Effect of chain unsaturation on the self-association of tri- and tetraethylene glycol octyl ethers obtained by butadiene telomerization

Article abstract of DOI:10.1021/la203644k

2,7-Octadienyl ethers of tri- and tetraethylene glycol (C _8:2E₃ and C_8:2E₄) have been synthesized by the atom-economical butadiene telomerization of the corresponding poly(ethylene glycols). On one hand, this synthetic path is attractive because it is expeditious and environmentally benign, and on the other hand, it provides unconventional amphiphiles for which the lipophilic chains possess two double bonds. These two unsaturations increase the global hydrophilicity of the compound, which is also highlighted by the modelization of the compounds using the conductor-like screening model for real solvents (COSMO-RS). The behavior of C_8:2E₃ and C_8:2E₄ in binary amphiphile/water and ternary amphiphile/oil/water systems is therefore greatly modified compared to that of the conventional fully saturated homologues (C ₈E₃ and C₈E₄) that are easily obtained after hydrogenation. This results in a lowered efficiency of the unsaturated compounds for oil solubilization. The usual Winsor-type microemulsion systems are formed, and for the same oil, the DLS investigation of the microstructure of the Winsor I microemulsion does not highlight any difference in the self-association between the unsaturated and saturated compounds.

Full text of DOI:10.1021/la203644k

ARTICLE
pubs.acs.org/Langmuir
Effect of Chain Unsaturation on the Self-Association of
Tri- and Tetraethylene Glycol Octyl Ethers Obtained by
Butadiene Telomerization
Jonathan Lai,†,‡ Valꢀerie Molinier,*,† Mathieu Sauthier,*,‡ Laurianne Moity,† Yves Castanet,‡ Andrꢀe Mortreux,‡
and Jean-Marie Aubry†
†E. A. 4478 Chimie Molꢀeculaire et Formulation, ‡UMR 8181 Unitꢀe de Catalyse et de Chimie du Solide, Universitꢀe Lille Nord de France,
USTL, ENSCL, Citꢀe Scientifique, 59652 Villeneuve d’Ascq Cedex, France
ABSTRACT:
2,7-Octadienyl ethers of tri- and tetraethylene glycol (C8:2E3 and C8:2E4) have been synthesized by the atom-economical butadiene
telomerization of the corresponding poly(ethylene glycols). On one hand, this synthetic path is attractive because it is expeditious
and environmentally benign, and on the other hand, it provides unconventional amphiphiles for which the lipophilic chains possess
two double bonds. These two unsaturations increase the global hydrophilicity of the compound, which is also highlighted by the
modelization of the compounds using the conductor-like screening model for real solvents (COSMO-RS). The behavior of C8:2E3
and C8:2E4 in binary amphiphile/water and ternary amphiphile/oil/water systems is therefore greatly modified compared to that of
the conventional fully saturated homologues (C8E3 and C8E4) that are easily obtained after hydrogenation. This results in a lowered
efficiency of the unsaturated compounds for oil solubilization. The usual Winsor-type microemulsion systems are formed, and for
the same oil, the DLS investigation of the microstructure of the Winsor I microemulsion does not highlight any difference in the self-
association between the unsaturated and saturated compounds.
exhibit amphiphilicity, albeit limited, while still possessing the
required properties to belong to the solvent family (low molec-
ular weight, volatility). They are also sometimes named chame-
leonic solvents to express their good solubility both in organic
solvents and in aqueous media. The shortest members of the
series are banned for being reprotoxic,6 and that is why the
scientific community tries to find biosourced alternatives to
glycol ethers.7 However, compounds such as C4E1, C6E2, and
C8E4 are considered to be safe and are still widely used because of
their unique properties.
Industrially, CiEj's are produced by reacting an anhydrous
alcohol (e.g., n-butanol) with ethylene oxide, which gives rise to
mixtures of compounds. Even if one can play on the reaction
conditions to enhance the production of glycol monoethers, they
1. INTRODUCTION
Poly(oxyethylene glycol) ethers of fatty alcohols (CiEj) are a
family of compounds that act as references in the physical
chemistry of nonionic amphiphiles. Depending on (i, j), they
cover a wide range of behaviors from weak to strong amphi-
philes1 and their phase behavior in water and oil/water systems is
well documented.2 They are also ubiquitous in industrial applica-
tions: the long-chain CiEj's are the main nonionic surfactants on
the market, and the short-chain CiEj's, together with their
homologues derived from propylene glycol (CiPj), are often
simply called glycol ethers and are used as solvents or additives in
a number of large-scale formulations. Their main applications as
solvents are in paints, inks, and cleaning fluids formulations,
whereas other applications include anti-icing agents in jet fuel,
fluids for hydraulic systems, and plasticizers.3 The world total
consumption of glycol ethers was approximately 1.3 million
metric tons in 2006.3 These glycol ethers are also often called
hydrotropes, protosurfactants,4 or solvosurfactants5 because they
Received: September 17, 2011
Revised:
October 21, 2011
Published: November 07, 2011
r
2011 American Chemical Society
242
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2.2. General Methods. Analytical NMR spectra were recorded on
Bruker AC spectrometers at 300.13 MHz for 1H and 75.47 MHz for 13C.
GC analyses were carried out on a Chrompack CP 9002 apparatus
equipped with a flame ionization detector and a CP-Sil 5CB column with
a 100% dimethylpolysiloxane internal phase (25 m  0.32 mm id).
Undecane was chosen as the internal standard for GC analysis. High-
resolution mass spectra were obtained at the Centre Universitaire de
Mesures et d’Analyses de l’Universitꢀe de Lille 2 on an Exactive instru-
ment (Thermo Fisher Scientific).
Figure 1. Octadienylethers of tri- and tetraethylene glycol (C8:2E3 and
C
8:2E4).
are never formed as well-defined compounds and the commercial
products are mixtures with an average degree of ethoxylation. To
obtain well-defined compounds, an alternative synthetic path is
based on the reaction of a given polyoxyethylene with a halo-
genoalkane under Williamson-type conditions.8 It is, however,
unsatisfactory with respect to limited yields, the use of unwanted
reactants (halogenated compounds, strong bases), the genera-
tion of side products (salts), and the required solvent extraction
procedures to isolate the desired products.
The palladium-catalyzed telomerization of 1,3-dienes with
nucleophiles was first discovered in 1967 by Smutny,9 and since
then, it has received great attention from both the academic and
industrial communities. This reaction pathway is indeed 100%
atom-efficient and can avoid the use of organic solvents. 1,3-
Butadiene has been the most studied diene, and monoalcohols
are usually used as nucleophiles, as, for instance, in the indus-
trially relevant reaction of 1,3-butadiene with methanol, which
gives access to 1-octene after hydrogenation and methanol
elimination. The use of polyfunctional nucleophiles raises issues
of selectivity enhancement to avoid obtaining complex mixtures.
Recent papers describe the butadiene telomerization with various
polyols including ethylene glycol,10,11 1,2-propanediol, 1,3-
propanediol, 1,2-butanediol, 1,4-butanediol,12 glycerol,13 isosorbide14
or even more complex sugars or polyols as pentoses,15 sorbitol, or
sucrose.16 The use of biphasic reaction systems increases the
selectivity and allows catalyst recycling.11
As well as being efficient, this synthetic pathway is also an
expeditious way to obtain unconventional amphiphiles that
possess a 2,7-octadienyl chain (C8:2) with an inner double bond
having a trans configuration (Figure 1). Subsequent hydrogena-
tion gives way to the usual saturated homologues (C8E3 and
C8E4). The effect of the presence of double bonds in the alkyl
chain of amphiphiles, particularly the short ones, is not widely
discussed in the literature.17 In this work, we have been interested
in evaluating the effect of the presence of these double bonds on
the behavior of tri- and tetraethylene glycol ethers obtained by
butadiene telomerization with the corresponding poly(ethylene
glycol). The self-association of C8:2E3 and C8:2E4 in water and
oil/water has been evaluated in comparison with that of well-
known C8E3 and C8E4. For C8E4 and C8:2E4, some DLS
(dynamic light scattering) measurements have been performed
in the microemulsion of Winsor I systems. The conductor-like
screening model for real solvents (COSMO-RS) has also been
used to model the compounds.
2.3. Synthesis. The typical procedures are described below for
C8:2E3 and its saturated homologue C8E3. C8:2E4 and C8E4 are obtained
through the same procedure.
2.3.1. Telomerization. Catalyst [Pd(OAc)2] (11.2 mg, 0.05 mmol),
phosphine ligand TPPTS (114 mg, 0.2 mmol), triethylene glycol
(11.1 mL, 83 mmol), and a soda degassed aqueous solution (1 M,
0.8 mL) were introduced into a 100 mL stainless steel autoclave that was
bolted and flushed with nitrogen. The base was dissolved in distilled
water, degassed under nitrogen flow, and then transferred to the
autoclave. The latter was cooled to À20 °C. A precise volume of
butadiene (19 mL, 207 mmol) was condensed in a Schlenk tube with
an acetoneÀdry ice mixture and transferred into the autoclave. Finally,
the reactor was heated to 80 °C and vigorously stirred (at a rate of about
1000 rpm) with a magnetic stirrer for 3 h. After the reaction, the system
was cooled and excess gaseous butadiene was vented. The crude product
was homogenized by methanol addition, and 250 μL of undecane was
added. The conversion and selectivities were calculated from the GC
analysis of the homogeneous mixture: 60% glycol conversion is achieved
with a selectivity of up to 80% mono-C8:2E3 (linear + branched < 5%),
7% dialkylated dienols, and 13% octadienols.
2.3.2. Separation of the Telomers. Twenty milliliters of water were
added, and the telomers were extracted with ethyl acetate (3 Â 40 mL).
The organic phase was dried by using Na2SO4 and evaporated to
dryness. Telomers were purified by several distillations under reduced
pressure (0.1 mbar). Eb ≈ 130 °C for C8:2E3 and 145 °C for C8:2E4.
2.3.3. Hydrogenation. To get the saturated homologues (C8E3 and
C8E4), hydrogenation was performed in an autoclave at 20 bars of
hydrogen and at room temperature for 24 h in the presence of Pd/C.
Total conversion was obtained, and filtration of the catalyst over Celite
afforded the pure product.
The final products have 97À98% purity (GC and NMR) with 2 or 3%
branched isomers as impurities.
2.4. C8:2E3. 1H NMR (300 MHz, CDCl3): δ 5.74 (ddt, 1 H, 3JHÀH
=
17.4 Hz, 3JHÀH = 10.8 Hz, 3JHÀH = 6.4 Hz, CH2ÀCHdCH2); 5.64 (dt,
1H, 3JHÀH = 15.4 Hz, 3JHÀH = 6.6 Hz, CH2ÀCHdCHÀCH2O); 5.51
(dt, 1H, 3JHÀH = 15.4 Hz, 3JHÀH = 6.1 Hz, CHdCHÀCH2O); 4.95 (d,
3
3
1H, JHÀH = 17.4 Hz, CHdCH2); 4.90 (d, 1H, JHÀH = 10.8 Hz,
CHdCH2); 3.92 (d, 2H, 3JHÀH = 6.7 Hz, CHÀCH2O); 3.50 À 3.71 (m,
12H, triethylene glycol moiety): 2.92 (t, 1H, 3JHÀH = 5.9 Hz, OH); 2.01
(dd, 4H, 3JHÀH = 7.2 Hz, CHÀCH2ÀCH2ÀCH2ÀCH); 1.43 (tt, 2H,
3JHÀH = 7.5 Hz, CH2ÀCH2 ÀCH2). 13C NMR (75 MHz, CDCl3): δ
138.5 (1C, CHdCH2); 134.4 (1C, CHdCHÀCH2O); 126.4 (1C,
CHdCHÀCH2O); 114.5 (1C, CHdCH2); triethylene glycol moiety:
72.5 (1C), 71.9 (1C), 70.6 (2C), 70.3 (1C), 68.9 (1C); 61.5 (1C,
CHdCHÀCH2O); 33.1 (1C, CH2ÀCHdCHÀCH2O); 31.6 (1C,
CH2ÀCHdCH2); 28.2 (1C, CH2ÀCH2ÀCH2).
2. EXPERIMENTAL SECTION
HRMS m/z: [M + H]+ calcd for C14H26O4H, 259.1904; found,
259.1905.
2.1. Chemicals. Triethylene glycol (99%) and tetraethylene glycol
(99%) were purchased from Sigma-Aldrich, and palladium acetate
(98%) was purchased from Strem. n-Octane and cyclohexane used for
the construction of “fish” diagrams were obtained from Sigma-Aldrich or
Fluka in the highest available grades. All chemicals were used as received.
For a comparison of DLS in the microemulsion phase, a sample of
tetraethylene glycol decyl ether (C10E4) was synthesized and purified
following known procedures.8,18
2.5. C8:2E4. 1H NMR (300 MHz, CDCl3): δ 5.72 (ddt, 1 H, 3JHÀH
=
17.0 Hz, 3JHÀH = 10.4 Hz, 3JHÀH = 6.6 Hz, CH2ÀCHdCH2); 5.61 (dt,
1H, 3JHÀH = 15.6 Hz, 3JHÀH = 6.6 Hz, CH2ÀCHdCHÀCH2O); 5.48
(dt, 1H, 3JHÀH = 15.6 Hz, 3JHÀH = 6.2 Hz, CHdCHÀCH2O); 4.92 (d,
3
3
1H, JHÀH = 17.0 Hz, CHdCH2); 4.87 (d, 1H, JHÀH = 10.4 Hz,
CHdCH2); 3.88 (d, 2H, 3JHÀH = 6.2 Hz, CHÀCH2O); 3.47À3.68 (m,
16H, tetraethylene glycol moiety); 2.81 (m, 1H, OH); 1.98 (dd, 4H,
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3
3JHÀH = 7.4 Hz and JHÀH = 6.6 Hz, CHÀCH2ÀCH2ÀCH2ÀCH);
1.40 (tt, 2H, 3JHÀH = 7.4 Hz, CH2ÀCH2ÀCH2). 13C NMR (75 MHz,
CDCl3): δ 138.5 (1C, CHdCH2); 134.4 (1C, CHdCHÀCH2O);
126.4 (1C, CHdCHÀCH2O); 114.5 (1C, CHdCH2); tetraethylene
glycol moiety: 72.5 (1C), 71.9 (1C), 70.6 (2C), 70.5 (2C), 70.2 (1C),
68.98 (1C), 61.5 (1C, CHdCHÀCH2O), 33.2 (1C, CH2ÀCHdCHÀ
CH2O), 31.6 (1C, CH2ÀCHdCH2), 28.2 (1C, CH2 ÀCH2ÀCH2).
HRMS m/z: [M + Na] calcd for C16H30O5Na, 303.2166; found,
303.2158.
2.11. Solubilization of Sudan Red III. Aqueous solutions of the
amphiphile to be tested were prepared at different concentrations, and
disperse red 13 was added until reaching saturation. The solutions were
stirred at room temperature for 24 h. After this period, the solutions were
filtered by means of a syringe equipped with a 0.45 μm pore-size filter.
The amount of hydrophobic dye dissolved was determined by UVÀ
visible absorption of the solutions at 503 nm (Varian Cary 50 spectro-
meter). Prior to the measurement, a calibration curve was established at
this wavelength. The solutions were diluted in absolute ethanol before
measurement.
2.12. Fish Diagrams. Ternary amphiphile/oil/water systems were
prepared in 2 mL test tubes with the studied compound and chosen oil.
Equal amounts (by weight) of water and oil were first introduced, and
increasing amounts of amphiphile were added. After each addition, the
test tubes were gently mixed and placed in a thermostated bath at the
desired temperature ((0.1 °C) until the attainment of equilibrium,
which generally required 2 to 3 h. The types of Winsor systems (IÀIV)
were determined by visual observation.
2.13. Dynamic Light Scattering (DLS). Dynamic light scatter-
ing measurements were performed with a Malvern Zetasizer NanoZ
using a 4 mW HeÀNe laser operating at a wavelength of 633 nm with a
detection angle of 173°. All samples were prepared with 100-nm-filtered
Millipore water (resistivity ≈ 18.2 MΩ/cm). Homemade sample cells
were used to perform measurements in the microemulsion phase of fully
equilibrated Winsor I samples at the desired temperature. These cells
were made up of a 5 mm internal diameter glass tube in which the
compounds were directly weighted (first water, then oil, and then
amphiphile). The tube was then flame-sealed, and the sample was gently
mixed and left to equilibrate in a thermostatted bath at the temperature
required to get a Winsor I system. Once the sample was fully equili-
brated, the test tube was placed in a 2 mL clear vial (typical vial for a GC
autosampler) to allow for good positioning of the sample at the center of
the Zetasizer measuring cell that was previously equilibrated at the same
temperature. Previous control experiments were performed on pure
solvents and polystyrene standards to make sure that the homemade
sample cells did not disturb the DLS measurement.
1
2.6. C8E3. H NMR (300 MHz, CDCl3): δ 3.47À3.68 (m, 12H,
triethylene glycol moiety); 3.36 (t, 2H, 3JHÀH = 6.9 Hz, CH2ÀCH2O);
3.24 (m, 1H, OH); 1.49 (tt, 2H, 3JHÀH = 6.9 Hz, CH2ÀCH2ÀCH2O);
1.19 (m, 10H, CH3ÀCH2ÀCH2ÀCH2ÀCH2ÀCH2ÀCH2ÀCH2O);
0.79 (t, 3H, 3JHÀH = 6.9 Hz, CH3ÀCH2).
13C NMR (75 MHz, CDCl3): triethylene glycol moiety: δ 72.5 (1C),
71.9 (1C), 70.4 (2C), 70.3 (1C), 69.0 (1C); 61.6 (1C, CH2ÀCH2À
CH2O); 31.8 (1C, CH2ÀCH2ÀCH2O); 29.6 (1C, CH2ÀCH2ÀCH2O);
29.4 (1C, CH2ÀCH2ÀCH2ÀCH2O), 29.3 (1C, CH2ÀCH2ÀCH2À
CH2ÀCH2O), 26.1 (1C, CH3ÀCH2ÀCH2), 22.6 (1C, CH3ÀCH2),
14.2 (1C, CH3).
HRMS m/z: [M + H]+ calcd for C14H30O4H, 263.2217; found,
263.2213.
1
2.7. C8E4. H NMR (300 MHz, CDCl3): δ 3.46À3.67 (m, 16H,
3
tetraethylene glycol moiety); 3.36 (t, 2H, JHÀH = 6.9 Hz, CH2À
CH2O); 2.91 (t, 3JHÀH = 6.9 Hz, 1H, OH); 1.49 (tt, 2H, 3JHÀH = 6.9 Hz,
CH2ÀCH2ÀCH2O); 1.18 (m, 10H, CH3ÀCH2ÀCH2ÀCH2ÀCH2À
CH2ÀCH2ÀCH2O); 0.79 (t, 3H, 3JHÀH = 6.9 Hz, CH3ÀCH2).
13C NMR (75 MHz, CDCl3): triethylene glycol moiety: δ 73.0 (1C),
71.6 (1C), 70.7 (2C), 70.6 (2C), 70.4 (1C), 70.1 (1C); 61.6 (1C,
CH2ÀCH2ÀCH2O); 32.1 (1C, CH2ÀCH2ÀCH2O); 29.6 (1C, CH2À
CH2ÀCH2O); 29.4 (1C, CH2ÀCH2ÀCH2ÀCH2O); 29.3 (1C, CH2À
CH2ÀCH2ÀCH2ÀCH2O); 26.1 (1C, CH3ÀCH2ÀCH2); 22.7 (1C,
CH3ÀCH2); 14.2 (1C, CH3).
HRMS m/z: [M + H]+ calcd for C16H34O5H, 307.2479; found,
307.2477.
2.8. Surface Tension Measurements. The surface tension was
measured with a Kruss K100 tensiometer equipped with a double dosing
system for the automatic determination of the critical micellar concen-
tration (cmc). Because of the high cmc values and the necessity to
perform dilutions over a wide concentration range, individual measure-
ments were also performed at given concentrations. The tension was
measured by the plate method to an accuracy of (0.05 mN/m.
2.9. Interfacial Tension Measurements. The oil/water inter-
facial tension of pre-equilibrated Winsor III systems was measured with a
Kruss Site 100 spinning-drop tensiometer. The heavy (water) phase was
introduced into the thermostated capillary that was then rotated at a
speed of about 6000 rpm to eliminate air bubbles. One drop of the light
(oil) phase was then introduced, and the rotational speed was increased
so that the deformation of the oil drop was sufficient. To avoid any
destabilization processes, the sample, the tensiometer capillary, and the
syringes used to extract each phase were thermostated at the same
temperature ((0.1 °C), that is, the optimal temperature of the system
studied.
2.10. Determination of the Amphiphile/Water Miscibility
Gaps. The binary diagrams of C8:2E3, C8:2E4, C8E3, and C8E4 were
determined visually by gradually increasing the temperature of individual
samples and noting the temperature at which the solutions became
cloudy. The experiments were performed on heating and cooling, and
the discrepancy in the cloud-point temperature was usually lower than
0.1 °C. The cloud-point temperature was taken as the average.
For some samples, particularly at low concentrations, the visual
observation was difficult; therefore, a transmission probe (Ocean Optics
T200-RT-VIS/NIR) was used to have a reliable limit between clear and
slightly turbid samples.
3. RESULTS AND DISCUSSION
3.1. Atom-Economical Synthesis of C8E3/C8:2E3 and C8E4/
C
8:2E4. The telomerization reaction has been applied to well-
defined triethylene glycol and tetraethylene glycol as telogenic
polyols. The reaction takes place under biphasic conditions,
the catalyst remains in the aqueous phase thanks to the water-
soluble ligand, and the monotelomers are extracted in the
butadiene organic phase as they are formed.14 This favors the
formation of monotelomers over ditelomers (only 1 to 2%).
Apart from the butadiene dimer byproduct that is also formed
in small amounts because of the biphasic conditions, the main
impurities are the octadienols that result from the reaction
with residual water.
In the monotelomer fraction, which is easily purified by
distillation under reduced pressure, small amounts of branched
products are also encountered (<3%) and cannot be separated
from the main linear products. Hence, they must be considered
to be inherent to the products under study (Figure 2).
After catalytic hydrogenation, saturated homologues C8E3 and
C8E4 are obtained and also contain the same amount of branched
isomers.
This synthetic path allows us to obtain in two steps using a very
easy procedure around 50 g of pure products on a laboratory
scale. It is thus a convenient and useful way to get polyeth-
oxylated octyl ethers for physicochemical studies.
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Figure 2. Simplified scheme of the catalytic cycle for the Pd-catalyzed
synthesis of octadienylethers of tri- and tetraethylene glycol (C8:2E3 and
C
8:2E4) (left) and products obtained and studied in this work (right).
Figure 4. Optimized geometries and σ surfaces of C8E3 (left) and
C
8:2E3 (right).
phenomenon is concerned. Because this behavior is particularly
sensitive to the presence of very small quantities of surface-active
compounds, the branched derivatives do not show very different
hydrophobicity.
The interesting point is to observe the large increase (more
than 10 °C) in the cloud-point value for the doubly unsaturated
samples. This increase should be related to the weakening of the
lipophilicity of the chain when two double bonds are present,
which tends to increase the global hydrophilicity of the com-
pound. Usually, the evaluation of the effect of the presence of
unsaturation in the alkyl chain of amphiphilic molecules focuses
on the cis-ethylene bonds found in the fatty chain of natural fats.
The general trend is that the presence of unsaturation lowers the
lowest temperature at which fluid states may exist (Krafft tem-
perature in the case of ionics). It was shown that the cis double
bond in oleate has a profound effect on the Krafft boundary but
the soap/water phase behavior is not very disturbed.17 The self-
assembly of long-chain sugar-based amphiphiles in the form of
liquid crystals is also modified by the presence of cis unsaturations
in the alkyl chain.20 However, the influence of chain un-
saturation on liquidÀliquid miscibility gaps in the case of
polyethoxylated amphiphiles is not described and is generally
assumed to be small.17
Figure 3. Cloud-point curves of the binary amphiphile/water systems
of octadienylethers of tri- and tetraethylene glycol (C8:2E3 (b) and
C
8:2E4 (2)) in comparison with those of their saturated homologues
(C8E3 (O) and C8E4 (Δ)).
Table 1. cmc and Cloud Points of C8Ej (j = 3, 4) and C8:2Ej
(j = 3, 4)a
cmc (mmol/L)
Tc (°C)
To get an idea of the conformational change induced by the
presence of the two unsaturations, the compounds were drawn
using ArgusLab 4.0.1021 and optimized with Arguslab UFF. Next,
DFT calculations were performed using Turbomole 5.122 and
DFT functional B88-PW86 with a triple-ζ valence polarized basis
set (TZVP). Continuum solvation model COSMO23 has been
applied in order to simulate a virtual conductor environment for
the molecule, and the COSMOconf extension24 has been used to
determine the most relevant conformers. In each case, only one
conformer has been found, and the optimized geometries are
presented in Figure 4 for C8E3 and C8:2E3, together with the
COSMO surface (σ surface) generated for each compound
showing the local positions of strongly polar sites. Similar results
were obtained for the tetraethylene glycol derivatives.
In the σ-surface representation, green to yellow codes the
weakly polar surfaces, blue represents positive polarity (δ+),
and red codes strongly negative polar surfaces (δÀ). In our
case, the positive polarity (blue) is held by the terminal hydro-
gen of the polyethoxylate moiety, whereas the strongly nega-
tive polar groups (red) are the oxygen atoms of the polyeth-
oxylate moiety.
amphiphile
tensiometry
solubilization
cloud point
C8E3
10b (8.5)c
14b
5b (8.5)c
15b
11b
38b
7b
10.0 (11.0)c
27.6b
40.5b (40.8)c
52.4b
C8:2E3
C8E4
C8:2E4
39b
a The cmc values were obtained either from dye solubilization experi-
ments at 25 °C or surface tension measurements at 20 °C for the
triethylene glycol derivatives and at 25 °C for the tetraethylene glycol
derivatives. b This work. c Reference 25.
3.2. Binary Systems CiEj/Water (i = 8 or 8:2, j = 3 or 4). 3.2.1.
Miscibility Gaps for C8E3/C8:2E3 and C8E4/C8:2E4. The miscibility
gaps of binary systems H2OÀC8Ej and H2OÀC8:2Ej (j = 3, 4) are
presented in Figure 3, and the coordinates of the critical points
are reported in Table 1 in comparison to the literature data for
C8E3 and C8E4. The data reported by Schubert et al.19 are
reference data because they have been obtained from ultrapure
samples via a three-phase extraction technique (3PHEX). The
data obtained in this work are in quite good agreement both for
C8E4 and C8E3, with a slightly higher discrepancy (1 °C) in the
case of this latter compound.
As expected, the main differences are found in the alkyl chains
ofthecompounds. FordoublyunsaturatedcompoundsC8:2Ej (j=3,4)
,
the molecular shape is bent because of the trans double bond at
carbon 2, and the two unsaturations increase the overall polarity
of the alkyl chain, as shown by the yellow-to-orange segments at
the location of the double bonds.
Nevertheless, it is interesting that the small quantity of
branched derivatives that are formed during the telomerization
reaction (<3%) and that cannot be removed by distillation does not
significantly impact the behavior in water, as far as the clouding
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Figure 5. σ profiles of C8:2E3 (À) and C8E3 (---).
Both the bent conformation and the presence of more polar
segments are responsible for the weakening of the hydrophobicity of
C8:2Ej (j = 3, 4) compared to that of the corresponding C8Ej.
The σ surface of the molecules can be expressed in a more
convenient way by using the σ-profile representations, as pre-
sented in Figure 5. They express on a 2D graph the proportion of
the surface with a given charge density σ. The comparison of the
σ profiles of C8E3 and C8:2E3 shows that for both compounds the
majority of the charge density is close to zero, expressing the low
polarity of the carbon and hydrogen atoms of the skeleton. The
oxygen atoms bear a negative polarity and are responsible for the
secondary maxima encountered at around 0.01 e.ÅÀ2. The
terminal proton imparts a slight hydrogen bond donor character
to the molecules. The main difference between the σ profiles of
C8E3 and C8:2E3 is the increase in density in the positive region
for the octadienyl derivative due to the electron-rich parts of the
hydrophobic chain (double bonds). The central peak is also
slightly shifted to more negative σ because of the decreased
number of hydrogen atoms in the skeleton compared to the
number in the saturated homologue, which is also an indication
of the decreased apolarity of the lipophilic chain.
3.2.2. Self-Association in Water by Surface Tension and
Solubilization Experiments. The concentration dependences of
the surface tension of aqueous solutions of the octadienyl ethers
of tri- and tetraethylene glycols C8:2Ej (j = 3,4) in comparison to
those of corresponding C8Ej are presented in Figure 6a,b. For
CiE3 (i = 8 or 8:2), the measurements were performed at 8 °C to
be below the cloud point of C8E3.
As expected for medium-chain amphiphiles, no clear break
is observed at the aggregation concentration, which in this
case cannot be properly called micellization. The decrease in
the surface tension with concentration is rather smooth and
is actually slightly more marked for the saturated compounds,
which tends to show again a rather stronger amphiphilic
behavior.
Figure 6. Surface tension vs concentration for aqueous solutions of
octadienylethers of tri- and tetraethylene glycol (C8:2E3 (b) and C8:2E4
(2)) in comparison to their saturated homologues (C8E3 (O) and C8E4
(Δ)) at T = 25 °C for tetraethylene glycol derivatives, and at T = 20 °C
for triethylene glycol derivatives. The dotted vertical lines indicate where
the cmc's are located, and the corresponding values are reported in
Table 1.
usual saturated CiEj. To estimate more precisely the aggregation
concentrations, solubilization experiments were performed, be-
cause the solubilization of a hydrophobic compound in aqueous
surfactant solutions is known to increase strongly at the con-
centration at which the surfactants start to aggregate.
Figure 7a,b show these solubilization experiments performed
with disperse red 13, a hydrophobic azo dye. They were carried
out at 25 °C, a temperature higher than the cloud point of C8E3,
which explains the noisier results for this compound.
Nevertheless, for both the tri- and the tetraethylene glycol
derivatives, it is clear from these results that aggregation takes
place at higher concentrations for the unsaturated compounds.
The aggregation concentrations can be clearly worked out from
these data and are consistent with the surface tension curves.
They are given in Table 1. The values for saturated C8E3 and
C8E4 are in fairly good agreement with those obtained from the
surface tension measurements and with the published values
from Ohta et al.25 For C8E3, the value is slightly higher, but that
can be due to the uncertainty in the data obtained once the
solutions are cloudy. The values worked out for the unsaturated
compounds are slightly higher than those obtained from the
surface tension curves, but still the important point to stress is the
increase in aggregation concentrations, which confirms the
reduced hydrophobicity of these compounds. Ambrosone et al.26
have studied even shorter amphiphiles and have shown that
This point is confirmed by the value of the saturated surface
tension that is also slightly higher for the octadienyl compounds
than for the octyl homologues: 31.2 versus 28.9 mN/m for the
triethylene glycol derivatives and 32.7 versus 27.9 mN/m for the
tetraethylene glycol derivatives. For all compounds and particu-
larly for the tetraethylene glycol derivatives, a minimum is
observed at the cmc, which indicates the presence of one or
several other surface-active compounds.
The concentration at which the saturated surface tension is
reached is higher for the unsaturated compounds than for the
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Figure 8. Fish diagrams of octadienylethers of tri- and tetraethylene
glycol (C8:2E3 (b) and C8:2E4 (2)) in n-octane/water 1/1 systems in
comparison with those of their saturated homologues (C8E3 (O) and
C8E4 (Δ)).
Figure 7. Dye solubilization by aqueous solutions of octadienylethers of
tri- and tetraethylene glycol (C8:2E3 (b) and C8:2E4 (2)) as a function
of concentration, in comparison with their saturated homologues (C8E3
(O) and C8E4 (Δ)) at T = 25 °C.
of these two phases in the middle-phase microemulsion is
maximized and the interfacial tension between the oily and
aqueous phases is minimized.
aggregation takes place at 0.00187 (molar fraction) for C6E3 and
0.00197 (molar fraction) for C6E4, which correspond to approxi-
mately 104 and 109 mmol/L, respectively. If we compare these
values to those determined in this work, then it appears that the
loss in hydrophobicity for the octadienyl compounds compared
to the octyl derivatives would thus correspond approximately to
the loss of one carbon in the alkyl chain.
These experiments also show that the solubilization capacities
of the aggregates formed are very similar for the octyl and
octadienyl derivatives because the slopes of the straight lines
obtained after aggregation are almost the same for the same
polyethoxylated polar head.
Finally, it should be stressed that for C8:2E3 a zone of turbidity
is found near the aggregation concentration (indicated as a gray
zone in Figure 7a). The same behavior had been observed for
C6E3 by Ambrosone et al.26 and had been attributed to the
“borderline” character of this compound, which has intermediate
behavior between that of completely insoluble C6E2 and that of
completely soluble and micellizing C6E4.
3.3. Behavior in Oil/Water 1/1 Systems. It is well known that
ternary mixtures of ethoxylated alcohols, n-alkanes, and water can
separate into a maximum of three liquid phases at equilibrium,
depending on the amphiphile, the alkane carbon number (ACN),
and the temperature.27 For a given oil and amphiphile, the three-
phase systems (Winsor III) appear within a given temperature
range, and the optimal temperature T* is defined as the tem-
perature at which the ethoxylated alcohol exhibits an equal
affinity for the aqueous or oily phase. At T*, the cosolubilization
The complete phase behavior of the CiEj/n-alkane/water system
is represented in a prism, but for the sake of simplicity and
depending on the properties that need to be enlightened, 2D
representations are preferred, namely, the delta (Δ) cut at con-
stant temperature, the gamma (γ) cut with a constant waterÀoil
ratio, and the chi (χ) cut at a constant surfactant concentration.28
3.3.1. Fish Diagrams of C8E3, C8:2E3, C8E4, and C8:2E4 with
n-Octane/Water. The γ cut at the 1/1 w/w water/oil ratio (also
called a fish diagram) gives access to important parameters that
allow the comparison of the amphiphilicity of various CiEj's if the
same n-alkane is chosen as the oil. Such diagrams have been
drawn for the compounds under study in the n-octane/water
system for which a number of literature data are available. They
are presented in Figure 8. The three-phase region is encountered
within the fish body in the (Tl À Tu) temperature range and for
CiEj concentrations between C0 and C*. For concentrations higher
than C*, a one-phase microemulsion (Winsor IV) is obtained,
and for concentrations lower than C0, two-phase systems are
formed. In the C0 À C* concentration range, for temperatures
lower than Tl, Winsor I systems are formed, whereas for tem-
peratures higher than Tu, Winsor II systems are encountered.
The coordinates of the X point of the fish diagram give
the optimal temperature T* and the efficiency C* of the
amphiphile.
The data are summarized in Table 2 in comparison with
literature data on the same systems.
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Table 2. Characteristic Data of the H2O/n-Octane/CiEj γ
Representations in Comparison with the Literature
Table 3. DLS Investigation of the Microemulsions of 10%
C8E4, 10% C8:2E4, and 1% C10E4 in n-Octane/Water 1/1a
TuÀ Tl
C*À C0
amphiphile
C8E4
T (°C)
Winsor system
Z average (nm)
PDI
amphiphile T* (°C) Tl (°C) Tu (°C) (°C) C* (%) C0 (%) (%)
25
40
55
25
55
20
25
30
WI
11
23
22
11
20
49
18
32
0.122
0.131
0.167
0.211
0.220
0.292
0.142
0.064
b
C6E3
C6E4
44.6
77.5
56.4
15.2
>80
42.5
41.9
35.8
nd
53.4
nd
17.6
nd
39.5
53.7
62.4
21.5
>60
25.3
24.4
2.9
nd
36.6
nd
WIII
WII
WI
c
a
a
C8:2E3
26.2
10.4
49.1
35.0
35.5
75.6
20.0
>90
47.6
48.2
49.4
9.6
4.7
1.8
2.5
2.0
1.7
57.7
19.7
C8:2E4
C10E4
a
C8E3
WIII
WI
C8:2E4
a
C8E4
12.6
12.7
22.2
22.7
WIII
WII
b
C8E4
a This work. b Reference 29. c Reference 30.
a The Z average is the mean hydrodynamic radius. PDI is the poly-
dispersity index.
The comparison of T* indicates again that the unsaturated
compounds exhibit very reduced hydrophobicity because for the
same polar head (E3 or E4) the optimal temperature is increased
by about 40 °C. For E3, it is interesting that T* for C8:2E3 is even
12 °C higher than that of C6E3, which indicates that the C8:2 alkyl
chain is even less hydrophobic than a saturated C6. Such a
conclusion could not be anticipated from the data obtained with
the binary systems CiEj/water. (See the comparison of aggrega-
tion concentration values in section 3.2.2.) The data presented in
Table 2 for C8E3 and C8E4 are in good agreement with the
literature values, which tends to indicate again that the presence
of small amounts of branched byproduct do not affect the
behavior in oil/water systems.
The second point that should be mentioned, which is linked to
the weakening of the hydrophobic chain, is the lower efficiency of
the unsaturated compounds, as indicated by the very high values
of C*. This reduced efficiency has been confirmed by measuring
the interfacial tension between excess oil and water phases in the
optimal three-phase systems of the four compounds (next section).
The most characteristic feature that needs to be discussed in
more detail is the large extension in temperature of the three-
phase region (fish body). Strey and co-workers29,31 have exten-
sively studied the phase behavior of polyethoxylated alcohols
ranging from weak to strong amphiphiles and have pointed out
that in a homogeneous series of CiEj and for the same n-alkane
the extension in temperature of the three-phase body is low for
the shortest amphiphiles (C4E1), passes through a maximum for
medium-chain amphiphiles (C6E3), and then diminishes again
for long-chain amphiphiles (C10E5, C12E6). This maximum in
the T extension of the fish body indicates the frontier between
weakly and strongly structured mixtures, also named the Lifshitz
line in other papers.32
To evaluate the degree of structuration of the microemulsions
formed by C8E4 and C8:2E4, some DLS (dynamic light
scattering) experiments were performed (Table 3) in the
amphiphile-rich phase of the different Winsor-type systems
formed depending on the temperature. In parallel, the same
measurements were carried out on similar systems formulated
with a stronger amphiphile having the same hydrophilic head,
namely, the decyl ether of tetraethylene glycol C10E4. The phase
diagram of this compound in n-octane/water has already been
studied in detail.28 The fish diagram at 1/1 n-octane/water shows
the existence of a three-phase system for C10E4 concentrations of
between 0.9 and 10.8% in the temperature interval between 22.0
and 29.2 °C. In this work, as a point of comparison, DLS measure-
ments were thus carried out at 1% C10E4 at 20, 35, and 30 °C for
WI, WIII, and WII systems, respectively. In the same way,
experiments were performed at 10% C8E4 and 10% C8:2E4 at
temperatures chosen to scan the different Winsor systems (cf.
Figure 8 and Table 3). Because of the extension of the fish body
in the case of C8:2E4, the Winsor II system of this compound
could not be investigated.
Only for C10E4 could the hydrodynamic radius worked out in
the microemulsion of the WI system be consistent with the
existence of discrete oil droplets (around 50 nm). For the C8E4
and C8:2E4 amphiphiles, average hydrodynamic radii close to
11 nm are found, which is too small compared to the molecular
lengths of the compounds and indicates that no real structuration
is found in this type of system. This was already pointed out by
Bodet et al.33 for similar systems formulated with C4E1 for which
DLS showed the existence of short-living, diffusive microdo-
mains but no real structuration. In the same way, all microemul-
sions in the WIII systems have an average hydrodynamic radius
that is close to 20 nm, which is an indication of the fluctuating
microstructuration. Again in the Winsor II systems, only in the
case of C10E4 could the average hydrodynamic radius support the
formation of discrete water droplets. Trying to establish a
distinction between weak and medium-strength amphiphiles
on the basis of the extension of the fish body or the DLS data
is somewhat unwise. In our case, we can say that both C8E4 and
C8:2E4 are fairly weak amphiphiles and no real structuration is
observed.
The large extension of the three-phase body in the case of the
unsaturated amphiphile might also be related to a loss of
temperature sensitivity. In conventional CiEj, the temperature
sensitivity is linked to the progressive dehydration of the poly-
ethoxylated polar head on heating. In the case of the unsaturated
compounds, a part, albeit small, of the hydrophilicity is held by
the double bonds (σ surface in Figure 4) and does not undergo
the same phenomenon when the temperature is increased.
Another alternative explanation of this enlargement of the fish
body in temperature could be a modification of the individual
solubilities of the amphiphiles in water and n-octane. The
increased solubilities of the unsaturated compounds in water
(higher cloud points, cpβ) have been established in the previous
section. A lower solubility in oil (higher critical point of the mis-
cibility gap, cpα) has been verified in the case of the tetrae-
thylene glycol derivatives: octylderivative C8E4 is completely
soluble in n-octane down to À20 °C, whereas octadienyl
derivative C8:2E4 presents a large miscibility gap in n-octane
(Figure 9).
Kilpatrick et al.4 pointed out that there are two distinct
patterns in which the three-phase system arises, depending on
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Table 4. Interfacial Tension between Excess Oil and Water
Phases at the Optimal Formulation for the H2O/n-Octane/
CiE3 and H2O/Cyclohexane/CiE4 Systems
system
T* (°C)
γmin (mN/m)
C
8:2E3/n-octane/watera
56.4
15.2
15.8
43.0
21.2
2
C8E3/n-octane/watera
C8E3/n-octane/waterb
C8:2E4/ cyclohexane/watera
C8E4/cyclohexane/watera
a This work. b Reference 34.
3 Â 10À2
2 Â 10À2
3 Â 10À1
2 Â 10À2
interfacial tension measurements were performed between the
excess oil and water phases at the optimal formulation (corres-
ponding to T*). It is indeed at this point that the minimum
interfacial tensions between the oil and water phases are en-
countered, when the affinity of the surface-active compound is
the same for both phases.34 The measurements were performed
in the n-octane/water system for the triethylene glycol derivatives
and in the cyclohexane/water system for the tetraethylene glycol
derivatives (Table 4). For C8E3, the results compare well with the
literature data and are another proof of the convenient purity of the
compounds prepared via the synthetic path presented here. In all
cases, it is clear that the addition of two double bonds to the
hydrophobic chain lowers the efficiencies of the amphiphiles
because the interfacial tension at the optimal formulation is divided
by approximately 100 in the case of the CiE3/n-octane systems and
by 10 in the CiE4/cyclohexane systems.
Figure 9. Miscibility gap of the octadienyl ether of tetratethylene glycol
C8:2E4 in n-octane.
4. CONCLUSIONS
Butadiene telomerization with polyols is a green atom-efficient
way to graft an alkyl chain onto hydrophilic synthons and is thus
convenient for obtaining amphiphilic species. The raw materials
obtained by this method possess a 2,7-octadienyl alkyl chain with
an inner double bond having a trans configuration. In this work,
the reaction has been performed on tri- and tetraethylene glycols,
leading to unconventional CiEj's. Further hydrogenation leads to
the well-known fully saturated homologues.
The behavior of these unsaturated derivatives in water and in
oil/water systems shows a weakening of the hydrophobicity and
a lower efficiency compared to that of the saturated homologues.
In γ representations with n-octane as the oil, the extent of the fish
body is greatly enlarged, which can be related to the weak
amphiphile character of the compounds and to the decreased
affinity of the unsaturated compounds with oil.
Even if this synthetic pathway cannot avoid the presence of a
small amount of branched products (<3%), it seems that it does
not strongly influence the physicochemical characteristics in
either binary or ternary systems. This has been proven by com-
parison with the literature data on tetraethylene glycol and
triethylene glycol octyl ethers.
Butadiene telomerization with polyols followed by hydroge-
nation is thus an interesting synthetic path to amphiphiles having
an octyl alkyl chain. The hydrogenation step is mandatory to
obtain the fully saturated compounds that are the real amphi-
philes because it has been shown that the presence of two double
bonds greatly impacts the efficiency of the compounds. Because
this synthetic pathway can be easily scaled up, it could represent
an efficient and more environmentally friendly way to obtain
well-defined poly(ethylene glycol) ethers.
Figure 10. Fish diagrams of the octadienylether of tetraethylene glycol
(C8:2E4 (2)) in cyclohexane/water 1/1 systems in comparison with that
of its saturated homologue (C8E4 (Δ)).
the strength of the amphiphile, the relative sizes of hydrophilic
and lipophilic moieties, the n-alkane chain length, and the
temperature. In the case of low-molecular-weight CiEj such as
the one under study (weak amphiphiles), they postulated that
the third phase arises from the fusion of two different two-phase
regions. We can assume that the reduced solubility of the un-
saturated compounds in n-octane is responsible in this case for
the enlarged three-phase region observed.
3.3.2. Fish Diagrams of C8E4 and C8:2E4 with Cyclohexane/
Water. To determine a complete γ plot for C8:2E4 in comparison
with C8E4 in the experimentally available temperature range,
n-octane has been replaced by cyclohexane, which has a lower
equivalent alkane carbon number (EACN = 1.518). The corre-
sponding γ plots for cyclohexane/water 1/1 are presented in
Figure 10.
This system allows us to draw the same conclusions as in the
n-octane systems regarding the values of T* and C* for the
saturated and unsaturated compounds (i.e., a weakening of the
hydrophobicity of the chain (higher T*) and a lower efficiency of
the unsaturated compounds).
With regard to the extension in temperature of the fish body, it
can be noticed here as well that there is a widening of the
temperature interval (Tu À Tl) for C8:2E4 but this widening is far
less important than in the case of n-octane.
Finally, to support the fact that the unsaturated amphiphiles
present a reduced hydrophobicity and a lower amphiphilicity,
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’ AUTHOR INFORMATION
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(25) Ohta, A.; Takiue, T.; Ikeda, N.; Aratono, M. J. Sol. Chem. 2001,
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Corresponding Author
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The French Centre National pour la Recherche Scientifique
(CNRS) is acknowledged for a grant to J.L. This work was
partially financed by Rꢀegion Nord-Pas de Calais through
European FEDER funding no. 33785 for the emerging project
Green Chemistry: Catalytic Synthesis and Functional Properties
of New Amphiphiles Derived from Biomass.
Genie Procedes 2001, J2157/1–J2157/20.
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