D-galactose-based organogelator for phase-selective solvent removal and sequestration of cationic dyes

Article abstract of DOI:10.1016/j.reactfunctpolym.2020.104766

Naturally occurring sugar-based monomers are attractive substrates in the design of functional glycopolymers. In this study, we report on the development of D-galactose based organogels of varying crosslink density capable of selective adsorption of dyes and solvents. Free radical polymerization of 6-O-methacryloyl-1,2;3,4-di-O-isopropylidene-D-galactose in the presence of nonamethylene glycoldimethacrylate generated novel crosslinked polymers OG10, OG15 and OG20 with 10, 15 and 20 wt% crosslinker, respectively. Depending on the nature of the solvent, OG10 undergoes swelling upto 935%. The negative zeta potential, as determined from DLS measurements, and the gelation ability suggested the potential utility of the polymer for dye removal from water. OG10 displayed significant adsorption of rhodamine B (RhB) (>95%), crystal violet (>93%), and methylene blue (>70%) dyes as well as the selective adsorption of >90% RhB from a solution containing both RhB and methyl orange. These porous organogels are also found to be suitable for phase-selective removal of organic solvents.

Full text of DOI:10.1016/j.reactfunctpolym.2020.104766

Reactive and Functional Polymers 157 (2020) 104766
Contents lists available at ScienceDirect
Reactive and Functional Polymers
journal homepage: www.elsevier.com/locate/react
Perspective Article
D-galactose-based organogelator for phase-selective solvent removal and
sequestration of cationic dyes
*
Shubhra Goel, Josemon Jacob
Department of Materials Science and Engineering, Indian Institute of Technology Delhi, New Delhi 110016, India
A R T I C L E I N F O
A B S T R A C T
Keywords:
Naturally occurring sugar-based monomers are attractive substrates in the design of functional glycopolymers. In
this study, we report on the development of D-galactose based organogels of varying crosslink density capable of
selective adsorption of dyes and solvents. Free radical polymerization of 6-O-methacryloyl-1,2;3,4-di-O-iso-
propylidene-D-galactose in the presence of nonamethylene glycoldimethacrylate generated novel crosslinked
polymers OG10, OG15 and OG20 with 10, 15 and 20 wt% crosslinker, respectively. Depending on the nature of
the solvent, OG10 undergoes swelling upto 935%. The negative zeta potential, as determined from DLS mea-
surements, and the gelation ability suggested the potential utility of the polymer for dye removal from water.
OG10 displayed significant adsorption of rhodamine B (RhB) (>95%), crystal violet (>93%), and methylene blue
D-galactose
Organogel
Free-radical polymerization
Dye adsorption
Swelling
(
>70%) dyes as well as the selective adsorption of >90% RhB from a solution containing both RhB and methyl
orange. These porous organogels are also found to be suitable for phase-selective removal of organic solvents.
1
. Introduction
gelators [23] have recently gained tremendous interest as new soft
materials [24] in the field of chemistry, materials, biology, environment,
Glycopolymers,
a
term used for both natural and synthetic
biomedicine, oil-spill recovery, dye adsorption, biotechnology, cos-
metics, food industry, and pharmaceuticals [16,21–23,25]. The con-
ventional polymers that gelate organic solvent reported in literature
consist of polyethylene, polyesters, poly(alkoxysiloxane), polyacrylates,
and polycarbonates [23,26–34].
carbohydrate-bearing polymers, have gained extensive attention over
the past few decades to generate diverse architectures including cross-
linked gels, amphiphilic block copolymers, dendrimers and other
branched polymers [1,2]. Functionalization of carbohydrates along
different carbon atoms for the synthesis of sugar-containing monomers
has re-emerged as an essential area of research in the field of polymer
technology [1,3]. Various polymerization methods such as free radical
United Nations have adopted certain Sustainable Development Goals
(SDGs) to address major global challenges in achieving a more sus-
tainable future, where access to clean water for everyone is highlighted
as one of the major concerns [35]. Water contamination from highly
toxic dyes released from various industrial sources has become a threat
to the environment as well as human health [36]. In order to protect the
quality of water, commonly used methods for dye removal include
membrane filtration, photochemical degradation, reverse-osmosis,
ozonation, ion-exchange, biodegradation, precipitation/oxidation,
electrocoagulation, and adsorption. Among these techniques, dye
removal by adsorption is preferable as no harmful substance is formed
[36–44]. Many LMWGs are widely used for selective gelation and dye
removal from water [36,45–48]. LMWGs based on dipeptides were used
for the removal of toxic dyes crystal violet (CV) and rhodamine 6G from
water using xerogels prepared from toluene [46]. Similarly, 48 gelators
based on the carboxybenzyl group were prepared by a two-step process
[
1–7], Reversible addition-fragmentation chain-transfer (RAFT) [8–11],
Atom-transfer radical polymerization (ATRP) [12–15], Ring-opening
metathesis polymerization (ROMP) [5], Ring-opening polymerization
(
ROP) [14], click chemistry [16,17] etc. have been reported in the
synthesis of linear glycopolymers as well as graft, block or random co-
polymers and crosslinked glycopolymers
[8,9,13–15,18,19]
4,5,18,20]. Chemical crosslinking leads to the synthesis of gels, which
[
renders the resulting biocompatible materials flexible and highly
porous. These crosslinked three-dimensional interconnected network
structure of polymers are capable of fluid absorption [21–23], based on
which they can be classified as hydrogels (water immobilized) or
organogels (organic liquid immobilized) [23].
The low-molecular weight gelators (LMWGs) and the polymeric
*
Corresponding author.
E-mail address: jacob@mse.iitd.ac.in (J. Jacob).
https://doi.org/10.1016/j.reactfunctpolym.2020.104766
Received 14 September 2020; Received in revised form 16 October 2020; Accepted 18 October 2020
Available online 20 October 2020
1
381-5148/© 2020 Elsevier B.V. All rights reserved.

S. Goel and J. Jacob
Reactive and Functional Polymers 157 (2020) 104766
that can instantly gel with crude oil to eliminate environmental pollu-
tion [48].
(TGA) were performed under nitrogen gas (flow rate as 20 mL/min) at
◦
◦
10 C/min from 25 to 750 C, using a Perkin-Elmer Pyris 6 TGA in-
strument. Swelling kinetics was recorded by immersing the solid dried
gel in an excess amount of solvent and then weighing the swollen gel
after 24, 48, and 72 h. The swelling was calculated using a standard
equation (Eq. 1). The swelling ratio (%) was obtained by measuring the
maximum amount of solvent taken by a fixed amount of the gelator.
As compared to LMWGs, the polymeric materials gelate even at low
concentrations, and their gelation ability can be tailored by modifying
the backbone or the pendant groups [23]. Recently, polymeric materials
[
37,38,40–42,49] as gels have gained special attention as economic and
efficient adsorbents for removal of dyes through hydrogen bonding,
hydrophobic interactions, electrostatic interactions, and interactions
41,44,46,47,50]. Ultra-light and superabsorbent cellulose nanofibril
π-π
W
s
[
Swelling (%) =
× 100
d
(1)
W
aerogels were assembled which were used to investigate the adsorption
and desorption of cationic dye malachite green [51]. Poly(vinyl alcohol)
where, W
s
is the weight of blotted swollen gel and W
d
is the weight of
(
PVA)-based ultra-fast self-healing dimethyl sulfoxide based organogels
dried gel taken before swelling. The swelling studies were performed in
different solvents. The gelator and the solvent were added in a sample
were synthesized, which can adsorb methylene blue (MB) dye selec-
tively with efficiency as high as 95.89% [41]. The β-D-glucose-based
hydrogels prepared by RAFT polymerization with di(ethylene glycol)
dimethacrylate as a divinyl crosslinker were used for the adsorption of
cationic dyes RhB and MB, along-with load and release of silver nitrate
◦
vial and heated at 80 C until the ‘sol’ state of the compound was visible.
It was then cooled to room temperature slowly. The vials were inverted
and kept undisturbed to observe the gel state of the mixture and the
images were recorded. Field-emission Scanning Electron Microscopy
[
40].
Although there are several reports in the literature on the develop-
(
FESEM) was performed using JSM-7800F Prime instrument. FESEM
micrographs were obtained for swollen and dried, cryofractured orga-
nogels. The absorption spectra were recorded on a PG instruments’
T90+ UV–Visible spectrophotometer.
ment of sugar-based hydrogels, the use of carbohydrate-containing
organogels for selective dye removal and phase selective gelation has
been less explored. The present study aims at establishing a simple,
scalable approach towards the synthesis of novel chemically crosslinked
polyacrylate-based organogels, bearing isopropylidene protected
galactose as a pendant group for diverse applications. We also report on
the application of these organogels in the selective removal of the
organic phase from an aqueous-organic mixture as well as in the selec-
tive removal of cationic dyes.
2
.3. Synthesis of 1,2;3,4-di-O-isopropylidene-D-galactose (IpGal) (2)
D-galactose (7 g, 38.8 mmol) and anhydrous ZnCl
mmol) were added to acetone (150 mL) followed by H
dropwise to the solution. It was allowed to react at room temperature for
h under nitrogen atmosphere until the starting material completely
disappeared. The solution was neutralized with saturated Na CO so-
2
(7.14 g, 52.4
2
4
SO (0.84 mL)
4
2
3
2
. Experimental
lution after the reaction. After filtration, the solvent was removed under
reduced pressure on a rotary evaporator. It was further extracted with
2
.1. Materials
2 4
diethyl etherand dried over anhydrous Na SO . The organic solvent was
evaporated, and the product 2 was obtained as a pale-yellow viscous oil
in 75.6% yield which was used for the next step without further
purification.
D-Galactose (extrapure, SRL), acetone (fisher scientific), 4-dimethy-
laminopyridine (DMAP, Alfa Aesar, 98%), methanol (rankem), and
methacryloyl chloride stabilized with BHT (MACl, chemlabs, 97%) were
1H NMR (400 MHz, CDCl
) δ 5.57 (1H, d, J = 5.2 Hz, anomeric
3
used as received. Zinc chloride (dry) (ZnCl
keeping under vacuum to remove traces of water. Benzoyl peroxide
BPO, Loba Chemie Pvt. Ltd.) was purified by recrystallization from
chloroform-methanol prior to use. Nonamethylene glyco-
2
, CDH, 97%) was used after
proton CH), 4.62 (1H, dd, J = 8.0 Hz, 2.4 Hz, CH), 4.34 (1H, dd, J = 5.2
Hz, 2.4 Hz, CH), 4.28 (1H, dd, J = 8.0 Hz, 1.7 Hz, CH), 3.87–3.75 (3H,
(
m, CH + CH
2
), 2.14 (1H, m, OH), 1.45 (3H, s, CH
3
), 1.33 (3H, s, CH
3
),
1
3
1
.25 (6H, s, 2CH
0.8, 68.4, 62.5, 26.2, 26.1, 25.1, 24.5; FTIR ( (cm ) 3489 (br, O
–
H stretch), 1069 (C O stretch).
3
); C NMR (100 MHz, CDCl ) δ 108.9, 96.5, 71.8, 71.0,
3
ldimethacrylate (NMGDMA, TCI chem. Pvt. Ltd.) was purified by pass-
ing through activated basic alumina twice. Triethylamine (TEA, Fisher
Scientific) was purified by distillation before use. Anhydrous tetrahy-
drofuran (THF) and N,N-dimethylformamide (DMF) were purchased
from Sigma Aldrich and used without further distillation. Silica gel
ꢀ
1
7
ν
–
H
stretch), 2985, 2933 (C
–
2.4. Synthesis of 6-O-methacryloyl-1,2;3,4-di-O-isopropylidene-D-
galactose (MAIpGal) (3)
(
mesh size 60–120 mm, Merck) was used for column chromatography.
2
.2. Instrumentation
2 (5 g, 19.2 mmol), TEA (8 mL, 57.6 mmol) and DMAP (2.35 g, 19.2
mmol) were added to anhydrous THF (60 mL) in a 250 mL round bottom
1H and 13C NMR measurements were performed on a Bruker DPX-
00 instrument operating at 400 and 100 MHz, respectively using
schlenk flask. The mixture was cooled down to 0 C and MACl (2.06 mL,
◦
4
21.1 mmol) was added dropwise over a period of 30 min. The reaction
◦
CDCl
3
as solvent and tetramethylsilane (TMS) as an internal reference.
mixture was kept at 27 C to react under nitrogen atmosphere for 1.5 h.
Fourier-transform infrared (FTIR) spectra were recorded on KBr pellets
using a Thermo Scientific Nicolet 6700 spectrometer to analyze different
The reaction was monitored by thin-layer chromatography (TLC) with
15% ethyl acetate/hexane as the mobile phase. On completion of the
reaction, THF was removed on a rotary evaporator, and it was redis-
solved in ethyl acetate. This was washed successively with 1 N HCl (2 ×
ꢀ 1
ꢀ 1
functional groups in the range 4000–400 cm at 4 cm resolution.
Molecular weights were determined using gel permeation chromatog-
raphy (GPC) on a Waters series 1515, equipped with Waters 1515 col-
30 mL), water (2 × 20 mL), saturated NaHCO (2 × 30 mL), brine (2 ×
3
◦
umn. THF was used as the eluent at a flow rate of 1 mL/min at 30 C.
20 mL), ammonia solution (2 × 20 mL) and brine (2 × 20 mL). The
organic layer was dried over anhydrous Na SO and the crude product
Calibration was done using polystyrene standards, and the samples were
prepared in THF at 1 mg/mL concentration. Wide-angle X-ray diffrac-
tion (WAXD) patterns of finely powdered samples were obtained using
2
4
obtained on removal of solvent was purified by column chromatography
(5% ethyl acetate in hexane as the eluent) to obtain 3 in 70% yield.
1
–
Rigaku ultima IV X-ray diffractometer, using CuK
α
radiation (0.154 nm)
H NMR (400 MHz, CDCl
–
–
3
) δ 6.12 (1H, s, HCH
C), 5.54 (1H, d, J = 4.8 Hz, anomeric proton CH), 4.63 (1H, dd, J
= 8.0 Hz, 2.4Hz, CH), 4.32–4.26 (4 H, m, 2 CH + CH ), 4.08–4.06 (1H,
m, CH), 1.94 (3H, s, C=C(CH )), 1.50 (3H, s, CH ), 1.45 (3H, s, CH ),
); C NMR (100 MHz, CDCl ) δ 166.20, 135.05,
–
C), 5.56 (1H, s,
◦
◦
◦
in the 2θ range of 5 to 50 at a scan rate 6 /min. Differential scanning
HCH
calorimetry (DSC) thermograms were recorded using a DSC Q200 in-
2
◦
strument in the temperature range 25–200 C at heating/cooling/heat-
3
3
3
◦
13
ing rate of 10 C/min under dry nitrogen. Thermogravimetric analyses
1.32 (6H, s, 2 CH
3
3
2

S. Goel and J. Jacob
Reactive and Functional Polymers 157 (2020) 104766
Scheme 1. Schematic representation for: (A) Synthesis of galactose-based monomer (3) and linear chain polymer (4) using BPO initiator, and (B) crosslinking
reaction using galactose based monomer with nonamethylene glycol dimethacrylate crosslinker.
1
2
24.78, 108.62, 107.74, 95.27, 70.10, 69.69, 69.51, 65.09, 62.62,
2.6. Synthesis of organogels
ꢀ 1
8.67, 24.94, 24.93, 23.96, 23.44, 17.26; FTIR (
ν
(cm ) 2978, 2926
–
H stretch), 1716 (C
–
O stretch), 1637 (C
–
C stretch), 1167 (C
–O
3 (2 g), 0.25 wt% benzoyl peroxide (5 mg) and NMGDMA (10, 15 and
(
C
stretch).
20 wt%) were added to DMF solvent (10.8 mL) under an argon purge
◦
and heated at 80 C to synthesize the crosslinked polymers (labelled
2
.5. Synthesis of poly(6-O-methacryloyl-1,2;3,4-di-O-isopropylidene-D-
OG10, OG15, and OG20, respectively). The reaction was kept overnight
for gelation under argon. The gel was washed with cold methanol and
dichloromethane to remove any unreacted reagents and kept for drying
galactose) (PMAIpGal) (4)
◦
3
(4.2 g, 12.7 mmol) and freshly recrystallized BPO (10.7 mg, 0.04
under vacuum at 50 C to obtain dried gels.
mmol) were dissolved in anhydrous DMF (21.4 mL) under argon and
◦
heated at 80 C for 72 h. The resultant polymer was obtained by pre-
2.7. Preparation of organogel
cipitation in cold methanol, filtered and dried (yield = 85%).
1
H NMR (400 MHz, CDCl
3
) δ 5.51 (1H, s, sugar anomeric proton CH),
1 mL of solvent was taken in a 5 mL sample vial, and finely crushed
powder of gelator was added slowly with stirring at room temperature.
The gelator was added till complete gelation has taken place and used
for further characterization.
4
1
2
.63 (1H, s, CH), 4.29–3.97 (5H, m, CH + CH
.50–1.25 (12H, m, 4 CH ), 0.88 (3H, m, C-C(CH
989, 2930 (C H stretch), 1720 (C
2
), 1.9–1.8 (2H, m, CH
2
),
ꢀ
3
3
)); FTIR (
ν
(cm 1)
–
–
–
O stretch), 1167 (C–O stretch).
Fig. 1. (A) FTIR spectra of (a) IpGal, (b) MAIpGal, (c) linear polymer, (d) OG10, (e) OG15, (f) OG20; (B) XRD patterns for (a) linear polymer, and (b) OG10.
3

S. Goel and J. Jacob
Reactive and Functional Polymers 157 (2020) 104766
Fig. 2. (A) DSC thermograms of (a) OG20, (b) linear polymer, (c) OG10; (B) TGA and dTG thermograms of linear polymer, OG10, OG15 and OG20 representing
thermal degradation of polymers.
3
. Results and discussion
confirmed by their ability to swell in a variety of solvents. The cross-
linking of MAIpGal with NMDMA (10, 15 and 20 wt%) shows no sig-
nificant change in the FTIR spectra, indicating no evidence of any new
functional group upon the formation of a crosslinked product. The small
The presence of equally reactive secondary hydroxyl groups in car-
bohydrates restricts many reactions to be done selectively at the primary
hydroxyl group. In galactose, hydroxyl groups are arranged in such a
manner that they can be protected together with two isopropylidene
units making the resultant product soluble in organic solvents. The
synthesis of compounds 2 and 3 in Scheme 1(A) were done according to
ꢀ 1
stretch at 1682 cm which appears after crosslinking is attributable to
the ester linkage of crosslinker present in the chains. The molecular
packing and orientation were investigated by XRD patterns recorded for
the linear and crosslinked polymers, represented in Fig. 1(B). The
prominent peak shift from 2θ ~16.5 to 17.40 indicates the change in
microstructure after crosslinking. The microstructural studies exhibit
the broad peak clearly indicating the completely amorphous nature of
both the polymers. This is attributed to the irregular random arrange-
ment of gelator molecules in the dried state.
1
0
literature [5,7,10] with slight modifications and were confirmed by H
NMR. Further, compound 4 was synthesized by free-radical polymeri-
zation of 3 as shown in Scheme 1(A), and the crosslinked polymers ac-
1
cording to Scheme 1(B). The formation of polymer was confirmed by H
1
3
and C NMR, and FTIR spectroscopic techniques. Chemical structure of
linear and crosslinked polymers were investigated by NMR, FTIR, XRD,
DSC and TGA techniques.
3.2. Thermal analysis
The absence of melting and crystallization peaks in DSC thermogram
of the linear polymer indicates its amorphous nature, which is in
3
.1. Spectroscopy
accordance with XRD data. The glass transition temperature (T
g
) for the
1H NMR signals in Fig. S1(a) show three distinct singlets at 1.3 (6H),
.4 (3H) and 1.6 (3H) ppm attributable to isopropylidene protecting
◦
linear polymer was observed at 125 C [7] in the second heating curve,
1
g
as shown in Fig. 2(A). No prominent T was observed in the second
groups in IpGal. The protection is also confirmed by FTIR spectrum of
IpGal as shown in Fig. 1. After protection of D-galactose, a broad O
heating curve of the crosslinked polymer due to irregularity of cross-
linking, which is attributed to the restricted segmental mobility of
chains of an amorphous polymer. As shown in Fig. 2(B), the TGA ther-
mograms reflect the high thermal stability of the polymer backbone. The
dTG curves in the same image show four main stages of degradation of
crosslinked polymers, and the curve recorded for linear polymer chain
shows degradation in three significant stages. The first and second stages
in linear polymer partially overlap. The first stage indicates 46%
–
H
ꢀ
1
stretch was observed at 3489 cm along with C
–
H stretches at 2985
O stretch at 1069 cm [7]. Subsequent reaction
with methacryloyl chloride results in the monomer as a white solid
ꢀ 1
ꢀ 1
and 2933 cm and C
–
1
(
MAIpGal) after purification. This was confirmed by H NMR in Fig. S1
b) with olefinic protons appearing at 6.1 and 5.6 ppm and the signal for
(
allylic methyl protons appearing at 1.9 ppm.
Also, from the FTIR spectrum in Fig. 1(A), it can be seen that the -OH
stretch completely disappears in the protected galactose unit. The
◦
degradation of the polymer at 304 C, which corresponds to the com-
bined degradation of isopropylidene groups, pendant methyl unit, and
the carboxylate group. The second step shows 49% degradation, which
ꢀ 1
monomer shows a characteristic band at 1716 cm attributable to the
–
C O functional group of the ester linkage. The absorption bands at
–
◦
corresponds to the degradation of the pendant sugar moiety at 377 C.
ꢀ 1
ꢀ 1
2
978 and 2926 cm corresponding to C
–
H stretch and at 1167 cm
◦
The third step at 440 C exhibited 5% degradation in the polymer.
corresponding to C
–
O stretch are observed in the spectrum as reported
This is similar to other thermally stable polymeric acrylate backbone
by Besenius et al. [5]. The free-radical polymerization of the synthesized
monomer yields ~85% polymer (PMAIpGal) after precipitation in cold
methanol. The formation of the polymer is accompanied by the disap-
pearance of olefinic proton signals at 6.1 and 5.6 ppm as observed in
Fig. S1(c).
as reported for other commercially available acrylate polymers [52–54].
◦
The first peak around 200 C in OG10, OG15 and OG20 is not observed
in the linear analogue suggesting the degradation of the crosslinked
network. Further, the degradation of the pendant sugar moiety was
◦
followed by the polymer backbone degradation at 440 C. Moreover, the
A GPC instrument was used to determine the molecular weight of the
polymer. The results depicted the GPC trace in low molecular weight
thermograms indicate a decrease in network stability with increasing
crosslinker percentage owing to the enhanced rigidity due to tighter
packing.
(
LMW) region, which found to be ~10,000 g/mol for polymer prepared
with 0.63 M solution concentration. All the crosslinked polymers were
prepared with the parameters optimized for linear chain polymer. The
NMR spectrum of crosslinked gels was not observed due to their insol-
uble nature in all the solvents. The formation of crosslinked gels was also
4

S. Goel and J. Jacob
Reactive and Functional Polymers 157 (2020) 104766
swelling capacity [49].
As the crosslinking in OG10 with 10 wt% of crosslinker amount
shows the maximum swelling ratio among the three, it was used to study
swelling behavior in other organic solvents. The gelation test for OG10
was carried out in 16 different organic solvents of various dielectric
constants ( ) using the vial inversion method as represented in Fig. 4, to
ε
determine the solvent uptake amount by the polymer. The stable gel
formation is evident with 1,4-dioxane, DMF, THF, chloroform,
dichloromethane, toluene, and ethyl acetate, whereas in DMSO, aceto-
nitrile, and diethyl ether, no such swelling was observed. Also, the
material was not swellable in highly polar (water and methanol), and
non-polar (n-hexane) solvents. The swelling ratio data in Table S1 shows
the swelling to be highest in chloroform (935%); the solvent with
neither highest nor lowest
ε
. Thus, the observed swelling did not show
any direct correlation to dielectric constant [40].
3
.4. Morphological studies
The morphologies of the chloroform-based gels prepared from OG10,
Fig. 3. Swelling ratio study of OG20, OG15, and OG10.
.3. Swelling studies
The swelling or gelation capability of a polymeric gel depends on
OG15, and OG20 were investigated using FESEM, and the images are
shown in Fig. 5. The images of cryo-fractured lyophilized xerogels
indicate a three-dimensional co-continuous, entangled fibrous bulk
morphology. The surface of the dried gels was found to be macroporous
with a random distribution of pores. The formation of such macropores
occur as the solvent enters the network of the gelator upon swelling. The
absorption of solvent by the gelator expands the polymer matrix where
the solvent pockets are formed after lyophilization, which appear as
pores in the microscopic images. A high crosslink density leads to a
compact network in the gel as the solvent absorption is difficult leading
3
various factors, including, the structure of the polymer chains, as well as
the crosslink density [40]. The swelling behavior is also based on the
type of interaction of gelator with the solvent or gelator-gelator inter-
action, such as H-bonding, Van der Waal forces or dispersion forces,
which include dipole-dipole, dipole-induced, and instantaneous dipole-
induced forces [55]. The swelling studies were performed for OG10,
OG15 and OG20 crosslinked polymers in DMF, and the weight of
swollen gel was recorded until no increase in weight was observed after
blotting the surface of the gel. The experiments were performed for 72 h,
though the weight of the swollen polymer did not increase after 24 h. In
Fig. 3, the results represent the swelling ratio values as 915%, 834%, and
to a smaller mesh size in the matrix as observed for OG20 with ~0.1
pore size as seen in Fig. 5(a). The pore size measured for OG15 was
found to be ~0.5 m and ~ 0.9 m for OG10 as estimated from the
FESEM images in Fig. 5. The size of pores decreased from 0.9 to 0.1
μm
μ
μ
μm
with the increase in crosslinker content from 10 wt% to 20 wt%. As the
crosslinker amount increases, the number of crosslinks increase, thereby
leading to the formation of a more compact polymer matrix.
7
3
02% respectively in CHCl . The swelling of gels was enhanced with a
decrease in crosslinking ratio due to increased porosity of the cross-
linked network structure, which is also evidenced from morphological
studies. The diffusion of the solvent into the gel is slower and prevented
when the matrix is highly crosslinked that leads to a reduction in
Fig. 4. Swelling studies of OG10 in (a) toluene, (b) THF, (c) chloroform, (d) 1,4-dioxane, (e) diethyl ether, (f) DMSO, (g) DMF, (h) DMAc, (i) 1,2-dichloroethane, (j)
o-dichlorobenzene, (k) o-xylene, (l) ethyl acetate.
Fig. 5. FESEM images of cryofractured lyophilized organogels (a) OG20 (b) OG15 (c) OG10.
5

S. Goel and J. Jacob
Reactive and Functional Polymers 157 (2020) 104766
Fig. 6. (a) chloroform absorbed by the gelator, (b) water separated from the chloroform-water mixture; (c) toluene absorbed by the gelator, (b) water separated from
the toluene-water mixture.
Fig. 7. UV spectra demonstrating dye adsorption by OG10 gel in 24 h (a) RhB (b) CV (c) MB, and (d) RhB/MO.
3
.5. Phase-selective organogelation studies
mg with toluene which accounts for nearly 100% organic solvent sep-
aration from water. Hence, the hydrophobic nature of the organogelator
can be used for selective removal of chloroform or toluene from their
aqueous mixtures.
Phase-selective organogelation has always been challenging in
mixtures of solvents when water is one of the components. The first
phase-selective gelator was introduced in 2001 by Bhattacharya et al. to
address recovery after oil spills. Further, on this basis, Debnath et al. also
developed few LMWGs as phase-selective organogelators in 2008 [46].
Interestingly, it was found that the organogels in the present study
selectively gelate organic solvent from oil/water mixtures. The studies
were carried out by taking 1:1 toluene/ water mixture with 100 mg of
OG10. The mixture was shaken vigorously and kept undisturbed at room
temperature. The gelation was observed in the toluene layer selectively,
as shown in Fig. 6, which could be easily separated from water, and
gelator can be reused as well. Also, after organic solvent separation, the
gel weight was observed as 771 ± 20 mg with chloroform, and 506 ± 10
3
.6. Dye-adsorption studies
The surface charge of the crosslinked polymer was measured in terms
of zeta potential using DLS instrument. The negative magnitude of the
surface charge suggested the potential use of the polymer towards the
adsorption of positively charged dyes. The preliminary studies were
conducted using cationic dyes as well as a mixture of cationic and
anionic dyes. The tests were performed with 10 mg of OG10 gel in 1.5
mL dye solution (0.0125 mg/mL) of RhB, CV, MB and RhB/MO each.
The solutions were kept at room temperature for 24 h for dye
6

S. Goel and J. Jacob
Reactive and Functional Polymers 157 (2020) 104766
Fig. 8. UV spectra showing a time-dependent dye adsorption studies using OG10 gel for: (a) RhB (b) CV (c) MB, and (d) RhB/MO.
adsorption. The initial and residual dyes in the solution were detected
using UV–Visible spectroscopy, as shown in Fig. 7. The UV curves clearly
indicate the adsorption of all the cationic dyes by the organogel. The
color of all the dye solutions nearly disappeared within 24 h. Also, the
data shows >90% of RhB and CV, and > 70% of MB were adsorbed by
the gel.
the compositions, and the pore size decreased with increase in cross-
linker amount due to tighter packing of the network. The zeta potential
of the crosslinked polymer was measured as ꢀ 5.53 mV, suggesting the
ability of the material to adsorb cationic dyes selectively. The adsorption
studies with OG10 on cationic dyes showed 90% ± 3% dye adsorption
for RhB, 36% ± 2% for MB, and 88% ± 2% for CV in 360 min. Addi-
tionally, the selective adsorption of 85% ± 2% of cationic RhB dye from
a mixture of RhB/MO was demonstrated. Since the gel adsorbed dyes
from water, this organogel can potentially be used for the removal of
various toxic cationic dyes from water. Also, it is found to undergo phase
selective organo-gelation from organic-aqueous solvent mixtures. The
adsorption is nearly 100% suggesting its potential for use in solvent
recovery.
Based on these preliminary results, further kinetic studies on the
adsorption of RhB, CV and RhB/MO were carried out with 10 mL dye
solution (0.00625 mg/mL), and that of MB with 10 mL dye solution
(
0.003125 mg/mL) using 40 mg of swollen gel. The time-dependent dye
adsorption was monitored using UV–Visible spectroscopy. As can be
seen in Fig. 8, the concentration of RhB and CV decreased with time with
almost complete adsorption in 360 min., whereas with MB, only 36% ±
2
% of dye was adsorbed in the same time. The gel took longer to adsorb
MB than RhB and CV, as observed in Fig. 7(c) with >70% of MB dye
Declaration of Competing Interest
adsorbed in 24 h. Also, adsorption studies with a mixture of anionic
(
MO) and cationic (RhB) dyes showed selective removal of RhB. This
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
was accompanied by a significant color change from orange to yellow,
the color of MO in water. UV–vis studies showed the disappearance of
the peak at 553 nm corresponding to the absorption maximum of RhB
dye (Fig. 8(d)). The gel selectively adsorbs cationic dyes due to the
negative charge associated with the surface of the crosslinked polymer,
which is measured in terms of zeta potential as ꢀ 5.53 mV.
Acknowledgements
The authors would like to thank the support of the Ministry of Ed-
ucation (MoE), India, formerly the Ministry of Human Resource Devel-
opment (MHRD), India.
4
. Conclusions
Protected D-galactose containing acrylate-based linear and cross-
Appendix A. Supplementary data
linked polymers were synthesized by free-radical polymerization. For
OG10, the swelling was found to be 412% in DMF, 480% in THF, 552%
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.reactfunctpolym.2020.104766.
3
in toluene, 619% in 1,4-dioxane, and 935% in CHCl solvent. No direct
correlation was found between the dielectric constant and swelling
ratio. The FESEM micrographs confirmed the porous morphology for all
7

S. Goel and J. Jacob
Reactive and Functional Polymers 157 (2020) 104766
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