Synthesis and properties of a branched short-alkyl polyoxyethylene ether alcohol sulfate surfactant

Article abstract of DOI:10.1016/j.molliq.2015.09.034

A branched alkyl polyoxyethylene ether alcohol sulfate, sodium isooctyl polyoxyethylene ether sulfate (i-OE₃S), was synthesized and its physic-chemical properties were investigated systematically. The experiment on Krafft point gave a result that the Krafft temperature of i-OE₃S was below 0 °C. The measurement of static surface tension showed that the critical micelle concentration (cmc) and surface tension at cmc (γ_cmc) of i-OE₃S were 21.67 mmol·L^{- 1} and 27.4 mN·m^{- 1}, respectively. From the results of dynamic surface tension (DST) measurements, we could obtain that the adsorption of i-OE₃S at air/liquid interface is controlled by the mixed diffusion-kinetic mechanism. Spherical assemblies with a diameter in the range of 263 to 606 nm in water were observed by transmission electron microscopy (TEM). The spreading performance of i-OE₃S aqueous solution on paraffin film was investigated by dynamic contact angle and the results demonstrated that i-OE₃S possessed an excellent spreading ability.

Full text of DOI:10.1016/j.molliq.2015.09.034

Journal of Molecular Liquids 212 (2015) 597–604
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Synthesis and properties of a branched short-alkyl polyoxyethylene
ether alcohol sulfate surfactant
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:
A branched alkyl polyoxyethylene ether alcohol sulfate, sodium isooctyl polyoxyethylene ether sulfate (i-OE S),
3
Received 12 August 2015
Received in revised form 16 September 2015
Accepted 21 September 2015
Available online xxxx
was synthesized and its physic-chemical properties were investigated systematically. The experiment on Krafft
point gave a result that the Krafft temperature of i-OE
3
S was below 0 °C. The measurement of static surface
S were
1.67 mmol·L− and 27.4 mN·m , respectively. From the results of dynamic surface tension (DST)
measurements, we could obtain that the adsorption of i-OE S at air/liquid interface is controlled by the mixed
diffusion–kinetic mechanism. Spherical assemblies with a diameter in the range of 263 to 606 nm in water
were observed by transmission electron microscopy (TEM). The spreading performance of i-OE S aqueous
tension showed that the critical micelle concentration (cmc) and surface tension at cmc (γcmc) of i-OE
2
3
1
−1
3
Keywords:
Branched tail
Polyoxyethylene groups
3
SO
3
vapor
3
solution on paraffin film was investigated by dynamic contact angle and the results demonstrated that i-OE S
Krafft point
Surface activity
possessed an excellent spreading ability.
©
2015 Elsevier B.V. All rights reserved.
1
. Introduction
that the introduction of EO units in the anionic surfactants might en-
hance the Ca -tolerance of original surfactants through molecular dy-
namics simulation.
2
+
Anionic–nonionic surfactants are widely used in the field of petro-
leum recovery, washing industry, environmental improvement of
water, coatings technology and other similar areas owing their excellent
properties in salt tolerance, Krafft points, surface properties and biode-
gradability [1–3]. Alkyl polyoxyethylene ether alcohol sulfates (AES)
are the most popular domains in anionic–nonionic surfactants. The
strong and close relationships between the structure of AES and their
physicochemical properties have been studied in recent years [3–7].
For instance, compared with common alkyl sulfates, both of the cmc
and Krafft points of the anionic–nonionic surfactants are much smaller
due to the existence of poly (ethylene oxide) groups, while their
temperature tolerance, salt tolerance and solubility are quite larger [3,
Compared to straight chain surfactants, the branched tail surfactants
possess excellent properties, including lower γcmc and Krafft points
which favor the practical applications [8,15]. The branched hydrophobic
groups are highly resistant to electrolyte, which results in an excellent
salt tolerance [16]. In addition, the branched products show superior
performance in the efficiency of decreasing surface energy and have
good natural biodegradation behavior at low temperature. These advan-
tages above make the branched alkyl ethoxy sulfates become research
hotspots in a number of fields. Varadaraj et al. [5,8,17,18] synthesized
a series of alkyl-branched ethoxy sulfates and studied their static
surface tensions, DST, dynamic contact angles, Krafft point and other
interfacial properties in details. In addition, they discussed the
structure–activity relationships of different surfactants through study-
ing thermodynamics of adsorption and micellization to answer their
superior surface properties and lower Krafft points. Huang et al. [19]
synthesized Guerbet hexadecyl sulfate by esterifying branched alcohol
7
–11]. Weil et al. [12] synthesized a series of sulfated ethenoxylated
tallow alcohols by esterifying ether alcohol with chlorosulfonic acid as
the sulfating agent. They found that the increase of average number of
ethylene oxide (EO) units (less than 5) leaded to a decrease in the
cmc, Krafft point, foam height and detergency of long straight chain
ether alcohol sulfates. Shinoda and co-workers [9,13] obtained a variety
of dodecyl polyoxyethylene sulfuric acid and discovered that the intro-
duction of EO groups could observably reduce the Krafft points of these
surfactants in the presence of multivalent cations and the variation of
cmc coincided with Weil's conclusion. Chen and Xu [14] researched
3
with liquid sulfur trioxide (SO ) solution in dichloroethane as the sulfat-
ing agent. They noted that the branched tail surfactant showed better
surface-active property, wetting ability and emulsifying ability than
sodium dodecyl sulfate (SDS). Jin et al. [20] discovered that Guerbet
tetradecyl polyoxyethylene ether sulfates possessed higher surface
activities than linear chain surfactants. They also showed that the
surfactant, with branched lipophilic chain and EO units being chosen
as hydrophobic and hydrophilic groups at the same time, possess even
more excellent surface activity than surfactants with only branched
tail but without EO groups.
⁎
Corresponding authors at: China Research Institute of Daily Chemical Industry, 34
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.2015.09.034
0167-7322/© 2015 Elsevier B.V. All rights reserved.

598
W. Wang et al. / Journal of Molecular Liquids 212 (2015) 597–604
The surfactants involving in the reports above are all focus on the
2
replaced by N three times. Then, EO (5 g) was added into the autoclave
long hydrophobic chain. But for the synthesis of sulfated ether alcohols,
long chain alcohols are easy to be oxidized or may present solid phase at
room temperature, which make the insertion of EO or propylene oxide
more different than that of branched short-tail alcohols [21]. Further-
more, the sulfating agents mentioned in the literatures above are
to induce the reaction. When the mixture was reheated to 160 °C, EO
(127.15 g) was continuous to be inputted into the reactor under a pres-
sure of 0.35 MPa gradually. Finally, the system was aged to keep the
pressure constant, pumped vacuum to remove the free EO at tempera-
2
ture below 80 °C, and filled with N for discharge. The product, with
chlorosulfonic acid or liquid SO
3
dissolving into dichloroethane. The
an average of about 3 EO, was obtained after removing catalyst by
residues such as waste acid or chlorine-containing compounds can
cause a burden to the environment and economy. Hence, a new
sulfating method for the synthesis of branched short-tail AES becomes
a challenged and meaningful research.
filtration.
3
2.2.2. Synthesis of i-OE S
1
) Preparation of liquid SO
into a 250 mL round-bottom flask equipped with an air-cooled
condenser and heated to 90 °C. Liquid SO was collected through
the condensation of SO vapor evaporating from the fuming sulfuric
acid and stored in a constant-temperature funnel at 36°C.
) Synthesis of i-OE S: The sulfation took place in a set-up assembled
by research group as shown in Fig. 1. Liquid SO (21.5 mL) was
added drop wise into the leftmost flask within half an hour and the
droplet was evaporated at 140 °C. The SO vapor was diluted with
3
: Fuming sulfuric acid (45 mL) was added
In this article,
polyoxyethylene ether sulfate (i-OE
SO vapor as the sulfating agent, and in doing so, the problem caused by
waste acid or chlorine-containing compounds can be alleviated greatly.
The structure and physicochemical performances of i-OE S were inves-
tigated systemically by various measurements. Sodium octyl ethoxy
sulfate (OE S) and sodium isooctyl sulfate (i-OS) were also synthesized
for comparing with i-OE S on Krafft point, static surface tension, and
dynamic contact angle. Moreover, DST and aggregation behavior of
a
branched short-tail AES, sodium isooctyl
3
S) was synthesized using bubbling
3
3
3
3
2
3
3
3
3
3
3
nitrogen which has a constant velocity of 0.14 m /h. Then, the
mixed well gas in the middle flask was passed bubble by bubble
into the i-OE (131.12 g) placed in a three-necked 500 mL round-
3
aqueous solution of i-OE
TEM, respectively.
3
S were measured by bubble pressure and
bottom flask at (40 ± 1) °C. The esterified product was neutralized
by 30 wt.% aqueous sodium hydroxide to pH = 8 gradually in an
ice–water bath.
2
. Experimental
2
.1. Materials and characterization
The raw product was dried under vacuum at 55 °C after extracting
Isooctyl alcohol, n-octanol, sodium hydroxide, absolute ethanol and
3
the unreacted i-OE with petroleum ether and filtered out inorganic
petroleum ether were from Tianjin Kermel Chemical Reagent Co., Ltd.
China). Acetone and fuming sulfuric acid (65%) were supplied by Tian-
salts in absolute ethanol. Because of the multiple distributions of EO
units, the product cannot crystallize [22]. After removing the solvent
(
jin Shentai Chemical reagent Co., Ltd. (China). EO was from Sinopec
Yanshan Co., Ltd. (China). The chemicals listed above were AR grade
and used directly without further purification. The alkaline catalyst was
provided by China Research Institute of Daily Chemical Industry (Taiyuan,
China). The deionized water with a resistivity of 18.25 MΩ·cm was used
from a UPD-II ultrapure water purifier.
3
by reduced pressure distillation, i-OE S presenting a viscous but pour-
able light yellow liquid was obtained at 55 °C, then becoming wax-like
solid at room temperature.
2.3. Krafft point
FT-IR for i-OE
3
S was detected from the product absolute ethanol
The technique for studying the Krafft point of surfactant was
referred to literature [23]. One percent solution of the surfactant was
whisked to dissolution extremely, then the solution was cooled to
cloudy and reheated slowly to clear at a rate of 1 °C·min . The temper-
ature at which the solution became one homogeneous phase was
recorded as the Krafft point.
solution smearing onto KBr prisms with a Bruker Vertex-70 spectrometer.
H NMR spectra was determined in CDCl
spectrometer.
1
3
using a Varian INOVA-400 MHz
−
1
2
.2. Synthesis
The product, i-OE
3
S, was synthesized via a three-step reaction
2.4. Static surface tension
according to Scheme 1.
Static surface tension measurements were performed using a
processor tension meter K122 (Krüss Company, Germany, accuracy
2
.2.1. Synthesis of isooctyl polyoxyethylene ether alcohol (i-OE
Isooctyl alcohol (130.23 g) mixed with catalyst (0.79 g) was placed
in an autoclave and heated to 140 °C after the air in the autoclave was
3
)
−
1
± 0.01 mN·m ) by the plate technique at (25 ± 0.1) °C. Ultrapure
−
1
water with a surface tension of (72.1 ± 0.2) mN·m
was used for
3
Scheme 1. The Synthetic route of i-OE S.

W. Wang et al. / Journal of Molecular Liquids 212 (2015) 597–604
599
Fig. 1. The simplified diagram of sulfation reactor.
calibration before measuring to prove that the system was thoroughly
clean.
was prepared by coating i-OE
3
S ethanol solution on potassium bromide
tablet, the residue of ethanol solvent may also contribute to the result.
1
3
The H NMR spectrum of i-OE S is illustrated in Fig. 3. In view of the
2
.5. DST
DST was investigated by a Krüss BP100 bubble-pressure tensiometer
multiple distributions of EO units of the compound, the chemical shifts
of hydrogen atoms are piled up, meanwhile, the peaks can just measure
the number of hydrogen atoms of characteristic groups through integral
quantities and chemical shifts. The assignments of the peaks are as
Krüss Company, Germany, accuracy ± 0.01 mN·m− ) at 25 °C.
1
(
Ultrapure water was performed for determining the internal diameter
of capillary before measuring. The parameter of effective surface ages
was set ranging from 10 to 50,000 ms.
follows: δ: 0.873 (6 H, −CH
methylidyne), 3.320 (2 H, −CH
3.921 (2 H, −O–CH –), 4.280 (2 H, −CH
The analysis of FT-IR and H NMR indicated that i-OE
3
), 1.267 (8 H, −(CH
–O–), 3.62–3.78 (8 H, −OCH
OSO –), and 7.260 (CDCl
S was successfully
2
)
4
–), 1.546 (1 H,
CH O–),
).
2
2
2
2
2
3
3
1
3
2
.6. TEM
obtained.
The microstructures of i-OE
3
S aggregates in solution were observed
through a JEM-1011 transmission electron microscopy (JEOL Co, Japan)
by negative staining at an acceleration voltage of 100 kV. A carbon-
coated grid was used as the substrate to load a thin sample film by
3.2. Krafft point
For micellization of ionic surfactants to occur at a given pressure,
there is an equilibrium state at which micelles and monomers are con-
comitant with solid hydrated surfactant at a particular temperature
which is called Krafft point. With further increase of the certain temper-
ature, the solubility of ionic surfactants exceeds its cmc and has a rapid
3
adding i-OE S solution and then staining reagent (2 wt.% phosphotung-
stic acid). In the process of film preparation, the first and second
solutions all need to equilibrate for at least 3 min and blot the excess
liquid with a filter paper at room temperature.
2
.7. Dynamic contact angle
The spreading ability of surfactants at the air/liquid/solid interface
was measured by the contact angle of droplet on hydrophobic substrate
using a drop shape analyzer DAS-100 (Krüss Company, Germany, accu-
racy ± 0.1°). Paraffin film for water with a contact angle of (106 ± 2)°
was chosen as the solid substrate [24]. Each experiment was repeated
at least three times at an environmental humidity of (50 ± 5)%.
3
. Results and discussion
.1. Structure characterization
The FT-IR spectra of i-OE
3
3
3
and i-OE S are shown in Fig. 2. From
Fig. 2(A), we can see that the major characteristic peak (−OH) of
−
1
i-OE
3
(B) at 3400 cm
disappears basically while the strong peaks
−
1
assigning to the C–O–SO
3
group for i-OE
55 cm are presenting. The phenomenon announces that i-OE
synthesized through the insertion of SO into the –OH of i-OE . The
presence of the weak hydroxyl peak in Fig. 2(A) is mainly due to the
high water absorptivity of i-OE S. Furthermore, since the test sample
3
S (A) at 1245 cm
and
−
1
7
3
S is
3
3
3
3 3
Fig. 2. FT-IR spectra of i-OE S (A) and i-OE (B).

600
W. Wang et al. / Journal of Molecular Liquids 212 (2015) 597–604
Fig. 3. 1H NMR spectrum of i-OE
S.
3
increase so that it enhances its applications in oil recovery and washing
industry [15,20]. The system of i-OE S appeared clear solution at−4°C
in this study, thus, we believe its Krafft point is below 0 °C. The data
for OE S and i-OS was listed in Table 1. It can be seen that i-OE S con-
taining both branched tail and EO units has a lower Krafft point than
the surfactants with single branched tail or EO units. The case is caused
by two reasons: (1) the intermolecular interaction of solid surfactant is
weakened by the introduction of branched tail, and (2) the presence of
EO unit makes a contribution to the formation of hydrogen bonds be-
tween the polar groups of surfactant and water [20,25]. Because of its
structure superiority, the surfactant molecules can separate from its
solid state and then combine with water molecules easily so that its sol-
ubility has an increase while Krafft point decreases.
are shown in Fig. 4. It can be seen that the γ of these surfactants decreased
with the variation of concentration which indicated that the surfactant
molecules have spread to air/liquid interface. Owing to the multiple distri-
butions of EO units of the compounds, forming the premicellar aggrega-
tions, a double inflection point phenomenon is provided in the curves of
γ against lgC [26]. The values of cmc and corresponding surface tension
(γcmc) are determined by the crossover point of two fitted straight lines
of the downward sloping portion and the plateau portion of the plots,
and then summarized in Table 1. It can be seen that the values of γcmc
3
3
3
−
1
for i-OS, OE
their cmc were 20.65, 14.11, and 21.67 mmol·L , respectively. The
values for OE S show a basic agreement with those of Li et al. [27]. As a re-
3 3
S, and i-OE S were 31.5, 28.5, and 27.4 mN·m , while
−
1
3
sult of the reduction of the effective hydrocarbon chain length by the in-
troduction of branching and the formation of steric hindrance to
3
.3 Static surface tension
3
micellization, the cmc of i-OE S is larger than the value of straight chain
surfactant with equal carbon atoms [8]. Since the insertion of EO groups
lead to the separation between hydrophobic chain and hydrophilic
groups, causing an increase in the area of the polar head groups and a dis-
The curves of surface tensions (γ) versus the logarithm of the bulk-
phase concentration in moles per liter for aqueous solution of surfactants
perse molecular arrangement in solution [28], the cmc of i-OE
have a decline comparing with that of i-OS. However, the result showed
that the cmc of i-OE S was nearly equal to that of i-OS. Barry and Wilson
29] gave an explanation that the effect of the polar group of surfactant
3
S should
3
[
molecule on cmc have a limit.
The surface excess concentration (Гmax) and the minimum area per
molecule (Amin) at the cmc are calculated by the Gibbs adsorption
isotherm equations [30] (Eqs. (1) and (2)) and listed in Table 1.
ꢀ
ꢁ
1
∂γ
Γ max ¼ −
ð1Þ
2
:303nRT ∂ lgC
T
Table 1
Parameters obtained from Krafft point and static surface tension measurements for i-OS,
OE S, and i-OE S.
3
3
Surfactants Krafft point cmc
γ
cmc
Г
max
A
min
pC20
mmol·L−
1
mN·m−1 mmol·m−2 Å2
°
C
i-OS
OE
i-OE
1
6.5
b0
20.65
14.11
21.67
31.5
28.5
27.4
0.62
0.52
0.39
268.8 2.5
316.5 3.3
422.9 3.5
3
S
Fig. 4. Plots of surface tension against log molar concentration of i-OS, OE
3 3
S, and i-OE S at
3
S
25 °C.

W. Wang et al. / Journal of Molecular Liquids 212 (2015) 597–604
601
1
016
Amin
¼
ð2Þ
ð3Þ
NAΓ max
pC20 ¼ − lgC20
where n is the Gibbs prefactor (for anionic surfactants, n = 2), R is
−
1
−1
the gas constant (8.314 J·mol ·K ), T is the absolute temperature,
and N is the Avogadro's constant. From Table 1, we can see that the
min for i-OE S solution at the air/liquid interface is larger than those
of OE S and i-OS, while its Гmax is the lowest. There are three factors
A
A
3
3
that contribute to the result: (1) the increased bulkiness of hydrocarbon
chain by the presence of branching, (2) the increased repulsion between
the polar groups by the effect of EO groups, (3) the flexible
polyoxyethylene chains presenting in curling structure [3,7,8,20]. The
adsorption efficiency at the air/liquid interface is indicated by a
parameter pC20 proposed by Rosen et al. [31]. It denotes the negative
logarithm of the surface active agent concentration (Eq. (3)), at which
−
1
Fig. 6. Variation of DST with t1/2 for i-OE
at 25 °C.
the surface tension is 20 mN·m
With the increase of the pC20 value, the adsorption efficiency of the
surfactant increases. The value of pC20 for i-OE S is 3.5, which is larger
than 3.3 and 2.5 for OE S and i-OS, respectively. The reason may be
lower than that of pure solvent.
3
S aqueous solutions with different concentrations
3
3
coefficient, subsurface concentration, and dummy time delay variable,
that the curly EO units fill the interval of voluminous branched tails.
Then, the close ordered assembly of surfactant molecules at the air/liquid
interface can produce a visible increase in the efficiency of surface tension
reduction [20].
respectively. When t is approaching zero, so is the value of C . Then,
the latter item indicating the surfactant diffusing back from subsurface
s
to bulk phase can be omitted. Finally, we obtain Fig. 6 and Eq. (5) involv-
ing a linear relation between γ(t) and t1 inferred from Henry's equation
for analyzing the short-time adsorption behaviors. When t tends to
infinite, equilibrium appears between the diffusion from subsurface to
/2
3
.4 Dynamic surface tension
bulk and that from bulk to subsurface, which means that C
equal to C . As a simplification, the integral formula is also ignored. In
this case, Fig. 7 and Eq. (6) also presenting a linear relation between
0
is nearly
The kinetics of surfactant adsorption is frequently studied by
DST data measured by the maximum bubble pressure technique at the
air/liquid interface [32–35]. The variations of DST data with surface
s
−
1/2
γ
(t) and t
is developed by Gibbs adsorption equation to process
age for i-OE
3
S solutions at different concentration are plotted in Fig. 5.
S take shorter time
the long-time surface tension data.
Where γ(t) and γeq refer to the surface tension at time t and t
approaching to infinite respectively, Гeq is the equilibrium surface
It shows that the concentrated solutions of i-OE
3
than the dilute solutions to reduce the equal surface tension coinciding
with Fick's first law.
s l
excess concentration, D and D indicate the diffusion coefficients for
For the solutions with concentrations below the cmc, the transfer
process of surfactant molecules from bulk phase to subsurface can be
illustrated by Word–Tordai equation [36]. The diffusion adsorption
model is described as Eq. (4).
short-time and long-time adsorption processes respectively, n is 2 for
ionic surfactant [34]. Both of short-time and long-time adsorption
models presenting the linear relations can be deduced by fitting linear
slopes to calculate the diffusion coefficient D listed in Table 2. For
rffiffiffiffiffi
rffiffiffi pffi
s
short-time adsorption process, D increases with the increase of solution
Z
t
ꢂ
ꢃ
pffiffiffiffiffiffiffiffiffi
Dt
π
D
π
concentration due to the presence of concentration gradient between
subsurface and bulk phase. For long-time adsorption behavior, Dl
decreases with the increase of solution concentration. We believe the
ΓðtÞ ¼ 2C0
−2
Csd t−τ
ð4Þ
0
0 s
The parameters Г(t), t, C , D, C , and τ denote the dynamic surface
concentration, reference time, bulk concentration, monomer diffusion
Fig. 7. Variation of DST with t− for i-OE
1/2
3
S aqueous solutions with different concentra-
3
Fig. 5. DST for i-OE S aqueous solutions with different concentrations at 25 °C.
tions at 25 °C.

602
W. Wang et al. / Journal of Molecular Liquids 212 (2015) 597–604
Table 2
3
Diffusion coefficients and corresponding coefficient ratios of i-OE S aqueous solutions
with different concentrations at 25 °C.
Concentration (mmol·L−1)
D
2
(m ·s−1)
D
2
(m ·s−1)
s
l
s l
D /D
1.76 × 10−
1.92 × 10−
2.64 × 10−
2.85 × 10−
5.47 × 10−
11
11
11
11
11
3.49 × 10
1.69 × 10
5.08 × 10
7.54 × 10
1.00 × 10
−13
50.43
0.10
0.47
0.94
1.88
5.13
−14
−15
−16
−16
3
3
4
5
1.14 × 10
5.20 × 10
3.78 × 10
5.47 × 10
main reason is that the free motion among the molecules is limited
when the surfactant concentration of subsurface is approaching to
that in bulk phase for long-time adsorption. The ratios of D
s l
/D are also
listed in Table 2. The large ratios indicate that the adsorption process
3
of i-OE S is controlled by the mixed diffusion–kinetic adsorption.
rffiffiffiffiffiffiffi
Dst
π
γ
γ
¼ γ −2C0nRT
ð5Þ
ð6Þ
ðtÞ
ðtÞ
0
Fig. 9. Variation of dynamic contact angle of i-OE
centrations on parafilm surface at 25 °C.
3
S aqueous solutions with different con-
rffiffiffiffiffiffi
nRT
π
2
eq
¼ γeq
þ
Γ
2C0
D t
l
cooperation between the branched hydrocarbon hydrophobic forces
and hydrogen bonding on the EO hydrophilic shells.
3
3
.5 Aggregation behavior of i-OE S in aqueous solution
3.6. Dynamic contact angle
When the concentration of surfactant aqueous solution is above its
cmc, surfactant molecules can self-assemble into ordered architecture
to minimize the interfacial energy [37]. In order to investigate the
The contact angle (θ) is used to investigate the air/liquid/solid
interfacial properties reflecting a time-dependent adsorption balance
of surface active agent [39]. Fig. 9 shows the θ of i-OE S aqueous solu-
morphology and size of the aggregates formed in i-OE
3
S solutions,
3
TEM, an intuitive and reliable technique, was performed to visualize
their ultra-structure. The TEM images show that the self-assembled
tions on paraffin film as a function of time with different concentrations.
It indicates that the equilibrium value of θ decreases with an increase in
microaggregates of i-OE
diameters from 263 to 423 nm at a concentration of 100 mmol·L
and from 273 to 606 nm at 200 mmol·L in Fig. 8.
3
S present spherical structures with a range of
concentration. The dynamic spreading behaviors of i-OE
OE
3
3
S, i-OS, and
S with equal concentration (80 mmol·L ) were studied by the
variation of θ near the leading edge versus the time on paraffin film in
Fig. 10, in which θ of i-OE S reaches a lower value (51.51°) in a short
−
1
−1
,
−
1
Compared to the normal spherical micelles with diameters of less
than 100 nm, the aggregates shown in Fig. 8 are larger [38]. Aoudia M.
et al. [3] took the view that the micelle size of AES, increasing with the
EO units, has no relation with the average micelle aggregation number
but with the micelle structure alteration due to the rearrangement of
the surfactant molecules in the micelle. For the formation of micelles,
a loose structure is needed to accommodate the larger polar head
groups due to the introduction of EO units in the surfactant which lies
at the periphery of the micelle–water. Moreover, it is widely accepted
that the presence of condition near thermodynamic equilibrium and
the driven forces such as hydrophobic forces, hydrogen bonding and
van der Waals all make some degree of contribution to the formation
and isolation of aggregates [32]. In this case, we believe that the
amphiphilic aggregates formed in water were mainly driven by the
3
time. The spreading behavior can be divided into short-time process
and long-time process. The former is called fast spreading controlled
by the gradient of surface tension along the air/liquid interface, while
the latter is known as slow spreading driven by the transfer of
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[18] held the view that the rigid structure and increased Amin of
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the branching lower the θ of droplet and enhance the water spreading
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structure like bilayer is formed on a low energy substrate to spread
water rapidly. The hydrophobic chains of the first layer are absorbed
on the solid surface while polar groups extend into water, meanwhile,
S solutions: (A) 100 mmol·L− and (B) 200 mmol·L−1
1
.
Fig. 8. Negative-stained TEM images of aggregates formed in i-OE
3

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[
[
[
[
H )
2 4 i
3
3 3
Fig. 10. Dynamic contact angle for i-OE S, i-OS, and OE S on parafilm surface as a function
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4
. Conclusions
[
A branched short-alkyl ethoxy sulfate was synthesized using SO
3
1
vapor as sulfating agent and characterized by FT-IR and H NMR. The
measurements of Krafft point, static surface tension, DST, TEM, and dy-
namic contact angle gave a wealth of information involving the physico-
[
[
chemical performances and aggregation behaviors of i-OE
solution. Its Krafft point is lower than those of i-OS and OE
3
S aqueous
S. The cmc
3
[
[
−
1
−1
and γcmc of i-OE
3
S solution are 21.67 mmol·L
and 27.4 mN·m
and its adsorption mechanism is mixed diffusion–kinetic controlled
adsorption. The spherical aggregates with a range of diameters from
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2
63 to 606 nm are present in the solution with a concentration above
its cmc. The contact angle of its droplet can reach a lower value than
those of i-OS and OE S in a short time. We believe that the presence of
3
[
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3
S in agriculture
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Acknowledgment
The authors are grateful for the financial support of the Shanxi Prov-
ince Science Foundation for Youths (no. 2015021052).
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