New supramolecular amphiphiles based on renewable resources

Article abstract of DOI:10.1039/b924335f

The complexation between esters derived from sorbitol and β-cyclodextrin was studied. Isosorbide dioleate and sorbitan trioleate formed well-defined inclusion complexes with β-cyclodextrin and these inclusion complexes exhibited surfactant behaviour.

Full text of DOI:10.1039/b924335f

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New supramolecular amphiphiles based on renewable resources†
Ce´cile Machut,*a,b,c Fouzia Mouri-Belabdelli,d Jean-Paul Cavrot,a,b,c Adlane Sayedea,b,c and
Eric Monfliera,b,c
Received 19th November 2009, Accepted 4th February 2010
First published as an Advance Article on the web 3rd March 2010
DOI: 10.1039/b924335f
The complexation between esters derived from sorbitol
and b-cyclodextrin was studied. Isosorbide dioleate and
sorbitan trioleate formed well-defined inclusion complexes
with b-cyclodextrin and these inclusion complexes exhibited
surfactant behaviour.
inclusion complexes were found to be much more surface active
than the b-CD/acid complexes. Furthermore, the surface prop-
erties of the b-CD/alcohol complexes were strongly dependent
on the length of hydrocarbon chain and only alcohols having
10–12 carbon atoms exhibited surface activity. In fact, the sur-
face activity of these supramolecular complexes was attributed
to hydrophobic chain of the guest protruding outside of the CD
cavity.
In this work, we have investigated the possibility of form-
ing supramolecular amphiphiles by noncovalent associations
between b-cyclodextrin and highly hydrophobic polyesters con-
taining several alkyl chains. Indeed, contrary to previous studies,
it was expected that use of such compounds will make the
formation of micelles easier, as schematically represented in
Scheme 1 by using a guest containing three alkyl chains
Native cyclodextrins are oligosaccharides composed of six or
more D-glucopyranose residues attached by b-1,4-linkages in a
cyclic array. The most common cyclodextrins contain six, seven
or eight glucose residues and are named a-cyclodextrin (a-CD),
b-cyclodextrin (b-CD) and g-cyclodextrin (g-CD), respectively.
They have the shape of a truncated cone with a hydrophobic
inner cavity.1 This internal cavity can accommodate a wide range
of guest molecules, ranging from polar compounds such as
alcohols, acids, amines and small inorganic anions to apolar
compounds such as aliphatic and aromatic hydrocarbons.2
Numerous researchers have investigated the interaction between
surface active compounds and cyclodextrin.3 In particular, it has
been demonstrated that the formation of inclusion complexes
between cyclodextrin and surfactant greatly modifies the surface
active properties of the surfactant. In fact, this association
generally results in the hydrophilisation of surfactant leading to
an increase in CMC (critical micelle concentration).4 Indeed, the
inclusion of the hydrophobic part of the surfactant molecule into
the CD cavity postpones the micellization process.5 However,
surface active supramolecular entities can be formed in the
presence of cyclodextrin. For instance, it is well known that
cyclodextrins can be used as additives to stabilize emulsions.
In this case, cyclodextrins form in situ surface active agents by
interacting with the components of the oil phase at the oil/water
interface.6 The interaction between non-ionic polyethylene
oxide-based surfactants and b-cyclodextrin7 or amphiphilic
b-cyclodextrin8 gives also rise to noncovalent amphiphiles.
Interestingly, it has been recently demonstrated that surface
active inclusion complexes can be isolated by using a suspension
method. So, N. Laugh-de Viguerie et al. have reported that
inclusion complexes of cyclodextrin with fatty alcohols or fatty
acids behave as supramolecular surfactants.9 The b-CD/alcohol
Scheme 1 Formation of micelle from amphiphile inclusion complexes.
In fact, we have postulated that the presence of several
long alkyl chains on the guest should permit a redistribu-
tion of cyclodextrins, leading to well-defined hydrophilic and
hydrophobic parts. In order to validate our concept, two
sorbitol derivatives substituted by oleoyl chains were chosen as
guest. These compounds were isosorbide dioleate and sorbitan
trioleate (Span 85) (Scheme 2).
aUniv. Lille Nord de France, F-59000, Lille, France.
E-mail: cecile.machut@univ-artois.fr; Fax: (+33) 3 21 63 02 94;
Tel: (+33) 3 21 63 23 20
Scheme 2 Sorbitol-derivatives esters chosen for the elaboration of
supramolecular surfactant 1 and 2.
bUArtois, UCCS, IUT de Be´thune, 1230 route de l’universite´ BP 819,
62408, Be´thune Cedex, France
cCNRS, UMR 818, France
dL2MSM, Faculte´ des Sciences, Universite´ Djilali Liabe`s, 22000, Sidi
Bel-Abbe`s, Alge´rie
These compounds are environmentally friendly and produced
from renewable resources. So, the dehydration of sorbitol yields
a mixture of sorbitan and the further dehydration of sorbitan
† Electronic supplementary information (ESI) available: Experimental
section. See DOI: 10.1039/b924335f
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produces isosorbide.10 Esterification of these compounds gives
rise to isosorbide dioleate and sorbitan trioleate (Span 85).11
Inclusion complexes between b-CD and sorbitol-derivatives
substituted by oleoyl residues were prepared using the suspen-
sion method. Briefly, an aqueous equimolar suspension of b-CD
and esters was vigorously stirred at room temperature for 24 h.
The resulting suspension was filtered to collect a white powder.
This experiment was performed with isosorbide dioleate and
the Span 85 as guest and b-CD as host and the resulting solids
collected are named compounds 1 and 2, respectively.
the disappearance or shifting of endothermic peaks in DSC
thermograms are generally an indication of the formation of
inclusion complexes.12 For comparison, pure esters, b-CD and
b-CD/esters physical mixtures obtained by simple blending were
also analyzed by DSC. The experiments were conducted from
-40 ◦C to 220 ◦C as both esters were liquid at room temperature.
The results obtained with the b-CD and Span 85 are represented
in Fig. 2.
1
The collected precipitates were analyzed by H NMR spec-
troscopy in DMSO-d6. As an example, the 1H NMR spectrum
of compound 2 is displayed in Fig. 1.
Fig. 2 DSC curves of b-CD, Span 85 and physical mixture of Span
85/b-CD. First cycle of heating from -40 to 220 ◦C (flow 10 ◦C min-1).
As expected for b-CD, a large endothermic peak is observed at
about 150 ◦C when b-CD is heated from -40 ◦C to 220 ◦C for the
first time (Fig. 2). This broad peak disappears during the second
cycle of heating and is attributed to the loss of crystal water
(see ESI†). In this range of temperature, it should be noticed
that no supplementary peaks are observed as the melting and
◦
decomposition of the b-CD occur at 265 C (see ESI†).13 The
thermogram of pure Span 85 shows a broad endothermic peak
at about 0 ◦C, corresponding to the melting of the crystalline
forms of this ester mixture (Fig. 2). The DSC thermogram of the
physical mixture heated for the first time shows the same broad
endothermic peak at around 0 ◦C and a large endothermic peak
Fig. 1 1H NMR spectrum of the compound 2 collected after mixing
Span 85 and b-CD with suspension method (DMSO-d6).
◦
For the two samples, characteristic peaks of both b-CD and
esters were observed. Furthermore, integration of 1H NMR
signals of the methyl protons of esters (d = 0.84 ppm) and
H1 protons of b-CD (d = 4.85 ppm) allowed determination
of the relative amount of cyclodextrin and ester in the sample.
Interestingly, the amount of CD was found to be three times
more important than that of ester in the solids containing
sorbitan trioleate. In contrast, for isosorbide dioleate, b-CD
amount was twice as important than that of isosorbide dioleate.
It is worth mentioning that these ratios were also confirmed
by elemental analysis on the precipitates. Finally, we have
determined that the b-CD/ester ratios in the precipitates were
independent of the initial amounts of b-CD and ester. Indeed, all
preparations conducted with different b-CD/ester ratios (2 : 1;
1 : 1 and 1 : 2) have led to precipitates containing a CD/sorbitan
trioleate ratio and a CD/isosorbide dioleate ratio of 3 and 2,
respectively.
at about 155 C. In fact, it appears clearly that the DSC curve
of the physical mixture is a superposition of the DSC curves of
pure Span 85 and b-CD, confirming that no inclusion occurs by
physical blending.
DSC thermograms of compound 2 show special features
compared with the pure b-CD and Span 85, as well as the
b-CD/Span 85 physical mixture (Fig. 3).
These above experiments, and the fact that compounds 1 and
2 rapidly precipitate when esters are mixed with cyclodextrin
aqueous solution below the solubility limit of the macrocycle
alone, suggest that the precipitates are well-defined compounds.
In fact, it is reasonable to assume that these compounds are
inclusions complexes.
Fig. 3 DSC cu◦rve of compound 2. First cycle of heating from -40 to
220 ◦C (flow 10 C min-1).
For the first heating of compound 2, the peak corresponding
to the Span 85 at 0 ◦C has disappeared. Two new peaks at about
100 ◦C and at 195 ◦C are noticed (Fig. 3). Interestingly, the peaks
In order to confirm this hypothesis, the samples were an-
alyzed by differential scanning calorimetry (DSC). Indeed,
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◦
observed at about 100 C have disappeared during the second
about 8 ¥ 10-6 M until the limit of solubility is achieved. Further-
more, a breakpoint could be noticed at 3 ¥ 10-5 M in each case.
This surface activity cannot be attributed to the decomplexation
of inclusion complexes. Indeed, the aqueous solution remained
clear and the formation of an oil phase composed of sorbitan
trioleate or isosorbide dioleate was never observed. In these
conditions, it can be considered that sorbitan trioleate/b-CD
and isosorbide dioleate/b-CD inclusion complexes exhibit a
surfactant behavior with CMC values in the range 3 ¥ 10-5
to 5 ¥ 10-5 M. Furthermore, these inclusion complexes appear
as powerful surfactants, since they greatly reduce the surface
tension of pure water from 72 mN m-1 to values between 32 and
42 mN m-1.
In conclusion, we have demonstrated that new supramolecular
amphiphiles can be easily obtained from renewable resources.
Indeed, sorbitan trioleate and isosorbide dioleate form well-
defined inclusion complexes with the b-CD and these inclusion
complexes exhibit high surface activity. Works are currently
underway to determine precisely the origin of the surface activity
and especially the structure of the surface-active inclusion
complexes in aqueous solution.
cycle of heating, suggesting a dehydration process.
During the second cycle of heating, it also should be noticed
that the small endothermic peak at 195 ◦C is still observed and
◦
that an exothermic peak appears at 175 C when compound 2
is cooled (Fig. 4). The enthalpies DH for transitions at 175 C
◦
and 195 ◦C are found to be similar, indicating that these peaks
are associated with the melting and crystallisation processes◦of
the same compound. In fact, it seems that the peak at 175 C
corresponds to the crystallization of a supercooled liquid phase.
Finally, it is worth mentioning that the pure b-CD endothermic
peak at 265 ◦C was no longer present when compound 2 was
heated to 300 ◦C (ESI†). These DSC data undoubtedly indicate
that compound 2 is not a simple mixture of b-CD and Span
85. Indeed, the presence of a single peak at 195 ◦C on the
thermogram suggests the participation of both species in the
formation of a new crystalline phase such as inclusion complex.14
Acknowledgements
Roquette Fre`res (Lestrem, France) is gratefully acknowledged
for its financial support.
Fig. 4 DSC curve of compound 2. Second cycle of heating (down) and
Notes and references
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