl was used to adjust the ionic strength of the
aqueous solutions.
pH Titration
The isoelectric point (pI) of DDEAO was determined by a
pHs-3c pH meter from Shanghai Precision and Scientific
Instrument Co. using the method of acid–base titration.
Surface Tension Measurement
The surface tensions of DDEAO were measured from a
series of aqueous solutions with a platinum ring using a
Kru¨ss K12 Tensiometer. The surface tension of double
distilled water, 72.0 0.3 mN/m, was used for calibration
purposes. Surfactant solutions were prepared with the
double-distilled water. The samples were stabilized for
10 min in the instrument before measurements were taken.
12
DDEAO
10
8
Results and Discussion
6
Effect of pH on the Aggregation Behavior of DDEAO
4
The pH titration curve of DDEAO is shown in Fig. 1. The
isoelectric point (pI) is the titration jump point of the pH
titration curve. As can be seen from Fig. 1, the pI value of
DDEAO is 7.8. Amine oxide surfactant has been known to
be mainly nonprotonated at high pH, and protonated at low
pH [24]. DDEAO shows cationic behavior when the pH
value is below 7.8, while it exhibits nonionic behavior the
2
0
2
4
6
8
10
12
Amount of base/mL
Fig. 1 The relationship between the base amount and pH
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J Surfact Deterg
70
minimum surface area per surfactant molecule (Amin) was
obtained from the Gibbs Eq. (2):
ꢀ
pH=2
pH=4
pH=6
pH=8
pH=10
ꢁ
1
oc
60
50
40
30
20
Cmax ¼ ꢁ
ð1Þ
ð2Þ
n2:303RT ologc
T
1
Amin
¼
NACmax
where R is the gas constant, T is the absolute temperature, NA
is Avogadro’s number and c is theconcentration of surfactant
in solution. For nonionic and zwitterionic surfactant, n = 1
[25–28]. The values of Umax and Amin, which are considered
as a sign of packing densities of surfactant molecules at the
air/liquid interface, are presented in Table 1.
0.1
1
As shown in Table 1, there are higher Umax values and
lower Amin values for DDEAO with the increase of pH with
pH \ 6. At a lower pH, the strong electrostatic repulsion
between the hydrophilic groups leads to a looser com-
pactness of the aggregation at the air/liquid interface.
However, the amount of nonprotonated species increases
with a rise in pH, and the electrostatic repulsion at the air/
liquid interface is weakened. This encourages the DDEAO
molecules to arrange more closely. In pH [ 6, DDEAO
shows a decrease Umax value and a increase Amin value with
increasing pH. The effect of interaction between hydro-
phobic chains becomes a dominant factor in the determi-
nation of Umax and Amin values. With increasing pH, the
van der Waals interaction between the hydrophobic chains
becomes weak resulting in lower packing densities of
surfactant molecules in the surface adsorption layer. In
addition, in comparison with DDMAO [23], DDEAO has
larger Umax and smaller Amin values in aqueous solutions.
The hydroxyethyl group of DDEAO can form a hydrogen
bond between molecules. The intermolecular hydrogen
bonds can conduce the molecules to arrange more com-
pactly at the air/water interface monolayer.
c/mM
Fig. 2 Surface tension vs. concentration for DDEAO with different
pH values at 298 K
Table 1 Surface properties of DDEAO with different pH values at
298 K
pH
CMC
(mM)
cCMC
(mN/m)
Umax
Amin
(A )
pC20
P
(lmol/m2)
2
˚
2
4
1.20
0.73
0.38
0.45
0.48
26.0
25.7
25.4
26.6
27.8
4.83
5.18
5.20
5.04
4.69
34.4
32.0
31.9
33.0
35.4
3.91
4.02
4.36
4.25
4.25
0.60
0.64
0.65
0.62
0.58
6
8
10
1.2
1.0
0.8
0.6
0.4
The adsorption of surfactants can be expressed by the
pC20 value. It is the value of the logarithm of the reciprocal
C20, i.e., the concentration of surfactants to lead the surface
tension of water to decrease by 20 mN/m. The larger the
pC20 value, the higher is the adsorption efficiency of sur-
factants [29]. The values of pC20 obtained for DDEAO at
various pH values are also presented in Table 1. From
Table 1, with the increases in pH, pC20 values increase up
to pH = 6 and then slightly decrease. It is noteworthy that
the pC20 values are larger in the pH 6–10 range compared
with those of pH 2–4. The result indicates that DDEAO is
easily adsorbed at high pH. It is considered to be due to the
following two points. First, the electrostatic repulsion
between the hydrophilic groups is a dominant factor in the
determination of pC20 values at low pH, while the effect of
interaction between hydrophobic chains is dominant factor
at high pH. Second, the influence of electrostatic repulsion
2
4
6
8
10
pH
Fig. 3 The CMC values as a function of pH for DDEAO at 298 K
surface activity of DDEAO is somewhat superior to that of
DDMAO.
From the surface tension curves, the surface excess
concentrations of DDEAO (Umax) at various pH values
were calculated from the Gibbs adsorption Eq. (1) and the
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J Surfact Deterg
values of the packing parameter within the range of 0.5–1.
Based on the molecular structure of DDEAO, the sectional
area of the hydrophilic group is compressed because of the
attractive interaction between the hydrogen bond of head
groups. The value of the sectional area of the hydrophilic
group relative to the volume of the hydrophobic group
becomes small, which causes the P value to be in the range
of 0.5–1.
is stronger than the effect of interaction between hydro-
phobic chains.
The packing parameter (P) determines the geometry of
micelles. The packing parameter of DDEAO at various pH
values are obtained by using Eq. (3) [30].
Vc
P ¼
ð3Þ
lcAmin
where Vc is the volume of hydrophobic group, lc is the
chain length of hydrophobic group. Amin is the minimum
surface area per surfactant molecule. When the
hydrophobic group is the straight-chain alkane, Vc and lc
were calculated by Tanford’s equations [31]:
Effect of Temperature on the Aggregation Behavior
of DDEAO
Surface tension measurements were employed to study the
micellar aggregation behavior of DDEAO in aqueous
solutions at different temperatures. Figure 4 depicts the
surface tension isotherms of DDEAO in the temperature
range of 288–308 K. CMC and cCMC values determined by
surface tension measurements are listed in Table 2.
As shown in Table 2, CMC values decrease slightly
with the increase of temperature. In the case of DDMAO,
CMC values of the nonprotonated species decrease with
increasing temperature, while the values of protonated
species increase with temperature [9]. Although DDEAO
shows cationic characteristics in aqueous solution, the
variation of CMC with temperature is similar to the non-
protonated species of DDMAO. There are two opposite
processes for the effect of temperature on the CMC values.
Increasing temperature weakens the hydration of the
hydrophilic groups, which favors micelle formation and
results in a lower CMC. However, the orderly structure of
the water molecules surrounding the hydrophobic groups is
broken with increasing temperature, which is a disadvan-
tageous factor for micelle formation. It is clear from the
data that the former is a dominant factor in the determi-
nation of the CMC values of DDEAO in this work. There is
a slight variation with the value of cCMC with increasing
temperature.
Vc ¼ ð27:4 þ 26:9nÞ ꢂ 10ꢁ3 nm3
lc ¼ ð0:15 þ 0:1265nÞ nm
ð4Þ
ð5Þ
where n is the number of carbon atoms in the hydrocarbon
chain. The values of the packing parameter of DDEAO
with different pH values at 298 K are given in Table 1.
P B 1/3 indicates spherical micelle formation. 1/3 \
P B 1/2 shows symmetrical micelle formation such as
spheroidicity, oblate spheroids and rods. The vesicles for-
mation is generally indicated when the packing parameter
lies within the range of 0.5–1 [32]. From the P values in
Table 1, the various pH values of DDEAO at 298 K have
40
60
288K
293K
298K
35
303K
308K
50
40
30
30
25
288K
293K
298K
303K
308K
0.1
1
c/mM
Based on the single-stage equilibrium model of micellar
solution, the standard Gibbs free energy change of micelli-
zation (DGmic), the standard enthalpy change for the micel-
lization process (DHmic), and the standard entropy of micelle
formation (DSmic) for nonionic and zwitterionic surfactants
can be estimated from the following equations [33]:
0.1
1
10
c/mM
Fig. 4 Surface tension vs. concentration for DDEAO in aqueous
solution at different temperatures
Table 2 Key surface property parameters and thermodynamic parameters of DDEAO at different temperatures
T/K
CMC (mM)
cCMC (mN/m)
DGmic (kJ/mol)
DHmic (kJ/mol)
ꢁTDSmic (kJ/mol)
288
293
298
303
308
0.42
0.40
0.38
0.35
0.33
25.74
25.73
25.64
25.53
25.77
-28.23
-28.84
-29.46
-30.16
-30.81
8.41
8.71
9.01
9.31
9.62
-36.64
-37.55
-38.47
-39.47
-40.43
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J Surfact Deterg
Table 3 Key surface property parameters of DDEAO with different
NaCl concentrations at 298 K
60
c
c
c
c
c
=0.1M
=0.2M
=0.3M
=0.5M
=0.7M
NaCl
NaCl
NaCl
NaCl
NaCl
cNaCl
(M)
CMC
(mM)
cCMC
(mN/m)
Umax
Amin
(A )
2
˚
(lmol/m2)
50
40
30
0.1
0.15
0.2
0.3
0.4
0.5
0.6
0.7
0.30
0.29
0.28
0.26
0.25
0.22
0.20
0.18
25.61
25.55
25.53
25.63
25.71
25.79
25.60
25.78
5.56
5.47
5.33
5.26
5.25
5.21
5.14
4.95
29.9
30.4
31.2
31.6
31.6
31.9
32.3
33.6
0.1
1
c/mM
concentrations of NaCl at 298 K. The key surface property
parameters are summarized in Table 3.
Fig. 5 Surface tension vs. concentration for DDEAO with different
concentrations of NaCl at 298 K
As can be seen, the value of cCMC has little variation,
however, CMC values decrease with the increase of NaCl.
The decrease of CMC values depends upon the salting-out
effect on the hydrocarbon chain and the influence of the
hydrophilic group on water structure. In addition, the
increase in NaCl decreases the surface charge density and
electrical repulsion, which favors micelle formation and
decreases the CMC [36, 37]. A decreasing trend in Umax
and an increasing trend in Amin suggesting a lower packing
density at the air/liquid interface. This can be ascribed to
the increase in hydrophilic group’s volume with the addi-
tion of NaCl.
DGmic ¼ RT ln Xcmc
ð6Þ
ð7Þ
d ln Xcmc
DHmic ¼ ꢁRT2
dT
DHmic ꢁ DGmic
DSmic
¼
ð8Þ
T
where R is the gas constant, T is the absolute temperature,
XCMC is the CMC expressed as a mole fraction,
XCMC = CMC/55.4. All the calculated thermodynamic
parameters of micellization at different temperatures for
DDEAO are listed in Table 2.
Figure 6 shows the relationship between the logarithm of
CMC values and the logarithm of the concentration of NaCl
in aqueous solutions at 298 K. It is different from that of
ionic surfactants, e.g., sodium dodecylsulfate (SDS) which
exhibits a linear dependence. It is also incomplete similarity
to those of zwitterionic surfactants which show biphasic
curves [38, 39]. When the salt concentration is low, log CMC
shows a linear dependence on log cNaCl for DDEAO. How-
ever, a deviation occurs in the range of cNaCl[0.35 M. The
CMC value declines with a sharp slope. Similar, a deviation
occurs in the range of cNaCl [ 1 M in the case of the pro-
tonated species of DDMAO [9]. Compared with DDMAO,
DDEAO is more sensitive to salt concentration. The
observed dependence can be interpreted in terms of the
salting-out effect on the hydrocarbon tail. At the same time,
shape/size changes of micelles with the increasein NaCl may
be another reason. The result is useful for some applications,
such as emulsification of oil spills at sea and enhancement of
oil recovery from depleted reservoirs.
As it can be seen, the negative value of DGmic means that
the micelle formation of DDEAO is a spontaneous process in
the temperature range investigated. More negative DGmic
values are observed at high temperature, which indicates that
it is easier to form micelle than those at low temperature. The
values of DHmic are positive, so the micelle formation pro-
cess is endothermic. The data from Table 2 imply that the
negative values of DGmic are mainly determined by the large
negative values of ꢁTDSmic. Therefore, the micellization
process is mainly entropy-driven for DDEAO in aqueous
solutions. The driving force of the micellization process is
the hydrophobic group of surfactant transfer from solution
environment to the interior of the micelle. During this pro-
cess, the solvated water molecules are released, thus an
entropy increase is obtained [34, 35].
Effect of Ionic Strength on the Aggregation Behavior
of DDEAO
Surface tension measurements were employed to study the
effect of ionic strength on surface activity of DDEAO.
Figure 5 depicts the plots of surface tension versus sur-
factant concentration of DDEAO varied with different
Conclusions
The effects of pH, temperature and ionic strength on the
aggregation behavior of DDEAO were investigated. The
123

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experimental results demonstrate that DDEAO shows good
surface activity over a large pH range, especially under
weakly acidic condition. It can be exploited for utilization
as surfactants for acidic shampoo. Compared with
DDMAO, the surface activity of DDEAO is somewhat
superior to that of DDMAO [9, 10, 24]. The packing
parameters (P) ensure the formation of vesicles for
DDEAO at various pH values. The surface activity of
DDEAO increases with increasing temperature. This trend
is similar to the nonprotonated species of DDMAO [9].
CMC values decrease with increasing ionic strength.
Compared with DDMAO, DDEAO is more sensitive to salt
concentration [9]. Above all, it has a high surface activity
under a wide range of conditions, which makes it a suitable
candidate for industrial applications combining the multi-
ple functions of amine oxide surfactants.
Acknowledgments The authors acknowledge the financial support
of the National Science and Technology Support Project of China
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Author Biographies
Ruitao Wang received
a B.Sc, in chemical engineering and
technology from Baoji University of Arts and Sciences, People’s
Republic of China in 2011. He is currently an M.Sc. student in applied
chemistry, China Research Institute of Daily Chemical Industry,
Taiyuan, People’s Republic of China. His studies involve the
synthesis and applications of surfactants.
Yunling Li received her Ph.D. in applied chemistry from the Shanxi
University, People’s Republic of China. She is currently a Professor
of Engineering in the China Research Institute of Daily Chemical
Industry, Taiyuan, People’s Republic of China. Her research interests
include surfactants and industrial catalysis.
32. Mahajan RK, Sharma R (2011) Analysis of interfacial and
micellar behavior of sodium dioctyl sulphosuccinate salt (AOT)
with zwitterionic surfactants in aqueous media. J Colloid Inter-
face Sci 363:275–283
33. Chevalier Y, Storet Y, Pourchet S, Le Perchec P (1991) Ten-
sioactive properties of zwitterionic carboxybetaine amphiphiles.
Langmuir 7:848–853
34. Geng F, Liu J, Zheng L, Yu L, Li Z, Li G, Tung C (2010) Micelle
formation of long-chain imidazolium ionic liquids in aqueous
solution measured by isothermal titration microcalorimetry.
J Chem Eng Data 55:147–151
Yongbo Song received an M.Sc. in organic chemistry in the Chinese
Academy of Sciences in 2006. He is currently a Ph.D. student in
applied chemistry, Shanxi University, People’s Republic of China.
His studies involve the synthesis and applications of new surfactants.
35. Jiao J, Dong B, Zhang H, Zhao Y, Wang X, Wang R, Yu L (2012)
Aggregation behaviors of dodecyl sulfate-based anionic surface
active ionic liquids in water. J Phys Chem B 116:958–965
Lifei Zhi is currently a Ph.D. student in applied chemistry at Shanxi
University, People’s Republic of China. Her studies involve the
synthesis and applications of new surfactants.
´
36. Martınez-Landeira P, Ruso JM, Prieto G, Sarmiento F (2002)
Surface tensions, critical micelle concentrations, and standard
free energies of micellization of C8- lecithin at different pHs and
electrolyte concentrations. J Chem Eng Data 47:1017–1021
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