Increased Emulsion Stability for Reverse Y-Shaped Sugar-Based Surfactants

Article abstract of DOI:10.1007/s11743-015-1710-x

A series of reverse Y-shaped surfactants containing aromatic and aliphatic linkers to combine two short hydrocarbon chains and one carbohydrate head group was prepared. Liquid crystalline behavior, air-water interfacial properties, and efficiency as an em

Full text of DOI:10.1007/s11743-015-1710-x

J Surfact Deterg
DOI 10.1007/s11743-015-1710-x
ORIGINAL ARTICLE
Increased Emulsion Stability for Reverse Y-Shaped Sugar-Based
Surfactants
1
1
1
•
Tammar Hussein Ali
Thorsten Heidelberg
•
Hairul Anuar Bin Tajuddin
•
Rusnah Syahila Duali Hussen
1
Received: 24 November 2014 / Accepted: 17 June 2015
Ó AOCS 2015
Abstract A series of reverse Y-shaped surfactants con-
taining aromatic and aliphatic linkers to combine two short
hydrocarbon chains and one carbohydrate head group was
prepared. Liquid crystalline behavior, air–water interfacial
properties, and efficiency as an emulsifier was investigated
for each reverse Y-shaped surfactant. All reverse Y-shaped
surfactants mediated higher emulsion stabilities for water-
in-oil compared to common typical reference surfactants,
reflecting an improved ability to cope with a curvature
towards water. The introduction of a benzene ring into the
linker substantially increased the affinity of the surfactant
for hydrophobic media, resulting in improved emulsion
stability for both water-in-oil and oil-in-water.
Introduction
Glycolipids, whether natural or synthetic, are non-ionic
surfactants that are associated with independent environ-
mental performance, biological degradability, and reason-
able pricing due to the abundance of renewable resources
[1–11]. Their application potential is not much restricted
by salinity and pH variations [12]. During recent years,
there has been intense interest in the development of new
surfactants derived from natural resources. Moreover, the
synthesis of surfactants has dramatically increased,
reflecting the wide range of applications for these com-
pounds, such as in cleaning and personal care products,
cosmetics, emulsion paints, and polishes [6]. Therefore,
many studies are now focussed on the industrial processes
and formulation of these compounds, in an attempt to
optimize the economy of sugar-based surfactants [1, 2, 13,
Keywords Reverse Y-shaped surfactant Á Sugar-based
surfactant Á Double alkyl chain Á Water-in-oil emulsion Á
Click chemistry
1
4].
Almost all synthetic straight-chain alkyl glycosides
AG) and their phase behavior properties have been
(
extensively reported [15]. The same cannot be said for
branched-chain alkyl glycosides [16–18]. The latter are
considered as low-foaming surfactants compared to
straight-chain AG [17, 19]. Over the past few decades, only
a few reports had dealt with double-chain alkyl surfactants
such as disodium 1,8-bis (alkyloxymethyl)-3,6-dioxaoc-
tane-1,8-disulfates and disodium 5,12-bis (alky-
loxymethyl)-4,7,10,13-tetraoxahexadecane-1,16-disul-
fonates (alkyl = octyl or decyl). Their structure consists of
double-chain amphiphilic bearing two ionic head groups,
with the molecular structure apparently shaped as a bundle
of two typical single-chain surfactants. However, these
designs reduce surface tension and enhance interfacial
properties [20]. Besides the advantage of the double-chain
in a surfactant, the presence of different linkages between
Supporting information A supplementary data section is
provided, which includes full details of the synthetic procedures as
well as characteristic data for intermediated products. The material
also contains images of NMR spectra, enabling the evaluation of the
surfactant purity and pictures of the emulsions before and after
separation. Supplementary data can be obtained from the journal
webpage.
&
&
Hairul Anuar Bin Tajuddin
hairul@um.edu.my
Thorsten Heidelberg
heidelberg@um.edu.my
1
Chemistry Department, Faculty of Science, University of
Malaya, 50603 Kuala Lumpur, Malaysia
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J Surfact Deterg
the head group and the hydrocarbon chain affects the sur-
factant behavior, possibly due to intramolecular H-bonding
The lyotropic phase behavior of the glycolipids was
investigated based on a contact penetration study observed
under an optical polarizing microscope (OPM). The
investigation was carried out at room temperature (about
27 °C), applying two different solvents, one of which was
polar (water) and the other non-polar (1-undecanol).
[
21]. This refers not only to the functional group that
mediates the connection, but in particular to small molec-
ular spacers and linkers that may be introduced between the
two surfactant-antipodes, i.e., head group and tail. The
effect of different linkages including triazole and benzene
in double-chain glycolipids is not clearly understood.
The current work investigated the benzene and triazole
linkages with regard to flexibility on the phase behavior.
We successfully synthesized a new series of double-chain
glycolipids with different possibilities of linkage. This
design of surfactants exhibits greater ability to lower sur-
face tension and increase emulsion stability than could
comparable single-chain surfactants, like alkyl polygluco-
sides (APG) [22] and alkyl triazole glycosides (ATG) [23].
Experimental Procedure
General Method for Click Chemistry I
A solution of sugar azide (4.5 mmol) and terminal alkyne
compound (4.9 mmol) in 40 mL MeOH was treated with
CuCl (40 mg). The solution was stirred at room tempera-
ture overnight and subsequently filtered through Celite.
After the solvent had evaporated, the residue was purified
by filtration through 5 cm silica gel. Elution with 2:1 ethyl
acetate:hexane removed any remaining starting materials,
after which the product was eluted with 4:1
methanol:CHCl3.
Experimental Section
Chemicals
General Procedure for Deacetylation II
Chemicals were procured from various suppliers, as indi-
cated in brackets: 3,5-dihydroxybenzoic acid (Merck
Deprotection was carried out by using a catalytic amount of
NaOMe in methanol. The mixture was stirred for 4 h at
room temperature. The catalyst was removed by neutral-
9
7 %), dihexylamine (Merck 99 %), 2-bromoethanol
Merck 95 %), b-D-glucose pentaacetate (Merck 98 %),
sodium azide (Merck 99 %), 1-bromohexane (Merck
8 %), propargyl bromide (Sigma-Aldrich solution 80 wt%
in toluene), 3-chloroperbenzoic acid (Sigma-
(
?
ization with Amberlite IR120 (H ) before the solvent was
9
evaporated to give the final surfactant.
Aldrich B77 %), and Allyl alcohol (Merck C99 %). All
chemicals were used without further purification. The
reaction products were purified by column chromatography
using the flash technique on silica gel 35–60 mesh (Merck).
TLC was performed on pre-coated plates of silica gel 60
Emulsion Stability
The water-in-oil emulsion preparation applied a composi-
tion of 1:19 ratio of water and oil (methyl laurate) con-
taining 0.5 % (wt/vol) surfactant. The formulation was
mixed with a homogenizer for approximately 2 min at
room temperature at a speed of 14,450 rpm. The emulsion
samples were stored at room temperature and monitored
for phase separation over a few weeks. A similar procedure
was applied for the stability investigation of oil-in-water
emulsions with a 19:1 ratio of water and methyl laurate,
and containing about 0.5 % surfactant.
(
GF254 by Merck). Visualization was achieved by treat-
ment with 15 % ethanolic sulfuric acid and subsequent
heating.
Instrumentation
Structural identities of the reaction products were based on
1 13
NMR spectra ( H and C, recorded on a Bruker AVN-
4
00 MHz spectrometer).
Surface tension measurements were performed at 25 °C
Results and Discussion
Surfactant Synthesis
under atmospheric pressure using a KSV Sigma 702 ten-
siometer. This instrument applied the DuNo u¨ y ring
method. The critical micelle concentration (CMC) was
assessed as the intersection of the linear regressions of the
surface tension against the logarithmic surfactant concen-
tration for the concentration-dependent region as well as
the concentration-independent region, at high surfactant
concentration. The surface tension at this intersection is
called the surface tension at the CMC (cCMC).
Glucose was selected and fixed as a hydrophilic domain for
all reverse Y-shaped surfactants, whereas the hydrophobic
domain comprised two C -hydrocarbon chains. This dis-
6
tribution of a C12 domain was most economical with
respect to chemical synthesis. The investigated reverse
Y-shaped sugar surfactants (compounds 2, 4, 6 and 8) are
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J Surfact Deterg
Fig. 1 Reverse Y-shaped
sugar-based surfactants
Fig. 2 Synthesis scheme for the hydrophobic building blocks
displayed in Fig. 1. For purpose of comparison, two
structurally closely-related single-chain sugar surfactants
Fig. 3 Synthesis of hydrophilic building blocks
(
9 and 10) were chosen, resembling previously investigated
alkyl polyglucosides (APG) [22] and alkyl triazole glyco-
sides (ATG) [23], respectively.
route to the carbohydrate precursors is shown in Fig. 3. All
surfactants were obtained in overall yields ranging from 29
to 56 % based on glucose pentaacetate. Chromatographic
purification was required due to the presence of remaining
All surfactants were prepared with a sequential
approach, in which the alkyl chains were first attached to a
linker. The latter was subsequently coupled to the carbo-
hydrate by glycosylation [2], alkylation of an amine [24] or
CuAAC-based click-chemistry [25]. The synthesis of
hydrophobic precursors is presented in Fig. 2, while the
1
starting material and side products. The H-NMR spectra
of the surfactants indicated high purity for all products and
confirmed the complete removal of protecting groups in the
final surfactants.
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J Surfact Deterg
Liquid Crystalline Behavior
matched this expectation. The reason might be found in the
rather short alkyl chains applied in this investigation,
requiring substantial participation of the linker to balance
the cross section of the sugar head group.
The lyotropic liquid crystal phases were investigated by
optical polarizing microscopy (OPM) at room temperature
using a contact penetration technique [26, 27]. There was no
liquid crystalline phase observed for surfactants 2, 6 and 8
when in contact with water. On their own the compounds
appeared as viscous fluids (syrup) at room temperature.
While no texture was observed for compound 2, probably
reflecting an isotropic liquid based of rather low viscosity,
compound 6 exhibited dark coloring, which made it
impossible to observe liquid crystalline textures. In the case
of compound 4, low water solubility was observed. This
reflected the presence of a benzene ring, which increased
the hydrophobicity of the surfactant. However, at the con-
tact zone with water a phase showing birefringence
emerged. The texture, depicted in Fig. 4b, suggested a
hexagonal phase. The compound alone also exhibited
birefringence, as displayed in Fig. 4a. However, in contrast
to the water penetrated assembly, the fan shape texture for
the anhydrous compound was more in line with a smectic A
phase (lamellar). The observation of a smectic phase for the
pure compound and an La phase at low water concentration
suggested that the aromatic ring with its two short alkyl
chains could balance the surface area of the carbohydrate
head group, thus promoting the lamellar phase.
The presence of aromatic groups, reflecting both benzene
and triazole, reduced the water-solubility of the surfactants.
This was to be expected, because of a substantially
increased hydrophobicity of the aglycon comprising not
only the alkyl chains, but the linking unit as well. Surpris-
ingly the addition of aromatic rings did not substantially
enhance the interactions with an oil phase. This could be
due to the selection of the latter, which did not contain
aromatic components. The observed low interaction of
surfactants involving triazole linkages and oil might dis-
courage the application of click coupling for the preparation
of sugar-based surfactants. A more extensive study
involving different types of oil as well as a wider range of
surfactants would be required to investigate the effect.
Air–Water Interface Behavior
The behaviour of the surfactants at the air–water interphase
was investigated by systematic surface tension measure-
ments over a wide range of concentrations, displayed in
Fig. 5. With the exception of compound 2, for which a
CMC could not be determined despite measurements at
high concentration, data regarding the micellar assembly
for the surfactants are tabulated in Table 1.
Upon contact with the non-polar solvent 1-undecanol, sur-
factants 6 and 8 did not show visible solubilization. However,
the exposure led to a swelling of the surfactant. No liquid
crystalline phase was observed. Compounds 2 and 4, on the
other hand, exhibited significant solubility in 1-undecanol. Like
The CMC decreased by one decade upon introduction of
a benzene ring, as seen in the lower values for compounds
4 and 8. The drastic reduction in the CMC could be due to
the presence of the benzene ring, which had somehow
increased the hydrophobicity of the surfactant consider-
ably. On the other hand, the triazole affected the CMC
significantly less. This observation was in line with previ-
ous observations for ATG [23]. A comparison of com-
pounds 4 and 8 indicated a significant reduction of the
minimum surface tension (cmin) upon introduction of the
triazole linkage, whereas the CMC itself was not much
6
and 8, 2 did not form a liquid crystalline phase in undecanol. 4
however exhibited a texture that closely resembled that of the
pure surfactant, as shown in Fig. 4c, thus confirming the sta-
bility of the lamellar phase in the non-polar solvent.
In terms of molecular assembly, a preference for the
lamellar phase was expected for reverse Y-shaped surfac-
tants, owing to a balance of the domain surface areas, like
that found in natural lipids. However, only surfactant 4
Fig. 4 OPM texture for compound 4: a texture of the pure sample. b Water penetration scans showing hexagonal H1 and lamellar phases La at
room temperature. c After 2-h contact with 1-undecanol, indicating penetration of the solvent
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J Surfact Deterg
Table 2 Emulsion stability (O/W and W/O)
Surfactants no.
Time for phase separation
O/W (days)
W/O
2
4
6
8
9
\1
5
30 min
Gel
5
*1 h
3 h
5
6
15 min
10 min
1
0
\1
Fig. 5 Surface tension measurements; 4 (filled squares), 6 (filled
triangles), 8 (cross symbols)
ionic interactions originating from a partial protonation of
the amine in combination with the relatively weak
hydrophobic effects of the short alkyl chains. On the other
hand, all reverse Y-shaped surfactants exhibited improved
emulsion stability for water-in-oil compared to the single-
chained reference surfactants. This was expected, consid-
ering the molecular shape. The introduction of a second
chain reduced the curvature of a surfactant assembly
towards the hydrophilic domain, thus simplifying the
reverse curving required for an oil-based emulsion. Sur-
factant 4 exhibited a remarkably good water-in-oil emul-
sion; no separation was observed within an observation
period of 2 months. However, the initially fluid emulsion
formed a gel after about 2 months. The latter required
heating above 67 °C to liquefy again. The emulsion sta-
bility of 4 to that 8, which was remarkable low. It might
reflect the effects of interacting head groups at the inter-
phase, as well as enhanced flexibility in surfactant 8 with
the introduction of the triazole ring which could prevent
these head group interactions. Stability of an emulsion is
highly related to the size of the droplet and their viscosity
Table 1 Surfactant behavior
Compound
CMC (mM)
ccmc (mN/m)
4
6
8
0.7
3.7
0.4
34
37
28
affected. An explanation could be found in conformational
constraints associated with the linkage of the carbohydrate
head group to the hydrophobic domain, because of the
directly-linked pyranose ring. Surfactant 8 however, con-
sisted of additional carbon atoms and a triazole ring,
resulting in greater flexibility. The lower surface tension of
surfactant 8, compared to 6, could be attributed to the
benzene ring, which increased the efficiency of the rather
small hydrophobic domain. Surfactant 4 exhibited an
unusual decrease of the surface tension above the CMC.
Such behavior has been previously associated with the
presence of poly-disperse micelles or the formation of a
gel-monolayer at the air–water interface [28].
[
30, 31]. However, there is no significant difference in the
range of droplet size of our emulsions (additional infor-
mation is in the supplement), so the most possible factor to
describe the stability of emulsion of surfactant 4 may be
related to the viscosity. This is also consistent with the
separation time of the emulsion and gel form of the sur-
factant 4 as discussed above.
Emulsion Stability
With the exception of surfactant 2, all surfactants exhibited
reasonable good oil-in-water emulsion stability, requiring 5
days for the separation of a homogenized formulation in
the absence of polymeric stabilizers. The data are tabulated
in Table 2. The reverse Y-shaped surfactants exhibited a
more significant increase of emulsion stability than did the
single chain C -ATG 10 [23], approximately matching the
Acknowledgments The authors of this paper gratefully acknowl-
edge the financial support from the University of Malaya and the
Malaysian Ministry of Education, under Research Grants RP002-
1
2
3BIO, PV055/2012A, RG201-11AFR RG264-13AFR, and FP032-
013B.
1
2
emulsion stability obtained with lauryl glucoside 9, which,
as an APG model, was considered a good emulsifier [22].
These data suggested that the destabilizing effect of a tri-
azole in an O/W emulsion could be compensated by
splitting the hydrophobic chain into two units. A similar
observation has previously been reported for the intro-
duction of a second head group [29]. The poor emulsion
stabilization of compound 2 could be because of repulsive
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Tammar Hussein Ali obtained his bachelor of science degree in
organic chemistry from the University of AL-Muthanna and com-
pleted his master’s degree at the University of Sains Malaysia in the
heterogeneous catalyst field. Currently he is a Ph.D. student at the
Faculty of Science, Department of Chemistry, University of Malaya
under the supervision of Thorsten Heidelberg, Rusnah Syahila Duali
Hussen, and Hariul Anuar Tajuddin His interest areas are organic
synthesis, catalysis and materials.
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Hairul Anuar Bin Tajuddin is a senior lecturer at the University of
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Thorsten Heidelberg is an associate professor affiliated with the
Department of Chemistry at the University of Malaya, Kuala Lumpur,
Malaysia. He obtained his Ph.D. at the University of Hamburg,
Germany. His area of expertise includes organic synthesis, molecular
assembly behavior and nano-materials. His research emphasis is on
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