Synthesis and properties of two surfactants containing polyoxypropylene block and short branched alkyl chain

Article abstract of DOI:10.1016/j.molliq.2016.04.034

Short branched alkyl polyoxypropylene sulfates, sodium isohexyl polyoxypropylene sulfate (i-HPS) and sodium isooctyl polyoxypropylene sulfate (i-OPS) were synthetized via propoxylation, sulfation, and neutralization three-step reactions, and characterized by FT-IR and ¹H NMR. Their Krafft points were measured and the results were all below freezing point. Their critical micelle concentrations (cmc) and minimum surface tension (γ_cmc) determined by equilibrium surface tension were 54.74 mmol L^-1 and 32.12 mN m^{- 1} for i-HPS, 15.57 mmol L^-1 and 33.33 mN m^-1 for i-OPS, respectively. The measurement of dynamic surface tension indicated that both the equilibrium values of surface tension and the time required for reaching the equilibrium decreased with the increasing concentration of surfactant solutions. The spreading ability on paraffin film researched through dynamic contact angle revealed that the droplet of i-HPS solution presented a minimum contact angle of 58.9°at a concentration of 150 mmol L^-1, while the equilibrium contact angle of i-OPS droplet was 52.8°under the same condition. Their salt tolerance measurement showed that 1.0 wt% i-HPS solution and 1.0 wt% i-OPS solution could endure 183.1 g L^-1, 143.9 g L^-1 NaCl, 206.3 g L^-1, 198.4 g L^-1 MgCl₂, and 229.7 g L^{- 1}, 217.9 g L^-1 CaCl₂, respectively.

Full text of DOI:10.1016/j.molliq.2016.04.034

Journal of Molecular Liquids 220 (2016) 101–107
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Synthesis and properties of two surfactants containing polyoxypropylene
block and short branched alkyl chain
⁎
⁎
Wanxu Wang, Jianbo Li, Xiaoyi Yang , Ping Li , Chaohua Guo, Quanhong Li
China Research Institute of Daily Chemical Industry, Taiyuan, Shanxi 030001, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Short branched alkyl polyoxypropylene sulfates, sodium isohexyl polyoxypropylene sulfate (i-HPS) and sodium
isooctyl polyoxypropylene sulfate (i-OPS) were synthetized via propoxylation, sulfation, and neutralization
three-step reactions, and characterized by FT-IR and 1H NMR. Their Krafft points were measured and the results
Received 2 February 2016
Received in revised form 5 April 2016
Accepted 11 April 2016
Available online xxxx
were all below freezing point. Their critical micelle concentrations (cmc) and minimum surface tension (γcmc
)
determined by equilibrium surface tension were 54.74 mmol L−1 and 32.12 mN m−1 for i-HPS, 15.57 mmol L−1
and 33.33 mN m−1 for i-OPS, respectively. The measurement of dynamic surface tension indicated that both the
equilibrium values of surface tension and the time required for reaching the equilibrium decreased with the in-
creasing concentration of surfactant solutions. The spreading ability on paraffin film researched through dynamic
contact angle revealed that the droplet of i-HPS solution presented a minimum contact angle of 58.9° at a concen-
tration of 150 mmol L−1, while the equilibrium contact angle of i-OPS droplet was 52.8° under the same condi-
tion. Their salt tolerance measurement showed that 1.0 wt% i-HPS solution and 1.0 wt% i-OPS solution could
endure 183.1 g L−1, 143.9 g L−1 NaCl, 206.3 g L−1, 198.4 g L−1 MgCl2, and 229.7 g L−1, 217.9 g L−1 CaCl2,
respectively.
Keywords:
Branched tail
Propoxylation
Krafft point
Surface properties
Salt tolerance
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
surfactant through phase behavior observation and micelle structure in-
vestigation. Their experiments showed that the changes of the surfac-
Alkyl polyoxypropylene sulfates (APS), one kind of extended surfac-
tants, consist of hydrophobic alkyl chain, hydrophilic sulfate group, and
poly(propylene oxide) linker as intermediate polarity group to provide
a smooth transition between the hydrophobic and hydrophilic parts [1,
2]. The insertion of propylene oxide (PO) into hydrophilic head and hy-
drocarbon tail contributes to the surfactant not only extending its tail
length without sacrificing its own water solubility but also acting as
one component of the middle phase microemulsions which possess su-
perior solubilization capacity and ultralow interfacial tension (IFT)
[2–4]. The APS with such unique surfactant molecular structure have
been widely studied due to their extensive applications in soil and aqui-
fer remediation, enhanced oil recovery, pharmaceutical formulations,
nanoparticle preparations and froth flotation [5–7].
Phan T.T. et al. [8] researched the effect of structure of APS, which in-
volves the number of PO units and the branching of hydrophobic chain,
on the microemulsion formation and IFT values. They found that both
the optimum salinity and minimum IFT values decreased with the in-
crease of the inserted PO units or the degree of branching of the hydro-
carbon tail. Angelika K. et al. [2] studied the effects of a series of anions
and cations on the cloud point of a long hydrophobic chain extended
tant solubility caused by the addition of salts followed the same order
as Hofmeister series. Zeng J.X. et al. [9] synthetized a series of APS
with different PO adduct numbers through Williamson reaction, sulfat-
ing with chlorosulfonic acid and neutralizing with sodium hydroxide.
They determined the structure of synthesized compounds and found
out that APS could decrease the IFT between aqueous solution and
model oil to a low value.
The APS reported in the literatures above have shown excellent per-
formances in microemulsion, IFT, and salt tolerance. However, the sul-
fating agents used in the synthesis of APS cause a massive amount of
waste acid and other pollutants during the sulfonation reaction, which
lead to an increase in energy consumption further. Therefore, it is of
great importance to develop an environmental friendly and economical
sulfating method occupying vapor SO3. However, to the best of our
knowledge, the related research has not yet been reported. In addition,
the performances of APS were focused on their functions combined
with other components, while the physical and chemical properties of
single APS aqueous solution have not been investigated systematically.
Hence, an economical and practical sulfating method for synthetizing
APS is a pioneer of APS industrialization. The APS fundamental research
can expand its application to various fields such as coating technology,
mineral flotation and other surface science technology.
⁎
Corresponding authors at: China Research Institute of Daily Chemical Industry, 34
In this article, two APS with branching hydrophobic chain, sodium
isohexyl polyoxypropylene sulfate (i-HPS) and sodium isooctyl
Wenyuan Street, Taiyuan, Shanxi Province 030001, PR China.
E-mail addresses: yxywlyzx@163.com (X. Yang), yipingli_@126.com (P. Li).
http://dx.doi.org/10.1016/j.molliq.2016.04.034
0167-7322/© 2016 Elsevier B.V. All rights reserved.

102
W. Wang et al. / Journal of Molecular Liquids 220 (2016) 101–107
polyoxypropylene sulfate (i-OPS) were synthesized through sulfating
with vapor SO3. Their structures were confirmed by FT-IR and 1H
NMR. Sodium isooctyl polyoxyethylene sulfate (i-OES) and sodium
hexyl polyoxypropylene sulfate (HPS) were also synthesized through
the same method for comparing with i-OPS and i-HPS to show how
the differences between ethylene oxide (EO) groups and PO units,
branched chain and straight tail would impact on the Krafft point, min-
imum surface tension (γcmc), and critical micelle concentration (cmc).
Moreover, dynamic surface tension, dynamic contact angle and salt tol-
erance of i-HPS and i-OPS aqueous solutions were measured by bubble
pressure method, sessile drop technique, and ultraviolet-visible detec-
tor, respectively.
2.2.2. Structural characterization
The intermediate products (i-HP, i-OP) and target products (i-HPS, i-
OPS) were dissolved in respective absolute ethanol and each of the so-
lutions was smearing onto KBr prisms. Then, FT-IR for the products
was recorded via a Bruker Vertex-70 spectrometer, respectively. 1H
NMR spectra for the target product CDCl3 solutions was detected on a
Varian INOVA-400 Hz spectrometer, respectively.
2.3. Krafft point
The method for determining the Krafft points of experimental prod-
ucts and control products was described in literatures [10,11]. The solu-
tions with a concentration of 1.0 wt% were cooled to cloudy after the
process of using ultrasonic technique to form a homogeneous continu-
ous phase. Then, the heterogeneous solutions were heated gradually
to clear again under a heating rate of 0.5 °C min−1. The temperature
at which the system became clear was the Krafft point of surfactant.
For each measurement, the reported Krafft point was the average
value of three replications.
2. Experimental methods
2.1. Materials
Isooctyl alcohol (99%), n-octanol (98%), n-hexyl alcohol (98%), abso-
lute ethanol (99.7%), cyclohexane (98%), sodium chloride (NaCl 99.6%),
magnesium chloride (MgCl2 98%), and calcium chloride (CaCl2 96%)
were supplied by Tianjin Kermel Chemical reagent Co., Ltd. (China)
and used without further purification. Acetone (98%), sodium hydrox-
ide (96%), and fuming sulfuric acid (65% concentration) were purchased
from Tianjin Shentai Chemical reagent Co., Ltd. (China). Isohexyl alcohol
(98%) was provided by TCI Development Co., Ltd. (Shanghai). PO
(99.5%) was offered by Sinopharm Chemical reagent Co., Led. (China).
The catalyst was prepared by our workgroup in advance. The deionized
water with a resistivity of 18.25 MΩ∙cm prepared by a UPD-II ultrapure
water purifier was used in the preparation of all solutions.
2.4. Equilibrium surface tension
The surface tension (γ) was obtained by a Krüss K12 Processor Ten-
siometer (Krüss Company, Germany, accuracy 0.01 mN m−1) via the
Wilhelmy plate method at 25 0.1 °C. The recorded value of each con-
centration was the mean value determined from the three repeated
measurements with an interval of 30 s. The γ of ultrapure water
(72.0
0.3 mN m−1) was carried out before the measurement to
make sure that the silica dish had no impurity.
2.2. Synthesis and characterization
2.5. Dynamic surface tension
2.2.1. Synthesis of i-HPS and i-OPS
Two surfactants, i-HPS and i-OPS were synthesized by a three-step
reaction as shown in Scheme 1.
Dynamic surface tension was conducted on a Krüss BP-100 bubble
pressure tensiometer (Krüss Company, Germany, accuracy
0.01 mN m−1) with a range of effective surface ages from 10 to
The autoclave charged with isohexyl alcohol (102.17 g) or isooctyl
alcohol (130.23 g) and catalyst (0.70 g) was pumped and filled with ni-
trogen three times to remove oxygen under stirring and heating. Then,
PO (6 g) was put into the reactor to induce the reaction at a temperature
of 150 °C. When the system was heated to 165 °C by the energy of chem-
ical reaction, a required amount of PO (169 g) was gradually added into
the reactor under a stable pressure of 0.30 MPa. Finally, the mixture was
separated using filtration after aging and cooling. Thus, the intermediate
product with an average of 3 PO units, isohexyl polyoxypropylene ether
alcohol (i-HP) or isooctyl polyoxypropylene ether alcohol (i-OP), was
obtained.
200,000 ms at 25
0.1 °C. Ultrapure water was used for calculating
the internal diameter of capillary in advance.
2.6. Dynamic contact angle
Dynamic contact angle was measured using a drop shape analyzer
DSA-25 (Krüss Company, Germany, accuracy 0.1°) to judge the
spreading property of the target product aqueous solution on paraffin
film at 25
0.1 °C. The ultrapure water drop with a contact angle of
The target products, i-HPS and i-OPS, were sulfated, neutralized and
purified by the method described in our previous article [10]. The dried
surfactants, a light yellow wax-like solid at room temperature, cannot
crystallize owing to the polydispersity of the PO units [1].
(106 2)° on paraffin substrate [12] was performed before measuring
to ensure the system was clean. The final value of each sample was the
average data from three droplets recorded at an air humidity of (55
5)%.
Scheme 1. Synthetic routes of i-HPS and i-OPS.

W. Wang et al. / Journal of Molecular Liquids 220 (2016) 101–107
103
2.7. Salt tolerance
The results of structure characterization show that the target surfac-
tants, i-HPS and i-OPS, are synthesized successfully.
The transmittance of the 1.0 wt% surfactant aqueous solutions con-
taining different amount of salt at 600 nm was carried out via a UV-
1601 UV/VIS spectrophotometer (Beijing Rayleigh Analytical Instru-
ment Co., Ltd, China, photometric accuracy 0.3%T) at 15 2 °C to in-
vestigate the salt tolerance of the target products. The salts involving the
measurements include NaCl, CaCl2, and MgCl2. The solutions with differ-
ent content of salt were stirred before the measurement.
3.2. Krafft point
Krafft point is a specific temperature at which the ionic surfactant
aqueous solution presents an equilibrium among micelles, monomers,
and undissolved surfactant at a certain pressure. In other words, Krafft
point is the temperature where the solubility of ionic surfactant equals
to its cmc at a given pressure. The solubility will have a rapid augmen-
tation and the micelles are formed in a large amount when the temper-
ature of solution slightly exceeds its Krafft point [13,14]. The
temperature involving such useful properties exerts an increasing im-
portant effect on foam flotation and metal cleaning at low temperature.
In general, the Krafft point of surfactant will be viewed as below 0 °C if
its 1.0 wt% solution still remain clarify when ice and water coexist. The
Krafft points of i-OPS, i-OES, i-HPS, and HPS are listed in Table 1 and all
below freezing point.
The introduction of short hydrophobic chain or branched tail to the
target products can reduce the intermolecular force between the surfac-
tant molecules in solid state [15]. The PO units extend the effective
length of carbon chain to a certain degree, but they do not sacrifice
the solubility of surfactant and instead introduce the branching to
weaken the interaction of surfactant. For i-OES, the reasons lead to its
low Krafft point have been discussed in our previous literature [10].
The surfactant with low Krafft point means that it has a high solubility
at low temperature, which can extend its actual application under dif-
ferent environment conditions.
3. Results and discussion
3.1. Structure of the products
Fig. 1 is the FT-IR spectra of the intermediate products and target
products. We emphatically analyze the information shown in Fig. 1
(A) in which reveals the FT-IR of i-HP (a) and i-HPS (b). The strong ab-
sorption peak of hydroxyl stretching region at 3400 cm−1 in Fig. 1(A-a)
is weaken significantly in Fig. 1 (A-b) through sulfating reaction. And
meanwhile, the intense symmetric stretching vibration peak of sulfur-
oxygen double bond at 1245 cm−1 and the broad stretching vibration
peak of C\O\S at 770 cm−1 appear in Fig. 1 (A-b). The appearance of
these two phenomena at the same time indicates that the hydroxyl
has been substituted by sulfate group [9]. In addition, the weak peak
of hydroxyl in Fig. 1 (A-b) could be caused by the sample smeared on
the KBr plate with a residue of ethanol [10].
The 1H NMR spectra of the target products are shown Fig. 2, in which
a number of information revealed in the two spectra overlaps seriously
owing to the surfactants belong to a homologous series. As a result of
the polydispersity of polypropylene segments in the products, the
chemical shifts of hydrogen atoms in molecules are superposed in the
spectra, which would lead to the indistinction of the splitting peaks.
While the number of hydrogen atom of each group can be calculated
by the integrate areas of respective proton peaks. The assignment and
corresponding number of hydrogen atom of peaks in Fig. 2 are listed
below, respectively.
For i-HPS in Fig. 2 (A), δ: 0.835 (6H,\CH3 of the isohexyl), 1.131 (4H,
\CH2\), 1.306 (9H,\CH3 of the PO groups), 1.427 (1H, methylidyne of the
hydrophobic chain), 3.312 (2H,\CH2\O), 3.493 (6H, O\CH2\methylidyne\),
3.916 (2H, O\CH2\), 4.696 (1H, methylidyne of the PO unit closest to the
sulfate group), 7.260 (CDCl3).
3.3. Equilibrium surface tension
The surface activity of i-OPS, i-OES, i-HPS, and HPS is evaluated by
the equilibrium surface tension. In Fig. 3, the γ of the four surfactants
aqueous solutions gradually decrease with the increase of their concen-
trations and finally reach respective plateaus at 25 °C, indicating that the
amphiphilic molecules diffuse to the air/liquid interface and then attain
saturation on the interface [16]. The curves of γ show two inflection
points versus the logarithm of the concentrations of surfactants solu-
tions. The appearance of the first inflection point may be caused by
the formation of the premicellar aggregates [17,18]. The cmc and corre-
sponding γcmc are obtained by the intersection point of fitting straight
lines of the second decreasing stage and the plateau portion. The satura-
tion adsorption values (Гmax) and the minimum area occupied by a sin-
gle amphiphilic molecule (Amin) at the air/liquid interface are derived
by the Gibbs adsorption isotherm equations [16,18] (Eqs. (1) and (2)).
The parameter, pC20 advanced by Rosen et al. [19], is computed by the
For i-OPS in Fig. 2 (B), δ: 0.871 (6H,\CH3 of the tail), 1.142 (8H,\CH2\),
1.241 (9H,\CH3 of the PO units), 1.489 (1H, methylidyne of the isooctyl),
3.331 (2H,\CH2\O), 3.533 (6H, O\CH2\methylidyne\), 3.904 (2H, O\CH2\),
4.719 (1H, methylidyne of the PO group nearest the sulfate group),
7.260 (CDCl3).
Fig. 1. FR-IR spectra of i-HP (A-a), i-HPS (A-b), i-OP (B-a), and i-OPS (B-b).

104
W. Wang et al. / Journal of Molecular Liquids 220 (2016) 101–107
Fig. 2. 1H NMR spectra of i-HPS (A) and i-OPS (B).
negative logarithm of surfactant concentration at which the surface ten-
sion of solution has a 20 mN m−1 reduction compared with that of pure
solvent (Eq. (3)) to denote the adsorption efficiency of surfactant. The
large pC20 implies that the amphiphilic molecules can adsorb at the
air/liquid interface easily and the γ of solution can be reduced at low
concentration of surfactant. Their numerical values are listed in Table 1.
flexible PO units structure near the head group, the length of hydropho-
bic chain and the presence of branching in the hydrophobic chain may
all attribute to the ranked and explain the Гmax order of the four
surfactants.
3.4. Dynamic surface tension
Γ max ¼ −ð∂γ=∂1 g CÞT =2:303nRT
ð1Þ
The adsorption kinetics of surfactant at the air/liquid interface is
commonly studied through the dynamic surface tension data from the
maximal bubble pressure technique [22]. The curves of dynamic surface
tension against the surface age for surfactant aqueous solutions with
different concentrations are shown in Fig. 4. It can be noticed that
both the reducing extent and the reducing rate of the surface tension in-
crease remarkably with the concentration of surfactant solution increas-
ing. The time needed to reach the absorption equilibrium, meanwhile, is
drastically shortened. In addition, the dynamic surface tensions of i-HPS
in Fig. 4 (A) are higher than these of i-OPS in Fig. 4 (B) with the same
concentrations, which has the same trend compared to the equilibrium
surface tension of i-HPS and i-OPS, indicating that i-OPS molecules pre-
fer to adsorb at the air/liquid interface. The reason caused the dynamic
behavior may be the interaction between hydrophobicity of the
branched tail and molecular weight under the premise of presenting
the same hydrophilic content and PO units [23]. The strong hydropho-
bicity contributes to the molecules escape from the bulk phase to the
air/liquid interface, while the high molecular weight slows down the
diffusion of surfactant molecules. In this case, the promoting effect of
Amin ¼ 1016=NAΓ max
pC20 ¼ −1 g C20
ð2Þ
ð3Þ
Where R, T, NA, n, and ∂γ/∂lgC represent the gas constant
(8.314 J mol−1 K−1), absolute temperature, Avogadro's constant,
Gibbs prefactor related to the species of surfactant adsorbed at the inter-
face (for ionic surfactant, n = 2), and the slope of the premicellar region
in the curve of γ against log concentration.
From Table 1, we can see that the cmc of i-OPS and i-OES are
15.57 mmol L−1 and 21.67 mmol L−1. They are smaller compared to
those of HPS and i-HPS which are 50.59 mmol L−1 and 54.74 mmol L−1
because the chain length of isooctyl is longer than that of isohexyl and
the hydrophobicity of isooctyl is higher than hexyl's [20]. Compared
with EO groups, the PO units with intermediate polarity, in a manner,
extend the length of hydrophobic chain which makes the cmc of i-OPS
lower than i-OES's. Owing to the reduction of the effective length of hy-
drocarbon chain by the introduction of branching, the cmc of i-HPS is
greater than that of HPS which has leaner tail and equal carbon number
compared to i-HPS. The γcmc of branched surfactants are lower than that
of straight surfactant as a result of the increasing of effective hydrocar-
bon density at the air/liquid interface contributed by the more hydro-
phobic branching structure [21]. The γcmc of i-OES is lower compared
to other surfactants, which may be caused by the increasing dispersion
of the surfactant molecular arrangement through embedding EO repeat
units and expanding the area of hydrophilic group. It is observed that
the Amin is sorted from big to small: i-OPS, HPS, i-HPS, and i-OES. The
Table 1
Krafft points and adsorption parameters for HPS, i-HPS, i-OPS, and i-OES aqueous solutions
at 25 °C.
Surfactants Krafft point cmc
γcmc
Гmax
Amin
pC20
°C
mmol L−1 mN m−1 mmol m−2 Å2
HPS
b0
b0
b0
b0
50.59
54.74
15.57
21.67
33.47
32.12
33.33
27.40
0.23
0.24
0.21
0.39
722.2 2.7
i-HPS
i-OPS
i-OES
692.1 3.1
808.7 3.3
422.9 3.5
Fig. 3. Surface tensions of HPS, i-HPS, i-OPS, and i-OES aqueous solutions against their log
molar concentrations at 25 °C.

W. Wang et al. / Journal of Molecular Liquids 220 (2016) 101–107
105
Fig. 4. Dynamic surface tensions of i-HPS (A) and i-OPS (B) aqueous solutions with different concentrations at 25 °C as a function of surface age.
hydrophobicity may have an edge compared to the impeditive function
of molecular weight in the diffusion process for i-HPS and i-OPS.
with hydrophobic chain pointed outward of the droplet while the
head group extended into water phase. The outer layer of the droplet
is similar to the alkane film with an excellent wetting ability on sub-
strate with low energy when the adsorption of surfactant approaches
saturation [26]. Thus, the essential of wetting ability of solution on
solid surface is the interaction between of solid surface and hydrophobic
region of surfactant. The difference between the contact angles of i-OPS
and i-HPS in Fig. 5 may be caused by the different tightness of hydrocar-
bon chain on the surface adsorption layer. The tighter the molecules ar-
range, the better the wetting ability is.
3.5. Dynamic contact angle
In the practical applications, such as washing and mineral flotation,
the surfactant is needed to help the solvent wet the oil/water or solid/
liquid interface. The spreading ability is a fundamental and applied
property of surfactant solution between two phases. The spreading per-
formance of i-HPS and i-OPS solutions on the substrate is investigated
through the most useful dynamic contact angle measurement, which
determines the angles between the air/liquid interface and the solid/liq-
uid interface with a common solid/liquid spot [24]. The contact angles of
droplets with different surfactant concentrations on the parafilm versus
the time are shown in Fig. 5, respectively. As can be seen from Fig. 5, the
contact angles of droplets decrease rapidly and reach lower equilibrium
values with the increase of the concentration of surfactants. Both the
starting and equilibrium contact angles of i-HPS in Fig. 5 (A) are larger
than those of i-OPS in Figure5 (B), which responds the phenomenon
of dynamic surface tension mentioned above.
The low contact angle involving the molecular orientation is decided
by the low γ of the droplet attributed to the branched tail structure of
the surfactant molecule. The molecules with stronger hydrophobic
chain are more eager to transfer from solution to the solid/liquid inter-
face, which is the hydrophobic effect driven by entropy [25]. The intro-
duction of branching and PO units makes a contribution to the
formation of closely packed monolayer which presents the structure
3.6. Salt tolerance
The presence of moderate salt in surfactant solution can improve the
interfacial activity, affect the aggregation behavior of molecules at inter-
face or in bulk phase, and change the partitioning coefficient of surfac-
tant in oil and in water [27]. However, excess salt in solution will lead
to precipitation of surfactant and cloudy solution, which finally invites
the reduction of related surfactant activities. The maximum salt toler-
ance of surfactant is a significant critical value to grasp the supplemental
dosage of salt for maximal surfactant activities. In addition, the critical
concentration of salt at which lead to surfactant precipitate is enslaved
to the structure, composition, and concentration of surfactant, regulated
by the species and valence of the counter ion, and temperature of the so-
lution. Fig. 6 (A) and (B) show that the transmittance of solutions with
1.0 wt% surfactant at 600 nm against the amount of salt added at 15
2 °C. The specific tolerance values of i-HPS and i-OPS to NaCl, MgCl2,
Fig. 5. Contact angles with time for droplets of i-HPS (A) and i-OPS (B) aqueous solutions with different concentrations at 25 °C.

106
W. Wang et al. / Journal of Molecular Liquids 220 (2016) 101–107
Fig. 6. Transmittance versus electrolyte concentration for i-HPS (A) and i-OPS (B) aqueous solutions with a concentration of 1.0 wt% at 15 °C.
and CaCl2 in aqueous solution are listed in Table 2 when the transmit-
tance is 0.5. From it, we can see that the salt tolerance of i-OPS is
lower than that of i-HPS.
In addition, the dynamic surface tension data revealed that i-OPS mole-
cules were more disposed to adsorb at the air/liquid interface than i-
HPS molecules. Likewise, the measurement of dynamic contact angle
on paraffin film indicated that i-OPS molecules were more eager to
transfer from solution to the solid/liquid interface than i-HPS molecules.
These meant that surfactant molecules with stronger hydrophobic
chain would like to escape from the bulk phase to the hydrophobic in-
terface. However, the study on salt tolerance signified that i-HPS pos-
sessed an excellent stability of salt tolerance compared to i-OPS.
Hence, the introduction of branched tail and PO units to surfactant mol-
ecule can impact on the adsorption and aggregation behaviors of surfac-
tant, which will play a significant role in agriculture adjuvant, mineral
flotation, and other surface-related science.
Generally, when the aggregations form in the solution with a con-
centration above cmc, the salt will effect the aggregation behaviors of
ionic surfactant remarkably [27]. Such as at high salt concentration,
the aggregations are asymmetric micelles which will impact the rheo-
logical behavior and stability of the solution [28]. The hydrated layer
of head group of surfactant molecule and the electrical double layer of
micelle are compressed due to the strong electrostatic interaction
among counter ions, surfactant ions, and micelles, which leads to the
areas of head groups of surfactant in micelles decrease and the sizes of
micelles increase sharply. Several cations can be adsorbed at the sur-
rounding of one head group of surfactant because of the small size of in-
organic ions. The adsorption state may reduce the net charge of micelles
so that increase the size of micelles and decrease the surface charging
amount. Such changes may result in the reduction of electrostatic repul-
sion between micelles and the formation of larger aggregations. Fur-
thermore, the solution may become turbid and present phase
separation. While for sulfate surfactants, the two p-d π dative bonds be-
tween sulfur and oxygen of\SO3H group strengthen the electron with-
drawing ability of sulfur from hydroxyl, which makes hydroxyl
dissociate easily and free energy reduce. The stable\SO−3 has a weak at-
traction for cations, which hampers cations enter the hydrated layer of –\
O−3 easily. Thus, i-OPS and i-HPS present excellent Na+, Mg2+, Ca2+
tolerance.
Acknowledgements
This project is funded by the Shanxi Province Science Foundation for
Youths (No. 2015021052).
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Surfactant
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