Synthesis, Properties, and Aggregation Behavior of Tetrasiloxane-Based Anionic Surfactants

Article abstract of DOI:10.1021/acs.langmuir.8b00825

Three silicone surfactants, 3-tris(trimethylsiloxy)silylpropyl sulfonate with different alkaline counterions (lithium, sodium, and potassium), were synthesized for the first time. Their chemical structures were confirmed by FT-IR spectra, ¹H NM

Full text of DOI:10.1021/acs.langmuir.8b00825

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Interface-Rich Materials and Assemblies
Synthesis, Properties and Aggregation Behavior
of Tetrasiloxane-based Anionic Surfactants
Yue Huang, Lina Meng, Mengdong Guo, Peijian Zhao,
Hongyan Zhang, Shuwen Chen, Jie Zhang, and Shengyu Feng
Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00825 • Publication Date (Web): 22 Mar 2018
Downloaded from http://pubs.acs.org on March 23, 2018
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Synthesis, Properties and Aggregation Behavior of
Tetrasiloxane-based Anionic Surfactants
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Yue Huang, Lina Meng, Mengdong Guo, Peijian Zhao, Hongyan Zhang, Shuwen Chen, Jie
Zhang*, and Shengyu Feng*
Key Laboratory of Special Functional Aggregated Materials & Key Laboratory of Colloid and
Interface Chemistry (Shandong University), Ministry of Education; School of Chemistry and
Chemical Engineering, Shandong University, Jinan 250100, PR China
ABSTRACT: Three silicone surfactants, 3-tris(trimethylsiloxy)silylpropyl sulfonate with
different alkaline counterions (lithium, sodium and potassium), were synthesized for the first
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time. Their chemical structures were confirmed by FT-IR spectra, H NMR and ESI-MS, and
their behaviors in aqueous solutions were investigated by surface tensiometry, electrical
conductivity, dynamic light scattering, and different transmission electron microscopy
techniques. These anionic silicone surfactants exhibited remarkable surface activity, and could
reduce the surface tension of water to as low as 19.8 mN/m at the critical aggregate
concentration (CAC). The adsorption and aggregation behavior of these surfactants were
assessed by determining the adsorption efficiency, minimum average area per surfactant
molecule, and thermodynamic parameters. The cryo-TEM results verified these molecules could
form vesicles in water above CAC. Moreover the lowest surface tension, the smallest CAC value,
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and the biggest aggregate size have been reached with potassium counterion. Thus different
behavior of these surfactants in water can be explained by different size of hydrated ions.
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Keywords: Anionic silicone surfactants, counterions, aggregation in water, vesicles
Introduction
Silicone surfactants have performed important roles ranging from facilitating delivery of
drugs and agriculture ingredients to foam stabilization1-7. Like common hydrocarbon surfactants,
they have both hydrophobic and hydrophilic groups. The permethylated siloxane groups, which
act as hydrophobic groups, grafted with one or more hydrophilic group ensure the good surface
activity of silicone surfactants in both aqueous and nonaqueous solutions8-10. Their extraordinary
qualities attribute to some unique characteristics: (i) the flexible silicone bonds that make it
possible for the molecules to pack more efficiently at different interfaces; (ii) low cohesive
energy that contributes to greater volume and cross-sectional area of the siloxane unit; (iii) low
glass transition temperature even for high molecular weight surfactants. Due to the distinctive
chemical and physical properties, especially their low toxicity, wide operating temperature, and
high surface activities, silicone surfactants have been widely used and investigated in recent
decades1, 11. Particularly, trisiloxane surfactants are able to decrease the surface tension of water
to approximately 20 Nm/m12, 13
Cationic silicone surfactants have been commonly utilized as fabric softening agents14, and
also can be used as antibacterial agents due to the presence of abundant positive charges15, 16, 17
.
.
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However, the positive charges also limit their wide applications. Silicone surfactants with
different chemical structures show different aggregation behavior. Tan et al. investigated a series
of cationic silicone surfactants with the same hydrophilic groups but different hydrophobic
silicone groups. Their results indicated that the surfactant aggregation behavior highly related on
the length and spatial structure of the hydrophobic group18.
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Cationic silicone surfactants cannot be used as dyeing materials in the industry of yarn
spinning and textile manufacture for their positive charge19, 20. Alternatively, nonionic silicone
surfactants have been commercialized for textile, cosmetics, agriculture, printing, dyeing, and
other applications21-24
.
Comparatively, anionic silicone surfactants have yet to be investigated. Anionic
hydrocarbon surfactants have been used in a wide range of applications due to their special
properties, thus raising our research interest in anionic silicone surfactants. Recently, Xing et al.
reported a synthetic method for an anionic gemini silicone surfactant and tested its surface
activity25. Their obtained anionic silicone surfactant had remarkable surface activity in aqueous
solution. Ervithayasuporn et al. synthesized silicon-based organic-inorganic hybrid
silsesquioxane containing the maximum number of sulfonic acid groups through oxidation
reaction under mild conditions with a high yield26.
In this study, we synthesized three novel sulfonate anionic silicone surfactants (Si4ASO3K,
Si4ASO3Na, and Si4ASO3Li,) via thio-ester oxidation reaction. They all had the same chemical
structure on the tetrasiloxane chain (as the hydrophobic segment), and the sulfonate (as the
hydrophilic group), but different counterions were chosen. Our work present a convenient,
straightforward synthetic route to obtain sulfonate anionic silicone surfactants (Figure 1). We
investigated their structures and solution behaviors in detail. The tetrasiloxane-based structures
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are expected to orient themselves in an “umbrella-shaped” distribution of the hydrophobic
groups on the aqueous surface18, 27. Their surface activity was stronger (γCAC ≈ 20 mN/m) and
more efficient (CAC ≈ 4.5 m mol/L) than a conventional anionic surfactant, sodium
laurysulfonate (SLS) (γCMC ≈ 33 mN/m and CMC ≈ 12.4 m mol/L)28. DLS and TEM results
indicated that their aggregation behavior was influenced by the counterions, which provide a way
to obtain surfactant aggregations with controlled size and narrow distribution.
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Figure 1. Synthesis route of Si4ASO3M (M= K, Na and Li).
Experimental
Materials
Trimethylchlorosilane, γ-chloropropyltrichlorosilane, potassium thioacetate, sodium
thioacetate and lithium thioacetatewere purchased from Energy Chemical (shanghai).
Isopropanol, ethyl alcohol, N,N-dimethylethanolamine and dichloromethane were obtained from
Sinopharm Chemical Reagent Beijing Co.,Ltd. All the reagents were used as received. Triply
distilled water was used to prepare all the solutions.
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Methods
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1H NMR spectra were recorded by using a Bruker AV 300 spectrometer in chloroform-
d(CDCl3). FT-IR spectra were recorded by using a Nicolet FT-IR spectrometer, and samples
were dispersed in anhydrous KBr pellets for testing. ESI-MS data were recorded using an
Agilent Technologies 6510 Q-TF LC/MS and with methanol as solvent.
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Surface tension measurements were carried out on a model BZY-1 tensiometer (Shanghai
Hengping Instrument Co., Ltd, accuracy ± 0.1 mN/m) by Du Nouy ring method at 298.15 K. All
measurements were repeated until the values were reproducible.
Electrical conductivity measurements of the anionic sulfonate silicone surfactant solutions
were carried out using a S230 SevenCompactTM conductivity analyzer (Mettler Toledo
Instrument Co., Ltd., accuracy ±0.5%) with a thermostatic bath (DC-0506, Shanghai Hengping
Instrument Co., Ltd.).
The dynamic light scattering (DLS) measurements were performed using a Dynapro Titan
system (Wyatt Technology, Santa, Barbara, CA) at a scattering angle of 90o.
The morphologies of anionic silicone surfactants aggregations in aqueous solution were
examined with a JEM-1011 TEM (JEOL, Japan) at 100 kV. The samples were prepared by
dropping anionic silicone surfactant solution on a carbon coated grid. Phosphotungstic acid
solution (2 wt%) was used to stain the samples, and then the grids were dried at room
temperature.
Freeze-fracture transmission electron microscope (FF-TEM) (Leica, BAF060) was used to
investigate the microstructures and transition of self-assemblies formed by these amphiphilic
molecules in solutions. A little sample solution (normally 1 µL) was enclosed into a sandwich of
a copper support disk of 3 mm in diameter, thickness of 100 µm. The sandwich was held
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together by stainless steel tweezers. Then the copper sandwich was frozen by plunged into liquid
propane which had been cooled by liquid nitrogen. In order to maintain the structures of the
solutions and obtain the veritable microstructures of the solutions, a mechanical plunging device
was built for rapid transfer of the sandwich with sample solution into liquid propane. The
plunging rate and the depth under the liquid propane/air interface were controlled by the
mechanical device.
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Cryogenic transmission electron microscopy (cryo-TEM) was a vital way to investigate
the vesicles’ microstructures. The TEM-grid loaded with our sample solution was dropped into
liquid nitrogen instantaneous to maintain the microstructure of our products, and then the copper
was visualized by JEOL 1400 under frozen temperature.
Preparation of γ-chloropropyltri(trimethylsiloxy)silane (Si4ACl)
Chlorotrimethylsilane (0.12 mol, 13.03 g) and γ-chloropropyltrichlorosilane (0.02 mol,
4.23 g) were placed in a three-necked flask equipped with a Vigreux column and a dropping
funnel, and isopropanol (0.18 mol, 10.80 g) was added. Then, water (0.18 mol, 3.24 g) was
added drop wise into the resulting solution. After stirring at room temperature for 3 h, the
organic layer was separated from the hydrolysate, and washed with water till neutral reaction.
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The product was distilled in vacuum, and the yield was 95%. The H NMR (CDCl3) of Si4ACl
exhibits the following: δ (ppm) = 0.11~0.14 ppm (SiCH3, 27 H), 0.54~0.63 ppm (CH2CH2CH2Cl,
2H), 1.77~1.87 ppm (CH2CH2CH2Cl, 2H), and 3.50~3.55 ppm (CH2CH2CH2Cl, 2H).
Preparation of S-3-tris(trimethylsiloxy)silypropylethanethiolate (Si4ASAc)
A mixture of γ-chloropropyltri(trimethylsiloxy)silane (0.0134 mol, 5.0 g) and potassium
thioacetate (0.023 mol, 2.68 g) was added into a flask. Then, DMF (10 ml) was added to the
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mixture, which was stirred at room temperature for 16 h. Water (30 ml) was added into the
solution, and the organic layer was separated and washed twice. Then it was distilled in vacuum
to provide S-3-tris(trimethylsiloxy)silypropylethanethiolate (Si4ASAc) with the yield of 85%.
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The structure of Si4ASAc was confirmed by H NMR (CDCl3): δ (ppm) = 0.05~0.14 ppm
(SiCH3, 27 H), 0.49~0.55 ppm (SiCH2CH2, 2H), 1.54~1.64 ppm (CH2CH2CH2, 2H), 2.315 ppm
(CH3CO, 3H), 2.85~2.89 ppm (CH2CH2S, 2H).
Preparation of potassium 3-tris(trimethylsiloxy)silylpropyl sulfonate (Si4ASO3K)
Si4ASAc (0.01 mol, 4.128 g) was dissolved in ethanol (20 ml). Potassium carbonate (0.005
mol, 0.691 g) and hydrogen peroxide (H2O2) (0.03 mol, 3.4 g) was added and the mixture was
stirred in a neutral environment at ambient temperature for 20 h. The ethanol was removed by
vacuum distillation to give colorless oil. The oil was dissolved in 10 ml water and the solution
was extracted with CH2Cl2 (2 ml). The aqueous solution was dried by vacuum distillation to give
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a white solid, and the yield was 70%. Si4ASO3K. H NMR (CDCl3): δ (ppm) = 0.05~0.15 ppm
(SiCH3, 27 H), 0.48~0.53 ppm (SiCH2CH2, 2H), 1.73~1.84 ppm (CH2CH2CH2, 2H), 2.85~2.90
ppm (CH2CH2S, 2H); FT-IR (KBr, cm-1): 2,960, 2,901, 1,579, 1,414, 1,256, 1,198, 1,054, 844,
-
756, and 669. The stretching vibrational frequencies of –SO3 were observed in region of νmax
~1,198-1,054; ESI-MS (+ESI): 457.0888, calculated m/z (Si4ASO3K+H+): 457.07.
Synthesis of sodium 3-tris(trimethylsiloxy)silylpropyl sulfonate (Si4ASO3Na) and lithium 3-
tris(trimethylsiloxy)silylpropyl sulfonate (Si4ASO3Li).
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Si4ASO3Na and Si4ASO3Li were synthesized and characterized using the same method as
Si4ASO3K using lithium carbonate and sodium carbonate in place of potassium carbonate.
Si4ASO3Na: 1H NMR (CDCl3): δ (ppm) = 0.07~0.10 ppm (SiCH3, 27 H), 0.48~0.53 ppm
(SiCH2CH2, 2H), 1.73~1.84 ppm (CH2CH2CH2, 2H), 2.85~2.90 ppm (CH2CH2S, 2H); FT-IR
(KBr, cm-1): 2,961, 2,901, 1,579, 1,438, 1,257, 1,210, 1,056, 844, 756, 712 and 689. The
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stretching vibrational frequencies of –SO3 were observed in region of νmax ~1,210-1,056; ESI-
MS (+ESI): 441.1143, calculated m/z (Si4ASO3Na+H+): 441.1.
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Si4ASO3Li: H NMR (CDCl3): δ (ppm) = 0.08~0.10 ppm (SiCH3, 27 H), 0.48~0.52 ppm
(SiCH2CH2, 2H), 1.74~1.84 ppm (CH2CH2CH2, 2H), 2.83~2.90 ppm (CH2CH2S, 2H); FT-IR
(KBr, cm-1): 2,960, 2,900, 1,588, 1,449, 1,257, 1,205, 1,062, 847, 756, 702 and 606. The
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stretching vibrational frequencies of –SO3 were observed in region of νmax ~1,205-1,062; ESI-
MS (+ESI): 425.1266, calculated m/z (Si4ASO3Li+H+): 425.12.
Results and discussion
Surface tension results
The surface tensions of these anionic silicone surfactants in the aqueous solutions were
measured to evaluate the surface activity. Figure 2 shows plots of the surface tension (γ) versus
concentration (C) for the aqueous solutions of Si4ASO3K, Si4ASO3Na and Si4ASO3Li at 25 oC.
As shown in Figure 2, the surface tension decreased initially when the surfactant
concentration increased, suggesting the surfactants were adsorbed at the air/solution interface.
The plateaus which appear in the (γ-lg C) plots indicated that aggregates were formed. The
critical aggregation concentration (CAC) is regarded as the concentration at the break point of
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the two linear points of the γ-lg C plot. The estimation of these linear was based on Li’s papers29,
30, 31. In particular, we had examined how theγ-lg C curve behaves just below the CMC because
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this is the part that has the most influence on the value of ΓCMC obtained by ST-Gibbs. No
minimum appeared in the γ-lg C plot, indicating a high purity of these three surfactants.
The surface tension at CAC (γCAC) and surface pressure at CAC (πCAC) can also be used to
evaluate the surface activities of surfactants. The πCAC is defined by Eq. (1)32
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πCAC=γ0–γCAC
（1）
where γ0 is the surface tension of pure water and γCAC is the surface tension at CAC. The values
of γCAC and πCAC were obtained and listed in Table 1. The γCAC of Si4ASO3K, Si4ASO3Na and
Si4ASO3Li were calculated to be 19.8, 20.1 and 20.8, respectively. The values are lower than
those of conventional anionic surfactants, such as sodium laurysulfonate (SLS, γCMC = 33.0
mN/m) and sodium dodecyl benzene sulfonate (SDBS, γCMC = 35.0)28, 33. And the πCAC values of
these anionic silicone surfactants are larger than those of hydrocarbon anionic surfactants.
Si4ASO3K, Si4ASO3Na and Si4ASO3Li showed a significant capability to reduce the water
surface tension. This result was attributed to the flexible Si-O-Si bonds which enables the methyl
groups to orient in low energy configurations. Then the surfactant easily arranged itself on the
surface of the solution34, 35, 36. These silicone surfactants revealed smaller critical aggregation
concentration (CAC), (CAC ≈ 5.0 mM) than classical sulfonate anionic surfactants, such as
sodium undecane-1-sulfonate (SUS, CMC = 42.7 mM) and sodium laurylsulfonate (SLS, CMC =
12.4 mM)28. Obviously, the sulfonate anionic silicone surfactants exhibited remarkable surface
activity and their lower CAC values compared with classical sulfonate surfactants, which
indicate a good potential for future applications.
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Figure 2. Surface tension vs concentration for Si4ASO3K, Si4ASO3Na and Si4ASO3Li in
aqueous solutions at 25 oC.
The maximum excess surface concentration (Γmax) and the area occupied by a single
amphiphile molecule at the air/liquid interface (Amin) were calculated according to Eq. (2)37


1
dγ




(2)
max  
nRT dlnC


where γ is the surface tension, R is the gas constant, T is the absolute temperature, C is the
surfactant concentration, (dγ/d lnC) is the slope of the linear fit of the data below the CAC in the
surface tension plots. The value of n is theoretically dependent on the surfactant type and
structure, and it can also be affected by the extra electrolytes and impurities in the water solution.
Here, n = 2 was taken based on the anionic surfactant structure38. With the Γmax obtained, the
value of Amin is estimated using Eq. (3)32
1016
Amin

(3)
N A max
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where NA is Avogadro’s number. The values of Γmax and Amin are listed in Table 1.
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A greater Γmax value or smaller Amin value means a denser arrangement of surfactants at
the air/liquid interface. As shown in Table 1, the Amin values were 169 Å2 for Si4ASO3K, 177 Å2
for Si4ASO3Na and 189 Å2 for Si4ASO3Li, and they were larger than those of conventional
hydrocarbon anionic surfactants, i.e., SLS, Amin = 56.7 Å2 28. These surface activity parameters of
our products compared with SLS had been shown in Table 1. The larger Amin values than SLS
which with linear hydrophobic groups might be the result of the “umbrella” shaped
trimethylsilyloxyl groups arranging themselves with a greater distance between surfactant
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molecules, and thus resulting in the siloxanyl groups orienting parallel to the water surface27, 39
.
The Amin values of the anionic surfactants followed the order: Si4ASO3K < Si4ASO3Na <
Si4ASO3Li. The sample Gouy-Chapman-Stern theory was used as the adsorption model.
According to the model described by Warszynski i.e., the negative surfactant head groups were
adsorbed in the stern layer while the hydrated counterions were small enough to penetrate into
the stern layer, suggesting that the most surfactant charges in the stern layer could be
compensated by the smallest hydrated potassium ions40. This phenomenon indicated that in the
presence of the smaller counterions the adsorption layer performed densely packing. This
theoretical model well fitted to our results shown in Figure 241. The results showed the curve of
Si4ASO3Na was similar with Si4ASO3Li, far apart from Si4ASO3K, which could be explained by
hydration radius of the counterions (Table 2). The Amin value at water interface of Si4ASO3Li
was larger than that of Si4ASO3Na and Si4ASO3K, due to the largest size of hydrated counterions
bound to the surfactant molecule as shown in Table 2.
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Table 1. Critical aggregate concentration (CAC) and adsorption parameters of Si4ASO3K,
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Si4ASO3Na , Si4ASO3Li and SLS in aqueous solutions at 298.15 K.
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CACa
(m mol/L)
4.5±0.2
CACb
(m mol/L)
3.0±0.2
γCAC
πCAC
Γmax
Amin
(Å2)
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(mN/m) (mN/m) (μmol/m2)
Si4ASO3K
Si4ASO3Na
Si4ASO3Li
SLS
19.8±0.2 52.2±0.2 0.98±0.10 169±16
20.1±0.2 51.9±0.2 0.94±0.05 177±13
20.8±0.2 51.2±0.2 0.88±0.1 189±19
10.8±0.3
11.5±0.3
12.4
3.1±0.2
3.4±0.3
--
33.0
39.0
2.93
56.7
(a: Determined from surface tension b: Determined from electrical conductivity)
Table 2. Values for the nonhydrated and hydrated counterion radius 41.
nonhydrated ion radius (nm)
hydrated ion radius (nm)
Li+
Na+
K+
0.068
0.095
0.133
0.38
0.36
0.33
Conductometry results
The CAC of Si4ASO3Li was first determined by electrical conductivity measurements at
25 oC which plots the conductivities of surfactant solutions as a function of the concentrations, as
shown in Fig. 3. The CAC value was determined by the intersection of the two straight lines in
the к-C plot. Figure 3 shows that as the temperature changed, the conductivity value changed
slightly, indicating that the electrical characteristics were minimally influenced by temperature.
The counterion binding degree (β) is known to be influenced by the hydrophilicity of the
counterions. The increasing hydrophilicity of the K, Na and Li counterions can decrease their
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affinity for the surfactant molecules42, 43, 44. Therefore, our alkali salt products with different
counterions showed the similar conductivity results. As previously reported, the β values of
cationic silicone surfactants were always larger than those of our products, for most of which are
alkalescent salts, and the hydrophobicity of the counterions with surfactant head groups would
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increase their binding with each other18, 27, 45, 46
.
1000
Si4ASO3Li
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900
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35oC
800
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400
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0
0.000
0.002
0.004
0.006
0.008
0.010
C/(M)
Figure 3. Conductivity as a function of concentration for Si4ASO3Li at different temperatures.
As shown in Table 1, the CAC values of the three surfactants that determined by electrical
conductivity were similar, but it can be still seen that with the counterion hydrophilicity
decreased, the CAC values decreased correspondingly. Compared to the lithium ion (Li+), the
potassium ion (K+), with the smallest hydrated ion radius, could penetrate into the stern layer,
and reduce the charge repulsion of the head groups more efficiently. This behavior reduced the
thickness of diffuse part of the double layer. Thereby making Si4ASO3K form aggregates more
easily.
The degree of counterion binding (β) can be obtained from the conductivity measurement
with Eq. (4)47
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1
2
  1 -
(4)
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9
where α1 and α2 are the slopes of the straight lines above and below the CAC in the conductivity
plots, respectively. The electrical conductivity versus surfactant concentration for Si4ASO3Li at
different temperatures is shown in Figure 3, and the values of β at each temperature for
Si4ASO3K, Si4ASO3Na and Si4ASO3Li were obtained as summarized in Table 3.
As shown in Table 3, the β values of the three anionic surfactants were low, indicating that
the self-repulsion was stronger than the attraction between the head groups and the counterions48,
49, 50. And the β values of Si4ASO3K, Si4ASO3Na and Si4ASO3Li decreased with the increase of
temperature, which can be attributed to the increased motion of the surfactant molecules as
temperature increases.
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Table 3. Degree of counterion binding (β) and thermodynamic parameters of aggregate
formation for Si4ASO3K, Si4ASO3Na and Si4ASO3Li in aqueous solutions at different
temperatures.
ѳ
ѳ
ѳ
ѳ
surfactants
Si4ASO3K
T (K)
β
△Gm
(KJ/mol)
288.15 0.1856±0.05 -28.10±0.360 -9.16±0.770 -18.94±0.410 65.73±1.422
293.15 0.1853±0.05 -28.48±0.362 -9.48±0.792 -19.0±0.40 64.81±1.363
△Hm
-T△Sm
△Sm
(KJ/mol)
(KJ/mol)
(J/mol K)
298.15 0.1847±0.05 -28.85±0.365 -9.80±0.833 -19.05±0.483 63.89±1.620
303.15 0.1830±0.05 -29.19±0.368 -10.12±0.859 -19.07±0.491 62.91±1.616
308.15 0.1797±0.05 -29.50±0.375 -10.42±0.883 -19.08±0.508 61.92±1.649
Si4ASO3Na 288.15 0.1682±0.05 -27.89±0.388 -13.96±1.194 -13.93±0.814 48.34±2.825
293.15 0.1591±0.05 -27.95±0.406 -14.33±1.237 -13.62±0.831 46.46±2.835
298.15 0.1499±0.05 -28.00±0.427 -14.71±1.279 -13.29±0.852 44.57±2.858
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303.15 0.1493±0.05 -28.36±0.430 -15.20±1.223 -13.16±0.793 43.41±2.616
308.15 0.1369±0.05 -28.43±0.462 -15.53±1.367 -12.90±0.905 41.86±2.937
Si4ASO3Li 288.15 0.2665±0.05 -30.02±0.271 -12.06±0.952 -17.96±0.681 62.33±2.363
293.15 0.2355±0.05 -29.58±0.297 -12.18±0.986 -17.40±0.689 59.36±2.350
298.15 0.2274±0.05 -29.57±0.306 -12.51±1.019 -17.06±0.713 57.22±2.391
303.15 0.2164±0.05 -29.56±0.319 -12.81±1.053 -16.75±0.734 55.25±2.421
308.15 0.1902±0.05 -29.33±0.352 -12.95±1.118 -16.38±0.766 53.16±2.486
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Thermodynamic analysis of aggregation
ѳ
The thermodynamic parameters, such as the standard Gibbs energy (△Gm ), the standard
ѳ
ѳ
entropy (△Sm ), and the standard enthalpy (△Hm ), were estimated to understand the aggregation
of the novel sulfonate anionic silicone surfactants. The △Gmѳ can be calculated according to Eq.
(5)51
Gm  (1 )RT ln cmc
(5)
where χcmc is the mole fraction of silicone surfactant at the CAC, T is the absolute temperature,
and R is the gas constant. Then, the enthalpy of aggregate formation can be calculated using the
Gibbs-Helmboltz equation, Eq. (6)51


Gm /T





Hm

(6)




1 /T

and the entropy of formation is obtained by Eq. (7)51
H m  Gm
Sm

(7)
T
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ѳ
ѳ
ѳ
The values of △Gm , △Hm , and △Hm were obtained and are presented in Table 3. Over
the whole investigated range, all of the △Gmѳ values are negative, indicating that the aggregation
of the surfactants is spontaneous.
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Effect of counterion on the aggregation properties
It is known that compared to temperature, the counterion has a vital effect on the surface
activity parameters of these anionic surfactants, such as CAC and Amin. So we investigated the
counterion effect on the aggregation behavior of the surfactants. It can be seen in Figure 2 that
the number of Si4ASO3Li at the interface is less than Si4ASO3K and Si4ASO3Na due to the larger
hydrated lithium ion. Freeze-fracture transmission electron microscope (FF-TEM) is the first
choice to identify the surfactants microstructures in solution. Figure 4 (a, b) presents the FF-
o
TEM result of Si4ASO3Li in aqueous solution at the concentration of 0.01M and 25 C. It is
obvious that our products performed as vesicles. FF-TEM images illustrated that the vesicles size
was uniform and lower than 200 nm. The result indicated a spherical vesicles microstructure for
our tetrasiloxane-based surfactants. Therefore, the vesicles can provide their potential application
in delivery system etc. In order to clarify the structure of vesicles, cryo-TEM was further used to
obtain their instantaneous microstructure in aqueous solution (Figure 4 c-f). The cryo-TEM
results indicated that these tetrasiloxane surfactants could form multilaminar vesicles (Figure 4 c,
d), and multi-ventricular vesicles (Figure 4 e, f). It is deductive that a model of multilaminar
layer vesicles structure could be imitated and shown in Figure 5.
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Figure 4. a,b) FF-TEM images of Si4ASO3Li in aqueous solution above CAC (10.0 mM); c-f)
cryo-TEM images of Si4ASO3K in aqueous above CAC ((c,d) 6.0mM and (e,f) 10.0mM).
Figure 5. Model of multilaminar layer membrane structure of vesicles.
The counterions’ difference could also influence the aggregate behavior. Morphology and
size distribution of our series of surfactants were investigated by TEM and DLS. Figure 6 shows
the TEM images of Si4ASO3K, Si4ASO3Na and Si4ASO3Li in aqueous solution at the same
temperature (298.15 K) and the same concentration of 6.0 mM. Spherical aggregates were
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observed with diameters ranging from 180 nm to 400 nm. The aggregate sizes followed the order
of Si4ASO3K>Si4ASO3Na >Si4ASO3Li. And as all the TEM results were carried out under the
same condition (the same concentration and same testing temperature), the different
morphologies of the three anionic surfactants could only be affected by the type of counterions.
As for the order of size, it is hypothesized that hydrated K ions with smallest radius could
penetrate the double layer more easily and reduce the head group repulsion more efficiently, and
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then result in more monomer packing of surfactants (See figure 7)41, 52-55
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Figure 6. TEM images of Si4ASO3K (a, b); Si4ASO3Na (c, d); Si4ASO3Li (e, f) in aqueous
solution (6.0 mM).
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Figure 7. The effect of different hydrated counterions on surfactant packing.
DLS measurement was carried out to investigate the hydrodynamic diameters of
aggregates, which formed by the surfactant series above the CAC. Figure 8 shows the size
distribution of Si4ASO3K, Si4ASO3Na and Si4ASO3Li in aqueous solution at 298.15K and 6.0
mM (above CAC). All the product DLS curves showed that the aggregates exhibited a mono-
modal function and the size distribution ranged from 200 nm to 400 nm, similar with the TEM
results. Taking Si4ASO3Li as a sample, DLS result showed average hydrated radium of 99 nm,
very close to the FF-TEM result (~200nm). The vesicles size could also be measured through
TEM images (~180nm). Thus, it is concluded that the vesicles formed by the tetrasiloxane-based
surfactants had uniform and steady size above CAC. The hydrodynamic diameters of surfactants
showed the same trend when the counterion changed, following the order of Si4ASO3K >
Si4ASO3Na > Si4ASO3Li. The smaller sized hydrated counterions could penetrate into the stern
layer and reduce the charge repulsion of the head groups more efficiently, thereby allowing the
aggregates to pack more tightly55, 56. Therefore our study provides a convenient way to adjust the
vesicles size by changing the counterions of the anionic sulfonate surfactants.
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Si ASO Li
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3
Si ASO Na
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Si ASO K
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3
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100
1000
Rh (nm)
Figure 8. Size distributions of Si4ASO3K, Si4ASO3Na andSi4ASO3Li at 6.0 mM concentration in
aqueous solution.
Conclusion
Three sulfonate anionic silicone surfactants, Si4ASO3K, Si4ASO3Na and Si4ASO3Li were
successfully synthesized for the first time. This work provides a simple and mild route to obtain
these unique anionic sulfonate silicone surfactants. The product aggregation behavior in aqueous
solution was investigated systematically. The results showed that the silicone surfactant
demonstrated superior surface activity compared with common hydrocarbon surfactants because
of the special properties of the siloxane. These compounds not only showed a lower surface
tension that is equal to the cationic silicone surfactants, but also had lower CAC values than the
cationic silicone surfactants and classical hydrocarbon anionic surfactants27, 28, 45. The counterion
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affected the surface activity and the aggregation behavior obviously. The CAC values and the
Amin following the order: Si4ASO3K < Si4ASO3Na < Si4ASO3Li. Cryo-TEM images showed that
this sulfonate surfactant in aqueous solution could form spherical vesicles spontaneously, and the
size of the vesicles ranging from 200 nm to 400 nm could be adjusted by changing the
counterions. DLS and TEM results indicated that our products had stable and mono-distributed
aggregation sizes both in and out of aqueous solution.
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The anionic silicone surfactants with these characteristics possess the ability to replace the
conventional hydrocarbon anionic surfactants used in emulsifiers, cosmetics and oil extraction57-
60. Moreover, due to the nontoxicity and the negative charge, they are good candidates to
overcome the limitations of cationic silicone surfactants in biomaterials, as drug carriers and in
textile dyeing. This investigation contributes to understanding the aggregation of novel anionic
silicone surfactants and provides a convenient way to adjust the vesicles size. It will be helpful in
designing novel and efficient silicone surfactants with controlled and narrow vesicles size
distributions, which may expand the potential applications of silicone surfactants in
biotechnology, controlled release and electrochemistry.
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China
(No. 21774070) and Special Fund for Shandong Independent Innovation and Achievements
transformation (No.2014ZZCX01101).
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