Properties and sodium salicylate induced aggregation behavior of a tail-branched cationic surfactant with a hydroxyl-containing hydrophilic head

Article abstract of DOI:10.1039/c5ra22115c

A cationic surfactant with a Guerbet-type branched tail and hydroxyl-decorated head group was synthesized and characterized. Its properties including surface activity, dynamic surface tension, wetting ability, concentration/salt induced aggregation pattern transition and rheological responses of aqueous solutions were measured and analyzed. It was found that this new amphiphile possessed powerful surface activity (γ_cmc = 25.26 mN m^-1) and could enhance the spreading of an aqueous solution on a low energy solid surface (paraffin surface); while dynamic surface tension measurements implied that the diffusion rate of surfactant molecules, influenced by the presence of hydroxyl groups, had an impact on the wetting process. It was determined that the introduction of branching hydrophobes and hydroxyls into the amphiphilic material crucially contributed to the superior performances. Moreover, a visual transition with increasing concentration of its aqueous solution was observed, while the addition of the structure-forming additive sodium salicylate (NaSal) could highly improve the viscosity by inducing the micellar growth in the cationic system which was researched by rheological experiments. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were operated to investigate the transformation of aggregates which are responsible for the concentration/salt induced phase behavior transition or rheological responses.

Full text of DOI:10.1039/c5ra22115c

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Properties and sodium salicylate induced
aggregation behavior of a tail-branched cationic
surfactant with a hydroxyl-containing hydrophilic
head†
Cite this: RSC Adv., 2015, 5, 105952
*
Yongjie Zhang, Yunling Li, Yongbo Song and Jun Li
A cationic surfactant with a Guerbet-type branched tail and hydroxyl-decorated head group was
synthesized and characterized. Its properties including surface activity, dynamic surface tension, wetting
ability, concentration/salt induced aggregation pattern transition and rheological responses of aqueous
solutions were measured and analyzed. It was found that this new amphiphile possessed powerful
surface activity (gcmc ¼ 25.26 mN mꢀ1) and could enhance the spreading of an aqueous solution on
a low energy solid surface (paraffin surface); while dynamic surface tension measurements implied that
the diffusion rate of surfactant molecules, influenced by the presence of hydroxyl groups, had an impact
on the wetting process. It was determined that the introduction of branching hydrophobes and
hydroxyls into the amphiphilic material crucially contributed to the superior performances. Moreover,
a visual transition with increasing concentration of its aqueous solution was observed, while the addition
of the structure-forming additive sodium salicylate (NaSal) could highly improve the viscosity by inducing
the micellar growth in the cationic system which was researched by rheological experiments. Scanning
electron microscopy (SEM) and transmission electron microscopy (TEM) were operated to investigate the
transformation of aggregates which are responsible for the concentration/salt induced phase behavior
transition or rheological responses.
Received 22nd October 2015
Accepted 7th December 2015
DOI: 10.1039/c5ra22115c
www.rsc.org/advances
low values (gcmc ¼ 24–27 mN mꢀ1), which is even comparable
to certain uorocarbon surfactants.12,13 In addition, there are
Introduction
distinct advantages of branched hydrocarbon surfactants over
uorocarbon materials in environmental issues, since uoro-
carbons have been suffering limitations from its bio-
accumulation and physiological toxicity.14,15 Moreover, it has
been well cognized that the presence of hydroxyl groups on
hydrophilic moiety could bring improvement in performance
of surfactants,16–18 since intermolecular hydrogen bonding
formed between hydroxyl groups reinforces the interaction
between molecules and further inuences their aggregation
behavior in aqueous solution. Therefore, to incorporate
hydroxyl groups into molecules is an important and practical
methodology to create new amphiphiles with desirable nature
and performances.
In the past two decades, there has been tremendous amount
of studies focusing on the viscoelastic micellar solutions gener-
ated by cationic surfactant–salt solution systems19–21 because of
their fascinating evolution of nanoscale architectures manipu-
lated by halide anions or other structure-inducing counter ions
among which the sodium salicylate is the most extensively used
one.22–24 These facilely tuned self-assembly nanostructures are
believed to have potential applications in preparation of micro-
scopic functional materials where microstructured templates are
It has been an important issue to explore new surfactants with
novel structures, because alteration of chemical structures
could contribute to unique properties which might have
potential applications or theoretical research values.1 It has well
been perceived that nonionic surfactants are more vulnerable to
producing low surface energy in aqueous solutions due to the
absence of electrostatic repulsion between hydrophilic heads.
However, very low surface energy could also be achieved by
specially modifying the hydrophilic head or changing the
hydrophobic structure of ionic amphiphiles.2–5
Branched hydrophobic tail could generally enhance the
surface activity or other interfacial properties of surfac-
tants.6–11 There has been a few works that reported extremely
low surface energy achieved by introducing branched tail into
structure of surfactants. Alexander et al.5 synthesized a series
of anionic surfactants with highly branched single hydro-
phobic chains and reduced the surface tension to extremely
China Research Institute of Daily Chemical Industry, Wenyuan Str. No. 34, Taiyuan,
030001, P. R. China. E-mail: lyunl0068@sina.com; Fax: +86-351-4040802; Tel: +86-
351-4046827
† Electronic supplementary information (ESI) available: Experimental and
spectral data. See DOI: 10.1039/c5ra22115c
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needed.25,26 A variety of cationic surfactants27 have been engaged (Lambda-35, Perkin-Elmer) with wavelength set to be 650 nm.
in those studies including cetyltrimethylammonnium bromide TEM observation was carried out on a JOEL JEM-1011 (Japan) at
(CTAB), cetyltrimethylammonnium chloride (CTAC), cetylpyr- an accelerating voltage of 100 kV. Samples of TEM were
idinium chloride (CPC) et al. and the morphology of micro- prepared by immersing an ultrathin carbon-coated copper grid
structures or the rheological properties are generally modulated into the micellar solutions, which was then freeze-dried. Field
by varying the surfactant concentration, salt/surfactant molar emission scanning electron microscopy (FE-SEM) was carried
ratio and temperature.
out on a JEOL JSM-6700F. About 5 mL sample was dripped on
In recent years, various cationic surfactants with hydroxyl- a silica substrate and then freeze-dried for SEM observation.
contained head groups have been researched, yet the Guerbet- The rheological measurements were carried out on a HAAKE
type branched one decorated by hydroxyethyl has never been RS75 rheometer with a cone-plate system (Ti, diameter, 35 mm;
reported. In this paper, a high performance cationic surfactant cone angle, 1ꢂ). The samples were measured at 25 ꢁ 0.2 ꢂC with
Guerbet-cetyl dimethyl hydroxyethyl ammonium chloride (G- the assistance of a cyclic water bath.
CDHAC) was subtly constructed. Compared with our previous
Synthesis. The branched hydrophobic tail of G-CDHAC
work,28 where
a
Guerbet branching-contained quaternary (whose synthetic route was shown in Scheme S1†) stems from
ammonium salt (G-CTAC) without decoration of hydroxyl was Guerbet-cetyl alcohol (2-hexyl-1-decanol), which belongs to
reported, the presence of hydroxyl group on G-CDHAC a sufficiently researched category of alcohols known as Guerbet
improved the surface activity of the Guerbet-type amphiphile alcohol.29–32 The 2-hexyl-1-decanol in our work was synthesized
to an even higher degree. Therefore, this low surface tension from n-octanol via Guerbet reaction,33 which was described in
material might have promise in substituting some of the ESI.†
expensive and biologically toxic uorocarbon materials. In
In order to acquire nal quaternary ammonium surfactant,
addition, other typical features like wetting ability, dynamic the tertiary amine Guerbet-cetyl dimethyl amine was synthe-
surface tension and self-assembled microstructures, which are sized as intermediate compound by reacting 2-hexyl-1-decanol
correlated with the inherent structure of the new amphiphile and dimethylamine. The amination reaction, which has been
and the consequent intermolecular interactions, were also dis- a mature technology known as one-step amination,34 was cata-
cussed. In order to explore the potential research value of the lyzed by Cu–Ni composite catalyst prepared by co-precipita-
new material, we investigated the surfactant–NaSal mixed tion.35,36 Detailed synthetic procedure of tertiary amine was
systems of G-CDHAC and G-CTAC by electron microscopy and stated in ESI.†
rheological measurements and some encouraging results were
found.
The Guerbet-cetyl dimethyl amine was neutralized by
equivalent amount of HCl and subsequently reacted with
ethylene oxide at 60–70 ꢂC for 3 h in an autoclave to produce the
cationic surfactant G-CDHAC. Another quaternary ammonium
compound Guerbet-cetyl trimethyl ammonium chloride
(G-CTAC) was also synthesized by reacting Guerbet-cetyl
Experimental section
Materials and instruments
Chemicals and materials. Dimethylamine with purity higher dimethyl amine and chloromethane at similar conditions
than 99% was obtained from Taiyuan Fertilizer Plant, China. (60–70 ꢂC, 3 h) under the catalysis of Na2CO3 as was reported in
Ethylene oxide was obtained from Sinopec Yanshan Co., Ltd our previous work.28
(Beijing China). Hydrogen was purchased from Taiyuan Iron &
The nal products were rstly puried by extraction, for
Steel Co., Ltd of China. Sodium salicylate, n-octanol, ethanol, which the mixture of highly deionized water (75% of volume
petroleum ether (60–90), acetone and diethyl ether were of fraction) and ethanol (25% of volume fraction) was engaged as
analytical reagent grade (A.R.) and purchased from Tianjin aqueous phase while petroleum ether was used as oil phase,
University Chemical Experiment Plant and Tianjin Kemiou and then recrystallized from acetone/diethyl ether mixed
chemical reagent Co., Ltd.
solvent.
The rened quaternary ammonium salt was detected by
Experimental techniques. FT-IR spectrum was recorded on
a Bruker Vertex 70 FT-IR spectrometer (Germany). 1HNMR water/chloroform bi-phase titration (titrated by sodium tetra-
spectra were detected by a Varian Inova-400 spectrometer phenylborate standard solution, indicated by bromophenol
(USA).
blue), the result of which determined that the average content of
¨
Equilibrium surface tension was measured by a KRUSS K12 cationic active substance is higher than 97 wt%. The molecular
Processor Tensiometer (ring method, single measurement) at structure of the G-CDHAC is shown in Fig. 1, and the corre-
1
ꢂ
temperature of 25 ꢁ 0.2 C. Every sample solution was equili- sponding HNMR and FT-IR signals were detected as below:
brated for 10 min before measurement. Contact angle test was
1HNMR (300 MHz, D2O): 3.98 (br s, 2H, VIII), 3.46 (br s, 2H,
¨
carried out on a KRUSS DSA255 Drop Shape Analyzer at room VII), 3.29 (br s, 2H, VI), 3.09 (s, 6H, V), 1.87 (br s, 1H, IV), 1.41 (br
temperature (25 ꢂC), employing paralm as solid substrate. s, 4H, III), 1.26 (br s, 20H, II), 0.84–0.80 (m, 6H, I); solvent peak
¨
Dynamic surface tension was tested by a Kruss BP100 bubble- appeared at 4.70 ppm. The spectrum was shown in Fig. S1.†
¨
pressure tensiometer (Kruss Company, Germany), whose
Chemical shi of the proton on –OH group was not detected
range of effective surface age varied from 10 to 200 000 ms. because the active hydrogen exchanges rapidly with D2O. Thus
Temperature was maintained to be 25 ꢁ 0.1 ꢂC. Transmittance in order to verify the presence of –OH, 1HNMR was again
measurement was operated on a UV-vis spectrophotometer operated using deuterated DMSO as solvent.
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Fig. 1 Molecular structure of G-CDHAC and the corresponding signal
designation of 1HNMR.
1HNMR (300 MHz, DMSO-d6): 5.77 (br s, 1H, IX), 3.84 (br s,
2H, VIII), 3.44 (t, J ¼ 4.8 Hz, 2H, VII), 3.28/3.29 (d, J ¼ 3.6 Hz, 2H,
VI), 3.10 (s, 6H, V), 1.95 (br s, 1H, IV), 1.32 (br s, 4H, III), 1.26 (br
s, 20H, II), 0.88–0.84 (m, 6H, I); solvent peak appeared at 2.510
ppm. as a quintet. The spectrum was shown in Fig. S2.†
FT-IR: characteristic peak of O–H stretching vibration
appeared at 3369 cmꢀ1; peaks at 2955, 2925 and 2856 cmꢀ1 are
attributed to the symmetric and asymmetric C–H stretching
vibrations of CH, CH2 and CH3. Bending vibrations of CH2 and
CH3 were detected at 1465 and 1378 cmꢀ1. C–O stretching
signal was located at 1048 cmꢀ1. The spectrum was shown in
Fig. S3.†
Preparation of G-CTAC/G-CDHAC–NaSal mixed micellar
solutions. Stock solutions of 50, 100, and 200 mmol Lꢀ1 G-CTAC
and G-CDHAC were prepared as well as those of sodium salic-
ylate (NaSal). The concentrations of sodium salicylate solutions
were calculated in advance to make the molar ratios of NaSal/G-
CTAC (CS/CA) and NaSal/G-CDHAC (CS/CB) to be 0.2, 0.4, 0.8, 1.0
and 2.0 aer mixing. Then stock solutions of surfactants and
sodium salicylate were mixed into vials to generate systems with
concentrations of 25, 50 and 100 mmol Lꢀ1 surfactant, while for
each concentration the NaSal/surfactant molar ratios were set to
be 0.2, 0.4, 0.8, 1.0 and 2.0. All the samples were equilibrated for
at least 2 weeks at 25 ꢂC before phase observation and other
measurements.
Fig. 2 Equilibrium surface tension plot versus concentration.
water–air interface, which endows the solution surface with
feature like surface of liquid hydrocarbon. On the other hand,
intermolecular hydrogen bonding highly enhanced the attract-
ing interaction between interface-adsorbed surfactant mole-
cules due to the presence of hydroxyl group on hydrophilic
head, which partially counterbalanced the electrostatic repul-
sion of positively charged heads (as is illustrated in Scheme 1).
Therefore, the interfacial amphiphiles are more tightly packed
and consequently minimize the dipolar effect on surface
tension because the electrostatic charge on ionic head is greatly
shielded.
Other basic parameters of G-CDHAC including critical
micelle concentration (CMC), maximum surface excess
concentration (Gm) and area per molecule at interface (A0) were
analyzed through linear tting or calculated via equations
below
Gm ¼ (ꢀ1/2.303nRT)(dg/dlog c)
A0 ¼ 1/GmNA
where NA represents Avogadro's constant, n is set to be
2 considering the subject is a thoroughly dissociated ionic
surfactant without addition of other electrolyte. The results are
presented in Table 1.
Results and discussion
Equilibrium surface tension
Surface activity is the most basic property of surfactant. The
equilibrium surface tension of G-CDHAC aqueous solution as
a function of concentration was measured and the result was
depicted in Fig. 2. It could be interpreted that the G-CDHAC has
a powerful ability to decrease the water surface tension to 25.26
mN mꢀ1 (gcmc). In fact, there should be a limit to which the
hydrocarbon surfactants can decrease the water surface
tension, and that limit is undoubtedly the air–liquid interfacial
tension of the pure liquid hydrocarbon. Considering that the
gair–liquid of n-pentadecane and n-tetradecane are 26.2 mN mꢀ1
and 24.8 mN mꢀ1 respectively,37,38 the modication on molec-
ular structure in our work has greatly enhanced the potential
surface activity of cationic surfactant.
It is interesting to notice that the cross sectional area (A0) of
2
˚
G-CDHAC molecule is smaller than that of G-CTAC (116.5 A
according to ref. 28), which is absolutely ascribed to the pres-
ence of OH group on G-CDHAC forming hydrogen bonding and
further pulling the interfacial amphiphiles to be more closely
packed. As a consequence, a higher density of CH3 groups on
water–air interface is correlated with the smaller A0 value and
the contribution of hydroxyl to a lower gcmc of G-CDHAC than
that of G-CTAC is determined.
Wetting ability
Wetting ability of G-CDHAC on low energy solid surface was
On one hand, the branched structure of hydrophobic tail measured by contact angle analysis, the result of which indi-
leaves the water surface to be more densely covered by terminal cated that the new cationic surfactant could enhance the
hydrocarbon groups of surfactant molecules adsorbed on spreading of aqueous solution on paraffin surface (Fig. 3). As is
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Scheme 1 Illustration of mechanism for low surface energy created by G-CDHAC and for aggregation behaviors in aqueous systems affected by
sodium salicylate.
Table 1 Surface adsorption properties of G-CDHAC
which reects the decrease of surface tension. It took less time
for bubble surface tension to reach equilibrium value at higher
2
˚
Parameters
G-CDHAC
CMC (mmol Lꢀ1
2.63
)
Gm (10ꢀ6 mol mꢀ2
)
A0 (A )
concentrations (Fig. 5a) because of the accelerated bulk-to-
surface diffusion rate. Moreover, it was vindicated to nd that
the decreasing rate of surface tension for G-CDHAC was higher
than it was for G-CTAC in the initial seconds aer bubble
formation, as could be seen from the curves of dynamic surface
tension versus square root of time (Fig. 5b). It could be specu-
lated from the result of dynamic surface tension that G-CDHAC
molecules diffuse faster to interface, which is consistent with
the paraffin-wetting process.
We proposed that the effect of hydrogen bonding between
orderly assembled interface-adsorbed amphiphiles and those in
subsurface bulky phase reinforced the aggregation potential onto
interfacial phase, considering that amphiphiles in bulky solution
move in disorder and are in statistically force-balanced state.
1.84
90.25
shown in Fig. 4a, the contact angle could be decreased to lower
than 29ꢂ at concentration of 100 mmol Lꢀ1. As concentration
increased, the wetting ability of G-CDHAC solution was obvi-
ously improved and there was a rapid decrease in equilibrium
contact angle during concentration range of 0.5ꢃ5 mmol Lꢀ1
.
This phenomenon is in accordance with the description of
young's equation:
gsg ¼ glg cos q + gsl,
where q is contact angle, and gsg, glg and gsl represent interfa-
cial energy of solid–gas, liquid–gas and solid–liquid interfaces
respectively. While gsg and gsl could be presumed as constant
due to the internal nature of paraffin and water, cos q value
must increase to equilibrate the equation as the surface tension
of solution (glg) decreases, which leads to enhanced spreading
of liquid drops on solid surface.
Phase behavior observation modulated by concentration/salt
addition
Visual appearance change with concentration. It was inter-
esting to observe that the visual appearance of G-CDHAC
Contact angle curves of solutions at concentrations higher
than 5 mmol Lꢀ1 exhibited signicant decrease in the initial
a few seconds, and the decreasing rate was higher for G-CDHAC
than it was for G-CTAC,28 as is depicted in Fig. 4b. This is
supposed to be correlated with the diffusion of surfactant
molecules. Therefore, in order to gain insight into the under-
lying mechanism of time-dependent wetting process, dynamic
surface tension measurement was operated and recorded.
Dynamic surface tension
The dynamic surface tension was detected by bubble pressure
method. With time increasing, amphiphiles tend to aggregate
Fig. 3 Representative photographs taken by Drop Shape Analyzer
which illustrates gradually enhanced spreading of G-CDHAC solution
onto bubble–solution interface and reduce the bubble pressure, on paraffin surface with increased concentration.
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Fig. 4 Dynamic contact angle curves as a function of time for (a) different concentrations of G-CDHAC; (b) comparison between G-CDHAC and
G-CTAC on their spreading process.
Fig. 5 (a) Dynamic surface tension of bubble formed in G-CDHAC solutions of different concentrations and (b) comparison between G-CDHAC
and G-CTAC on their decreasing rate of bubble surface tension as function of [time]1/2
.
solution exhibited transparent–translucent–transparent transi-
To obtain the concentration and molar ratio where G-
tion as concentration increased. The aqueous solution became CDHAC/G-CTAC-NaSal mixed systems can form homogeneous
turbid (or blue) during concentration range of 1–10 mmol Lꢀ1 and stable micellar solutions, macroscopic states of samples
without addition of any other substances and Tyndall
phenomenon occurred to the turbid solutions irradiated by
laser light of 650 nm wavelength (Fig. 6). Their turbidity was
conrmed via transmittance measurement on a UV-vis spec-
trophotometer (as presented in Fig. 6). It could be determined
that the sample of 5 mmol Lꢀ1 exhibited lowest transmittance
and appeared in light blue color. The internal architecture
accounting for the turbidity was further investigated by micro-
scopic characterization, as would be discussed below. As the G-
CDHAC concentration increased to 20 mmol Lꢀ1 or higher, the
solution turned transparent again.
Phase behavior of G-CDHAC/G-CTAC–NaSal mixed system.
Sodium salicylate has been frequently employed to induce the
micellar growth of amphiphilic cations (CTA+, CP+, etc.),
therefore we presume that the newly synthesized cations
(G-CDHA+ and G-CTA+) could also be modulated by this
Fig.
6 Visual appearance of G-CDHAC solutions changing with
benzoate derivative counter ion to form valuable supramolec-
ular assemblies.
concentration and their transmittance for 650 nm light detected by
UV-vis spectrophotometer.
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were observed and recorded by photography (Fig. 7) and phase
diagram (Fig. 8). It could be seen that the increase of concen-
tration and molar ratio (CS/CA for NaSal/G-CTAC, CS/CB for
NaSal/G-CDHAC) would lead to the creation of two-phase.
Moreover, the NaSal-surfactant mixed system was more
inclined to be homogeneous while the molar ratio is main-
tained at 0.4. Fig. 7 showed the typical transformation of sample
state.
Fig.
9
SEM images of micrometer-scaled spherical aggregates
formed in 5 mmol Lꢀ1 G-CDHAC solution.
Microscopic investigation
Concentration induced transformation of aggregates. In
order to make sense of the visual appearance transition of the
G-CDHAC single-component solutions, transmission electron
microscope (TEM) was operated on the sample of 5 mmol Lꢀ1
G-CDHAC solution which exhibited lowest transmittance.
Micrometer-scaled spheres were prevailingly observed in TEM
visions (data not shown). Scanning electron microscope (SEM)
was also conducted on the freeze-dried sample to conrm the
TEM results, and similar spherical assemblies of micrometers
were basically detected as expected (Fig. 9). Moreover, the SEM
images indicated that the micrometer-scale spherical aggre-
gates are composed of nanoparticles incorporated in networks
fabricated by nanowires, which is similar with the microstruc-
ture found by Chen et al. in their FcM-AOT ionic self-assembled
complex.39 The large size of aggregates explained the trans-
lucent appearance of the solution. Besides, the translucent
solution stayed unprecipitated for more than 3 months, exhib-
iting a homogenous solution state, which might be attributed to
the effect of Brownian motion and electrostatic repulsion.
As the G-CDHAC concentration increased to 20 mmol Lꢀ1 or
higher, the solution turned transparent again and the aggre-
gation pattern of G-CDHAC molecules was conrmed by TEM
on concentrated solution of 100 mmol Lꢀ1. As can be seen from
TEM visions (Fig. 10a and b), the amphiphiles self-assembled
into vesicles with diameter of about more than 100 nm. The
visions observed by SEM conrmed the formation of vesicles
(Fig. 10c and d), since the wrinkles are clear evidence of
collapsed vesicles. It is an extraordinary feature for single-
component aqueous solution of mono-alkyl ionic surfactant to
self-assemble into vesicles without induction of any other
additives, because most of vesicles are formed by double-
chained amphiphiles or in systems containing multiple kinds
of amphiphiles.40 Few reports have been issued on spontaneous
formation of vesicles by mono-chained surfactant in aqueous
solution without addition of negatively charged additives.41
Therefore, the self-assembly pattern of vesicles is a unique
character of the new material.
Microscopic visions of G-CDHAC/G-CTAC-NaSal mixed
systems. TEM was employed to characterize the structural
transformation of supramolecular assemblies of G-CDHAC and
G-CTAC with the tuning of sodium salicylate.
Fig. 7 Typical samples of phase behavior transition of NaSal-G-
CDHAC mixed systems modulated by concentration and CS/CB.
Fig. 8 Phase behavior record of mixed systems of (a) G-CTAC/NaSal and (b) G-CDHAC/NaSal.
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at higher concentration, as would be discussed in rheological
properties below.
In G-CDHAC/NaSal systems, there appeared different
aggregating behaviors as the concentration or molar ratio
varied. In the mixture of 25 mmol Lꢀ1 surfactant with molar
ratio (CS/CB) being 0.4, densely packed nano-particles were
observed, some of which aligned in rows and showed
a tendency of integrating into nanorods (Fig. 12a and b). While
the concentration CB increased to 50 mmol Lꢀ1 with a molar
ratio of 0.4, wormlike micelles were generated as observed in
TEM visions (Fig. 12c and d). Further increase of molar ratio to
0.8 could also promote the formation of wormlike micelles
(Fig. 12e and f). Scheme 1 illustrated the effect of sodium
salicylate in inducing the transformation of G-CDHAC
aggregates.
Rheological properties. The elongated micellar aggregates
might lead to signicant rheological responses. For instance,
a widely studied viscoelastic system of CTAB/NaSal micellar
solutions have been determined to display shear-thinning or
Fig. 10 Vesicles formed in 100 mmol Lꢀ1 G-CDHAC solution
observed by (a and b) TEM and (c and d) SEM techniques.
In G-CTAC/NaSal systems, only spherical aggregates could be shear-thickening properties depending on concentration or
observed (Fig. 11). As the concentration increased from 50 to salt/surfactant ratios. Therefore we conducted shear viscosity
100 mmol Lꢀ1, there occurred only increase in the amount of
measurements on G-CDHAC/G-CTAC-NaSal mixed systems
nano-spheres but no further growth or transformation of
aggregates. Nevertheless, the shear viscosity of the mixed
system was enhanced with the nanoparticles becoming denser
which showed homogeneous and stable state in phase behavior
observation.
It could be seen that the viscosity of the G-CDHAC/NaSal
mixed systems was enhanced with the increase of surfactant
concentration, while the increasing addition of NaSal (CS/CB)
could also improve the viscosity (Fig. 13b). In addition, all the
samples measured exhibited a shear-thinning behavior as the
shear rate increased. Cooperation of G-CTAC and NaSal could
also enhance the viscosity of aqueous system to different levels
depending on the concentration of the mixed system, as is
shown in Fig. 13a. We speculated that the reinforced aggrega-
tion behavior of the cationic surfactants induced by structure-
forming agent sodium salicylate, which was discussed in
microscopic investigation, should be responsible for the
occurrence of rheological responses.
Fig. 11 Microscopic characterization of (a) 50 and (b) 100 mmol Lꢀ1
G-CTAC/NaSal mixed systems with molar ratio (CS/CA) being 0.4.
Fig. 12 Microscopic characterization of G-CDHAC/NaSal mixed systems with concentrations and molar ratios (CS/CB) being (a and b) 25
mmol Lꢀ1 and 0.4; (c and d) 50 mmol Lꢀ1 and 0.4; (e and f) 25 mmol Lꢀ1 and 0.8.
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Fig. 13 Shear viscosity (h) curves of (a) G-CTAC/NaSal and (b) G-CDHAC/NaSal systems as a function of shear rate.
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A Guerbet type tail-branched quaternary ammonium surfactant
with a hydroxyl-decorated hydrophilic head was synthesized for
the rst time. This novel cationic amphiphile (G-CDHAC) was
found to be an excellent low surface energy material with a gcmc
of 25.26 mN mꢀ1, while the correlated wetting ability on low
energy solid surface was also distinct. The introduction of
branching hydrophobe and hydroxylated hydrophilic head was
proposed to be responsible for the outstanding performances of
the new amphiphilic material. The G-CDHAC molecules formed
complex aggregates in dilute solutions with slightly higher
concentration than CMC, and formation of vesicles in relatively
concentrated solutions without addition of any other inductive
agents was conrmed by TEM and SEM techniques. Further
investigations concerning salt (NaSal) induced transition of
supramolecular assemblies and the consequent response of
viscosity properties were also studied, and it was found that the
micellar growth in G-CDHAC aqueous system could be achieved
by modulating the addition of structure forming agent NaSal.
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Acknowledgements
We gratefully acknowledge nancial support from the National
Science and Technology Support Project of China (No.
2014BAE03B03) and Project for innovation capacity of system-
transformed scientic research institute, Ministry of science
and technology (2014EG111212).
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