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Syntheses and properties of ethoxylated double-tail trisiloxane surfactants containing a propanetrioxy spacer.

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Full Paper
Received: 1 October 2010
Revised: 26 December 2010
Accepted: 26 December 2010
Published online in Wiley Online Library: 29 March 2011
(wileyonlinelibrary.com) DOI 10.1002/aoc.1775
Syntheses and properties of ethoxylated
double-tail trisiloxane surfactants containing
a propanetrioxy spacer
Zhongli Peng∗, Jianfeng Huang, Furong Chen, Qinghua Ye and Qiaoyu Li
A new type of ethoxylated double-tail trisiloxane surfactants containing a propanetrioxy spacer of the general formula
ROCH2 CH(OR)CH2 O(CH2 CH2 O)x CH3 [R = Me3 SiOSiMe(CH2 )3 OSiMe3 , x = 8.4, 12.9, 22] has been synthesized. Their structures
were characterized by 1 H-NMR, 13 C-NMR and 29 Si-NMR spectroscopy. The critical micelle concentration (CMC) values of these
double-tail trisiloxane surfactants were at the level of 10−5 mol l−1 , and the surface tension values of their aqueous solutions
at CMC were in the range of 21-24.9 mN m−1 . Only the double-tail trisiloxane surfactant with average ethoxy units of 8.4 (1P)
possesseda good spreading ability (SA) value. Its SA values of aqueous solutions (5.0 × 10−3 mol l−1 ) on parafilm and Ficus
microcarpa leaf surfaces were more than 15 (within 10 min) and 13 (within 3 min), respectively. The trisiloxane surfactant 1P
was also found to have the strongest hydrolysis resistant ability among all of the double-tail trisiloxane surfactants prepared.
Its aqueous solutions were stable for 130 days in an acidic environment (pH 4.0) and 59 days in an alkaline environment (pH
10.0) with surface tension values less than 23 mN m−1 . It is suggested that this surfactant can be used as a wetting agent or
c 2011 John Wiley & Sons, Ltd.
spreading agent in certain extreme pH environments. Copyright Keywords: double-tail trisloxane surfactants; spreading ability; hydrolysis resistant ability; propanetrioxy spacer; synthesis
Introduction
Appl. Organometal. Chem. 2011, 25, 383–389
Experimental
Materials
Polyethylene glycol monomethyl ether 400 [average degree of
polymerization (DP = 8.4)] was purchased from Shanghai Haojiong Assistant Co. Ltd, China. Polyethylene glycol monomethyl
ether 600 (DP = 12.9) came from the Zhejiang Huangma Chemical
∗
Correspondence to: Zhongli Peng, Department of Chemical Engineering,
Huizhou University, Huizhou 516007, China. E-mail: zsupzl@126.com
Department of Chemical Engineering, Huizhou University, Huizhou 516007,
China
c 2011 John Wiley & Sons, Ltd.
Copyright 383
The unique ability of certain low-molecular-weight siloxane
surfactants to promote rapid spreading of aqueous solutions
on low-energy hydrophobic surfaces such as Parafilm or
polyethylene was discovered in the 1960s. This ability, which
is called ‘superwetting’ or ‘superspreading’, is the basis of their
use as herbicide wetting agents. The trisiloxane surfactants were
commercialized as siloxane agricultural adjuvants because they
were the easiest to prepare on a large scale.[1] It is reasonable to
predict that the superspreading trisiloxane surfactants may find
great potential for use in other areas, such as water-based coatings
for plastics, cosmetics, etc. However, they are very sensitive to
environmental pH values. They hydrolyze rapidly when placed in
environmental pH values below 5 or above 9, and are only stable
for 40 days even in an environment of pH 7.0.[2 – 4] This limits their
application scope. However, in recent years research on trisiloxane
surfactants had focused on the relationship between structure and
spreading properties,[5 – 7] spreading mechanism[8 – 10] and other
application performances.[11,12] There are few studies on their
hydrolysis resistance.
To improve the hydrolysis-resistant ability of siloxane surfactants, Leatherman et al. synthesized hydrolysis resistant disiloxane
surfactants[2] and trisiloxane surfactants.[3,4] Nevertheless, the
structure of the siloxane needed to synthesize the above hydrolysis resistant siloxane surfactants is very special, and they
are not easily available on the market; hence the industrial production of these hydrolysis resistant siloxane surfactants is not
easy. Recently, using siloxanes easily available in the raw state,
we synthesized several types of double-tail trisiloxane surfactants, whose chemical structures are shown in Scheme 1, and
analyzed the relationship between their structures and spreading
and hydrolysis-resistance performances.[13 – 15] We proposed that
their spreading properties are relevant to the flexibility of the
spacer between the hydrophobic groups and hydrophilic groups.
Some of these double-tail trisiloxane surfactants[14] have both the
capacities of hydrolysis resistance and spreading, despite their
initial spreading rates not being very fast. However there is a rigid
spacer in these surfactant molecules and their aqueous solutions
are inevitably alkaline, which restricts their applications in some
places, since their hydrophobic groups and hydrophilic groups
are linked by an N atom. If hydrophobic groups and hydrophilic
groups in double-tail trisiloxane surfactants are linked by an O
atom, the above situation will change. However, so far no such
double-tail trisiloxane surfactants have been reported.
In this paper a new type of double-tail trisiloxane surfactant containing a propanetrioxy spacer of the general formula ROCH2 CH
(OR)CH2 O(CH2 CH2 O)x CH3 [R = Me3 SiOSiMe(CH2 )3 OSiMe3 , x = 8.4,
12.9, 22] is introduced. Its syntheses and properties are reported
in detail.
Z. Peng et al.
Scheme 1. Chemical structures of double-tail trisiloxane surfactants previously prepared.
384
Industry Group Co. Ltd, China. Polyethylene glycol monomethyl
ether 1000 (DP = 22) was obtained from the Shanghai Jinshan
Chemical Co. Ltd, China. Tetraethylammonium bromide (TEAB),
analytical grade, was purchased from Tianjn Yongda Reagent Development Center, China. 3-Chloropropylene was from Wuhan
Xinbao Chemical Co. Ltd, China. Siloxane agricultural adjuvant Silwet L-77 [Me3 SiOSi(Me)(R)OSiMe3 , R = −(CH2 )3 O(CH2 CH2 O)8 CH3 ]
from Momentive Performance Materials was provided by Huizhou
Yinnong Technology Co. Ltd, China. 1,1,1,3,5,5,5-Heptamethyl-3(3-glycidyloxypropyl) trisiloxane was provided by Jiangxi Haiduo
Chemical Co. Ltd, China. Speier-catalyst solution (isopropanol
solution of chloroplatinic acid) and catalyst ligand solution (isopropanol solution of triethylamine) were prepared in accordance
with the methods described by Wang et al.[16] Parafilm with a
thickness of about 0.5 mm was prepared by melting paraffin wax
on a clean glass and cooling to room temperature. The paraffin
wax was purchased from Shanghai Specimen and Model Fac-
wileyonlinelibrary.com/journal/aoc
tory, China. Polyethylene terephthalat (PET) film was obtained
from Stchaofa Packaging Materials Co. Ltd, China. Ficus microcarpa
leaves were directly pre-picked from a Ficus microcarpa. All of other
chemicals were of analytical grade. The water used was doubly
distilled.
Syntheses
The synthesis route to double-tail trisiloxane surfactants is shown
in Scheme 2. The specific methods are described in the following.
Procedure (a) was carried out in accordance with our previous
work.[13] The products are confirmed by electrospray ionization
mass (ESI-MS) spectroscopy.
In procedure (b), the product of procedure (a) (M) and a certain
volume of aqueous Na2 CO3 solution with pH value about 10.0 were
mixed in a single-neck flask equipped with refluxing condenser
and magnetic stirrer. The mixture was refluxed for 3 h, and then
c 2011 John Wiley & Sons, Ltd.
Copyright Appl. Organometal. Chem. 2011, 25, 383–389
Ethoxylated double-tail trisiloxane surfactants
Table 1. Data of distilled water drop diameter on substrates surfaces
Distilled water drop diameter (mm)
Parafilm
PET film
Ficus microcarpa leaves
5.5
4.9
4.5
Table 2.
1 H-NMR spectral data (δ) of compounds 1P, 2P and 3P
H
1P (δ)
2P (δ)
3P (δ)
a
b
c
d
e
f
g
h
i
j
k
−0.07
0.01
0.36
1.50
3.46
3.93
4.06
3.93
3.56
3.56
3.28
−0.04
0.04
0.38
1.51
3.49
3.82
3.96
3.82
3.59
3.59
3.32
−0.03
0.05
0.40
1.53
3.50
3.97
4.09
3.97
3.60
3.60
3.34
distilled to remove water. After the dissolution of the residue with
toluene, filtration of the mixture and removal of the solvent, the
product N was obtained.
The product N was also used as reactant in procedure (c),
and mixed with 3-chloropropylene, NaOH and TEAB according to
molar ratio of 1 : 5 : 4 : 0.08 in a three-neck flask equipped with
refluxing condenser and magnetic stirrer using tetrahydrofuran as
a solvent under nitrogen. The mixture was heated to 55–60 ◦ C for
8 h and then a crude product was obtained by vacuum distillation
to remove tetrahydrofuran and the excessive 3-chloropropylene.
The crude product was dissolved with ethyl acetate and filtered.
Then the filtrate was washed with saturated NaCl solution twice,
dried with anhydrous magnesium sulfate and filtered. After the
vacuum distillation of the final filtrate to remove the ethyl acetate,
the product O was ultimately obtained.
Similarly, in procedure (d) the product of procedure (c) (O),
1,1,1,3,5,5,5-heptamethyl-3-(3-glycidyloxypropyl)trisiloxane,
Speier-catalyst and catalyst ligand (isopropanol solution of triethylamine) were placed according to the molar ratio of
1 : 2.2 : 1.3 × 10−4 : 1.3 × 10−3 in one of the above-mentioned
three-neck flasks under nitrogen. The mixture was heated to
60–75 ◦ C for 3 h. A yellowish-brown viscous liquid product P was
obtained though vacuum distillation to remove the solvent and
the excessive 1,1,1,3,5,5,5-heptamethyl-3-(3-glycidyloxypropyl)
trisiloxane.
Structural Characterization
Appl. Organometal. Chem. 2011, 25, 383–389
13
C-NMR spectral data (δ) of compounds 1P, 2P and 3P
C
1P(δ)
2P(δ)
3P (δ)
1
2
3
4
5
6
7
8
9
10
11
−0.01
2.19
13.84
23.55/23.98
69.64
73.40
72.50
74.39
70.80
70.80/72.14
59.19
−0.01
2.18
13.84
23.55/23.98
69.70
73.47
72.19
74.46
70.80
70.80/71.65
59.26
0.04
2.25
13.91
23.61/24.04
69.69
73.49
72.51
74.64
70.77
70.86/72.19
59.68
Table 4. 29 Si-NMR spectral data (δ) of selected substructures of
compounds 1P, 2P and 3P
Substructures
(CH3 )3 SiO
(CH3 )(CH2 )SiO2
1P(δ)
2P(δ)
3P (δ)
7.09
−21.68
7.03
−21.62
6.91
−21.78
Surface Activity, Spreading Ability and Hydrolysis-resistant
Ability Determination
Basically, properties experiments were carried out in the light
of the methods described in the works of Zhang et al.[7] and
Peng et al.[13] Aqueous solution surface tension (γ ) values were
obtained by the Wilhelmy plate method using a BZY-1 completely
automatic surface tension meter (Shanghai Equity Instruments
Factory, China) and under constant atmospheric conditions (23 ◦ C
room temperature, 75% relative humidity). The critical micelle
concentration (CMC) values were taken at the intersection of the
linear portions of the plots of the surface tension against the
logarithm of the surfactant concentration. The surface tension of
the intersection point is called the surface tension of surfactant
solution at CMC (γcmc ). All the surfactant solutions used to
determinate CMC value were tested within 1 h after preparation.
Surfactant solutions were prepared with distilled water. Prior to
measurements on surfactant solutions, the surface tension of the
distilled water was measured. These surface tension values of
water were in the range of 70.0 ± 0.3 mN m−1 .
The spreading ability (SA) of surfactant solutions was evaluated
by the following procedure. By means of a plastic head
dropper, about 23 µl of surfactant solution prepared with
Na2 HPO4 –KH2 PO4 buffer solution (pH 7.0) was deposited on
substrate surface. All substrates used for the spreading experiment
were first washed with tap water and then with distilled water twice
and naturally dried before use. After a certain time, the average
diameter of the drop was measured by means of a vernier caliper.
Tests were run in triplicate. SA was calculated by the equation SA =
(D/D0 )2 , where D = diameter of drop of test solution (or emulsion)
after a certain time and D0 = diameter of drop of distilled water
applied to the surface in the same manner. The distilled water drop
diameters on the substrate surfaces were found having no change
within the experimental time. Their data are listed in Table 1.
The hydrolysis resistant ability (HRA) of surfactants was
measured by the following procedure. Three surfactants solutions
c 2011 John Wiley & Sons, Ltd.
Copyright wileyonlinelibrary.com/journal/aoc
385
Mass spectroscopy was carried out on a Shimadzu LCMS-2010A
liquid chromatography mass spectrometer (LC-MS) using electrospray ionization in positive-ion mode. Proton nuclear magnetic
resonance (1 H-NMR) and carbon-13 nuclear magnetic resonance
(13 C-NMR) spectroscopy were recorded with a Varian Mercury-plus
300 spectrometer in CDCl3 . Silicon nuclear magnetic resonance
(29 Si-NMR) spectroscopy was determinated with a Bruker Avance
400 spectrometer in CDCl3 .
Table 3.
Z. Peng et al.
Scheme 2. Synthesis route to double-tail trisiloxane surfactants.
(1.0 × 10−3 mol l−1 ) with the pH value of 4.0, 7.0 and 10.0 were
prepared with potassium hydrogen phthalate, Na2 HPO4 –KH2 PO4
and Na2 CO3 –NaHCO3 buffer solutions, respectively. Their γ -values
within 10 min after their preparation were measured. The first
measurement time was used as starting time (time = 0), and then
the γ -values of surfactants solutions were measured at a certain
time until they reached 26 mN m−1 . The faster the γ -value of
surfactant solution rose, the poorer the HRA of the surfactant.
These tests were measured under natural atmospheric conditions.
Results and Discussion
Syntheses
386
The double-tail trisiloxane surfactants (1P, 2P and 3P) were
structurally characterized with 1 H-NMR, 13 C-NMR and 29 Si-NMR
wileyonlinelibrary.com/journal/aoc
Table 5. Aqueous
surfactants
Surfactant
1P
2P
3P
surface
CMC
(mol l−1 )
2.75 × 10−5
4.11 × 10−5
7.24 × 10−5
activity
of
double-tail
trisiloxane
as m
(Å 2 )
G0 mic
(kJ mol−1 )
31.6
39.5
49.73
−35.7
−34.7
−33.3
γcmc
max
(mN m−1 ) (mol cm−2 )
21.7
23.6
24.9
56.65
42.08
33.40
spectroscopy. Their spectra data of the double-tail trisiloxanne
surfactants are listed in Tables 2–4, respectively. The atom
numbering scheme is shown on the structure in Scheme 2. In
all cases, the spectra are consistent with the assigned structures of
the compounds.
c 2011 John Wiley & Sons, Ltd.
Copyright Appl. Organometal. Chem. 2011, 25, 383–389
Ethoxylated double-tail trisiloxane surfactants
Table 6. Spreading ability of double-tail trisiloxane surfactants solution on parafilm, PET film and Ficus microcarpa leaf surfaces
Surfactant
SA
concentration
(mol l−1 )
1P
2P
3P
5.0 × 10−3
1.0 × 10−3
5.0 × 10−3
1.0 × 10−3
5.0 × 10−3
1.0 × 10−3
Parafilm
PET film
Ficus microcarpa leaves
3 min
10 min
3 min
10 min
3 min
10 min
5.8
3.6
2.6
2.4
2.0
1.9
15.5
5.6
3.2
2.8
2.4
2.4
3.7
2.9
2.6
2.1
1.8
1.7
7.9
3.8
3.2
2.2
1.9
1.8
14.0
5.3
3.2
2.7
2.2
2.2
–
9.1
5.3
3.2
2.5
2.5
Table 7. The HLB values and molecular volume of surfactantsa
HLB values
Molecular volume (cm3 mol−1 )
Appearance of solution (1.0 × 10−3 mol l−1 )
a
1P
2P
3P
Silwet L-77
9.4
598.3
Turbid
11.4
711.3
Turbid
13.9
938.8
Clear
11.8(EO8)
384.4
Turbid
Calculated with the Molecular Modeling Pro.
Interfacial Properties
The surface excess concentration (max ), the surface area per
molecule (as m ) and the standard free energy of micellization
(G0 mic ) of double-tail trisiloxane surfactants were acquired
according to Zhang et al.[7] All the data of CMC, γcmc , max , as m and
G0 mic of double-tail surfactants prepared are listed in Table 5.
The CMC values of the double-tail trisiloxane surfactants were at
a level of 10−5 mol l−1 , and their γcmc values were all higher than
those of other types of double-tail trisiloxane surfactants[13 – 15]
having the same ethoxy units previously prepared. Only the
γcmc value of surfactant 1P was below 22 mN m−1 ; the other
two surfactants’ γcmc values were all over 23 mN m−1 . This
may be due to the presence of a high-energy glyceryl in their
molecules. The CMC, γcmc and as m values of double-tail trisiloxane
surfactants (1P, 2P and 3P) increased with the increase in the
number of ethoxy units, which was attributed to a stronger
hydrophilicity and a more voluminous hydrophilic group of
surfactants.[17]
Spreading Ability
Appl. Organometal. Chem. 2011, 25, 383–389
Hydrolysis-resistant Ability
The HRA of the double-tail trisiloxane surfactants is influenced by
the volume of their hydrophobe and the external environment
(Figs 1–3). Obviously the HRA of the double-tail trisiloxane
surfactants was far stronger than that of Silwet L-77. In a
neutral environment (pH 7.0) the HRA of the double-tail
trisiloxane surfactants was better than that in acidic or alkaline
environments. This is due to the promotion of the hydrolysis
c 2011 John Wiley & Sons, Ltd.
Copyright wileyonlinelibrary.com/journal/aoc
387
As shown in Table 6, the SA values of double-tail trisiloxane surfactants solutions on the three substrates increased with the increase
in the solutions’ concentration and the spreading time. This confirms the existence of a Marangon-flow-driven process[18,19] in the
spreading behavior of double-tail trisiloxane surfactant solutions.
Despite the critical surface tension of wetting (γcw ) of paraffin
wax being less than that of PET,[20] the SA values of double-tail
trisiloxane surfactants solutions on parafilm were higher than on
PET (Table 6). This result is similar to that obtained by Zhang et al.[21]
Maybe the hydrophobic groups of double-tail trisiloxane surfactants can be absorbed more effectively on the parafilm surface and
form a tighter surfactant monolayer[21] or bilayer.[22] The SA values
of double-tail trisiloxane surfactants solutions on the Ficus microcarpa leaves were better than on the parafilm (Table 6), indicating
that the ingredient of paraffin wax in parafilm is slightly different
from that on Ficus microcarpa leaves, and the paraffin wax in Ficus
microcarpa leaves is more conducive to adsorbing the hydrophobic groups of the double-tail trisiloxane surfactants. Obviously this
difference cannot be explained by the view of Zhang et al.[21] that
the thickness of paraffin wax in the substrate surface influences
the spreading rate of aqueous solution of siloxane surfactants.
Among the three double-tail siloxane surfactants only the
aqueous solutions of surfactant 1P exhibited better spreading performance on the parafilm and Ficus microcarpa leaf
surfaces (Table 6). Nevertheless, as shown in Table 7, only the
hydrophilie/lipophilie balance (HLB) value of 2P was close to that
of Silwet L-77, while the HLB values of 1P and 3P were far from that
of Silwet L-77. According the views of Zhang et al.,[8,21,23] the SA of
aqueous solution of 2P should be superior to the SA of 1P aqueous
solution. In fact, however, the SA of aqueous solution of 2P is far
poorer than that of 1P solution. This may be due to the higher γcmc
value of 2P solution, which is over the γcw value of paraffin wax
(γcw = 23 mN m−1 ).[1] Therefore, 2P solution can only partially wet
the parafilm instead of spreading on it. In contrast, the solution of
1P is able to spread on the parafilm as its γcmc value is less than
23 mN m−1 . The initial spreading rate of 1P solution on the parafilm
surface is not very fast, which may be attributed to its inappropriate
HLB value and larger molecular volume relative to Silwet L-77.
The fact that the SA value of surfactant 1P solution (5.0 ×
10−3 mol l−1 ) is >13 within 3 min on Ficus microcarpa leaf surfaces,
and its SA value is >15 within 10 min on parafilm surface, indicates
that, despite its initial spreading rates not being very fast, potential
applications may be found in the field of pesticide adjuvants,
water-based coatings for plastics and cosmetics.
Z. Peng et al.
Figure 1. Surface tension versus time plots of trisiloxane surfactants
solution (pH 7.0).
Figure 3. Surface tension versus time plots of trisiloxane surfactants
solution (pH 10.0).
suggesting that it may find applications in certain extreme pH
environments.
Conclusions
Figure 2. Surface tension versus time plots of trisiloxane surfactants
solution (pH 4.0).
Ethoxylated double-tail trisiloxane surfactants containing a
propanetrioxy spacer can be synthesized with chemicals easily
available on the market. The CMC values of all of the doubletail trisiloxane surfactants were at a level of 10−5 mol l−1 . Their
γcmc values were in the range 21–24.9 mN m−1 . The aqueous
solution of the double-tail trisiloxane surfactant 1P was found
to have the best SA on the parafilm and Ficus microcarpa leaf
surfaces. The double-tail trisiloxane surfactants showed a better
HRA, especially surfactant 1P, whose surface tension of aqueous
solution was still less than 23 mN m−1 over 130 days in an acidic
environment (pH 4.0). It is suggested that surfactant 1P can be
used as a wetting agent or spreading agent in certain extreme pH
environments.
Acknowledgments
388
reaction by the acid and base. Although the double-tail trisiloxane
surfactants are neutral and non-nonionic surfactants, their
HRAs in an acidic environment (pH 4.0) are superior to that
in an alkaline environment (pH 10.0). This also verifies our
previous inference[15] that the hydrolysis mechanism of double-tail
trisiloxane surfactants in an acidic environment is different from
that in an alkaline environment.
Surfactant 1P was found to have the best HRA among the
three double-tail trisiloxane surfactants, which was attributed to
its having the weakest hydrophilicity. In an acidic environment
(pH 4.0), surfactant 1P was stable for at least 130 days since the
surface tension values of its solution were less than 23 mN m−1 at
all times. In an alkaline environment (pH 10.0), however, the HRA of
surfactant 1P was not as good as that in an acidic environment (pH
4.0). However, it was also stable for 59 days with its surface tension
below 23 mN m−1 . In the meantime the HRAs of the doubletail trisiloxane surfactants (1P, 2P and 3P) were also found to be
better than that of the corresponding double-tail trisiloxane analog
surfactants[13 – 15] previously prepared. In particular, the surfactant
1P exhibited super-strong HRA in an acidic environment (pH 4.0),
wileyonlinelibrary.com/journal/aoc
The financial support of the Science and Technology Program of
Huizhou (no. 2009B020002023) and the Natural Science Project of
Huizhou University (no.C209.0402) is gratefully acknowledged.
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Appl. Organometal. Chem. 2011, 25, 383–389
c 2011 John Wiley & Sons, Ltd.
Copyright wileyonlinelibrary.com/journal/aoc
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tail, properties, containing, space, synthese, trisiloxane, double, surfactants, ethoxylated, propanetrioxy
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