Angewandte
Communications
Chemie
For cost-effective oil-spill recovery, the PSOG should be
(diesel, petrol, kerosene). This is not surprising in light of the
an easily available, very cheap, nontoxic, and biodegradable
oil solidifier that can congeal the oil phase very efficiently and
selectively. Many alkyl 4,6-O-benzylidene-glycopyranosides
competing solubilizing effect of the aromatic components
present in these oils; crude oil and its fractions are known to
contain aromatic constituents. It is interesting to note that
gelators 1 and 2 are both supergelators of crude oil, with CGC
values of 0.5 wt% and 0.6 wt% respectively, which is
a desirable property for efficient oil-spill recovery. Since
gelators 1 and 2 congealed crude oil more efficiently than the
other gelators, these two were chosen for detailed studies. The
stabilities of the gels formed by gelators 1–5 in various
solvents were measured by measuring their sol–gel transition
temperatures (Tgel; Table S1 in the Supporting Information).
The gels formed by 1 and 2 showed good thermal stability and
were stable for several months at ambient conditions.
[
8,9]
are known organogelators of nonpolar solvents.
Both the
benzylidene group and diol motifs are important structural
features for gelation; the vicinal diol motif is essential for
hydrogen-bond-mediated self-assembly and the benzylidene
group for rigidity (conformational pre-organization). How-
ever none of these alkyl 4,6-O-benzylidene-glucopyranosides
are known to be phase-selective gelators, presumably owing
to the partial solubility of these derivatives in water. We
envisioned that one of the strategies to convert them in to
phase-selective gelators would be to make them more hydro-
phobic by modifying structural features that are not crucial
for gelation, such that they become water-insoluble but still
congeal oils. For this, modification at the anomeric position is
presumed. We decided to use more hydrophobic thioalkyl or
thioaryl motifs in place of the O-alkyl motif at the anomeric
position. In order to check the feasibility, we synthesized
compounds 1–5 (Figure 2) from cheaply available d-glucose
by following straightforward reported procedures (Scheme S1
A comparison of the IR spectra of gelators 1 and 2 in the
self-assembled state (solution in gelling solvent) and disso-
ciated state (solution in non-gelling solvent) suggest that
intermolecular hydrogen bonding is the primary interaction
for self-assembly. The OÀH stretching appeared at a lower
wave number in the self-assembled state (gel) than in the
dissociated state (Figures S2,S3 in the Supporting Informa-
1
tion). In addition, concentration-dependent H NMR titra-
[10]
in the Supporting Information).
tion (Figures S4,S5) of the gelators in a gelling solvent
confirmed the involvement of intermolecular hydrogen
bonding in the self-assembly (gelation), as was evident from
the down-field shift of OH signals with concentration. The
higher gelation ability of gelator 1 (0.5 wt%) than gelator 2
can be attributed to the presence of a thiotolyl group in
gelator 1, which can engage in additional interactions such as
p–p stacking or CÀH···p interactions. Scanning electron
microscopy (SEM) analysis (Figure 2 and Figure S8 in the
Supporting Information) of the xerogels made from benzene
gels of 1 and 2 revealed that these gelators form an entangled
fibrous network in their gels.
We carried out rheology analysis to understand the
strength and stability of the gels formed by gelators 1 and 2
(Figure 3 and Figures S6,S7 in the Supporting Information).
Figure 2. A) Chemical structures of gelators 1–5. B) Transparent gel
stable to inversion) of gelator 1 in benzene at 0.5 wt%. C,D) SEM
(
Images of xerogels made from a benzene gel (0.9 wt%, wt/v) of
gelator 1 (C) and a benzene gel (1.25 wt%, wt/v) of gelator 2 (D).
Scale bars: 5 mm and 50 mm, respectiveley.
We tested the gelation ability of diols 1–5 in different
solvents. All of these diols form gels in nonpolar liquids such
as silicon oil, pump oil, and diesel. To our satisfaction, the
modification of the compounds did not compromise their
gelation ability despite structural changes at the anomeric
position. The critical gel concentration (CGC) values of these
gelators were measured (Table S1 in the Supporting Infor-
mation). It was observed that the gelation ability increased
with increasing length of the thioalkyl chain. This may be
attributable to an increased contribution from hydrophobic or
van der Waalꢀs interaction in the self-assembly process.
Although compound 2, which has a small alkyl chain,
congealed aromatic solvents, 3–5 were soluble in aromatic
solvents. It is worthy of note that compounds 3–5 took more
time to congeal crude oil and its higher-boiling fractions
Figure 3. A,B) Rheological data for gels made in benzene with 2 wt%
wt/v) of gelator 1. A) Frequency sweep (w) and B) stress sweep (s0)
are shown. C) A 5 wt% (wt/v) benzene gel (10 g) of gelator 1 holding
the weight of 1 L of water in an inverted round-bottomed flask (copper
sulfate was added to the water for better visualization).
(
Angew. Chem. Int. Ed. 2016, 55, 7782 –7785
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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