J. Coat. Technol. Res.
https://doi.org/10.1007/s11998-018-00177-z
Methyltrichlorosilane functionalized silica nanoparticles-treated
superhydrophobic cotton for oil–water separation
Love Dashairya, Dibya Darshan Barik, Partha Saha
American Coatings Association 2019
Abstract Water pollution due to oil spills has become
a significant concern in recent times for the marine
ecosystem. The use of oleophilic/hydrophobic sorbents
for oil–water separation has gained a lot of attention as
an economical and environment-friendly solution.
Herein, we developed a superhydrophobic/superoleophilic cotton by silica nanoparticles ( 800 nm)
treatment followed by surface functionalization with
methyltrichlorosilane (MTCS). X-ray diffraction, field
emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, and Fourier-transform infrared spectroscopy studies reveal that the formation of
pseudo-amorphous SiO2 NPs and a C–Si–O coverage
on cotton fiber render it superhydrophobic with
increased surface roughness. The MTCS/SiO2-treated
cotton exhibited contact angles of 173 ± 2 and 0
on the water–cotton and oil–cotton interface, respectively. Moreover, the MTCS/SiO2-treated cotton
demonstrated superhydrophobicity over the entire
pH range, with excellent absorption capacities for
various oil–water mixtures ranging from 30 to 40
times its weight.
Keywords Superhydrophobicity, Absorption
capacity, Contact angle, Methyltrichlorosilane
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s11998-018-00177-z) contains
supplementary material, which is available to authorized users.
L. Dashairya, D. D. Barik, P. Saha (&)
Department of Ceramic Engineering, National Institute of
Technology, Rourkela, Odisha 769008, India
e-mail: sahapartha29@gmail.comsahap@nitrkl.ac.in
Introduction
Water pollution due to oil spills, by the accidental
dumping of crude oil, petroleum by-products, and
hazardous organic solvents from chemical industries
into oceans, has become a significant concern for the
marine ecosystem.1–3 The negative impact of oil spills
is enormous and associated with an adverse effect on
the ecosystem.4 Oil spills during oil extraction, transportation, and storage account for more than one-third
of the accidental release of oil in the marine ecosystem.1,5 The above factors have led to an increasing
demand for an effective solution to remove oil from
oily wastewater. Traditional practices like gravitational
separation, skimming, flotation, centrifugation, flocculation, and coagulation have its shortcoming.6 The
major drawbacks including low separation efficiency,
prolonged separation time, high energy requirement,
and complicated steps warrant new technology for
separation of oil. In this regard, porous sorbents are
considered an economic and viable option for oil
recovery owing to their ease of operation and availability. A porous sorbent must have a high degree of
separation efficiency and low processing cost.7 However, preliminary research targeted to develop
hydrophobic/oleophilic sorbents demonstrated low
separation efficiency and concomitant absorption of
oil and water and calls for further research.6,8
Cotton, a natural plant fiber that is cheap and
possesses a three-dimensional macroporous structure,
is ideal from a sorbent’s point of view.6,9 Therefore,
there is opportunity to fabricate cotton-based
hydrophobic/oleophilic sorbents for oil–water separation.10 However, pristine cotton generally covered with
a wax layer has a low surface roughness which impairs
its hydrophobic/oleophilic behavior. It is to be borne in
mind that low surface energy and high surface roughness are essential parameters for sorbents to develop
Recently,
hydrophobic/oleophilic
properties.11,12
J. Coat. Technol. Res.
Fe2O3/Cr2O3 nanoparticles and TiO2-coated cotton
fabrics demonstrated surface roughness and hydrophobicity, showing water contact angles greater than
150.4,13 Silica nanoparticles-impregnated sponge has
shown high surface roughness and low surface energy
for developing sorbents for oil–water separation.7,9,14
Silica nanoparticles offer many benefits such as ease of
functionalization due to the presence of silanol groups
and compliance with a variety of surfaces (cotton,
fabrics, sponge).14,15 Qinwen et al. developed
hydrophobic cotton and polyester fabrics by silica sol
nanoparticles and nonfluorinated alkyl silane, which
showed excellent water repellency with a water contact
angle of 155 on cotton and 143 on polyester.16
Recently, Liu et al.9 modified cotton surface by silica
nanoparticle and octadecyltrichlorosilane demonstrating high oil absorption capacity (AC). However, many
synthesis routes use expensive 1H,1H,2H,2H-perfluorooctyltriethoxysilane for surface modification which
limits the commercial aspects and leads to the development of low-cost sorbents for oil spill cleanup.17
Also, usage of expensive and toxic fluorinated long
alkyl silane has its own drawbacks and poses a high risk
to human health and the environment due to the
emission of fluorinated compounds during synthesis.18,19
Methyltrichlorosilane (MTCS) is an inexpensive and
fluorine-free silane that possesses critical surface tensions below 35 dynes/cm and demonstrates outstanding superhydrophobicity properties.20,21 Khoo
et al.22 studied MTCS-functionalized glass and SiO2
substrates, which exhibited contact angle 160. To
the best of our knowledge, MTCS-functionalized silica
nanoparticles-treated cotton fibers have not been
reported for oil–water separation. In the present work,
we present a facile method to fabricate superhydrophobic/superoleophilic
MTCS-functionalized
silica
nanoparticle (MTCS/SiO2)-treated cotton. The cotton
is initially treated with stöber silica nanoparticles via
solgel route and subsequently modified with MTCS
through a simple dip coating method demonstrating
selective absorption of oils up to 30–42 times its
weight.
Experimental
Materials
All the chemicals were analytical reagents grade and
used without further purification. Deionized (DI)
water was used throughout the experiments. Cotton
balls were purchased from a local medical shop.
Tetraethylorthosilicate (C8H20O4Si, 99.9%) was purchased from Alfa Aesar. Ammonia solution (NH4OH,
25%) was purchased from RFCL Ltd. Cyclohexane
(C6H12, 99%) was purchased from Emplura, Merck
Ltd. 2-Propanol (C3H8O, 99%) was purchased from
Rankem. Methyltrichlorosilane (CH3Cl3Si, 98%) was
purchased from Avra Synthesis Pvt. Ltd. Sodium
chloride (NaCl), magnesium chloride (MgCl2), citric
acid, sodium sulfate (Na2SO4), and sodium hydroxide
pellets (NaOH) were acquired from Loba Chemie.
Engine oil, pump oil, kerosene, diesel, vegetable oil,
and hydraulic oil were obtained from local stores.
Pretreatment of cotton
The raw cotton was ultrasonically washed with DI
water. A 2% (wt./wt.) NaOH aqueous solution was
prepared in a beaker. The cotton washed with DI water
was placed in the NaOH aqueous solution for 10 min.
Then, the cotton was taken out from the NaOH
solution and washed several times with DI water until
the pH of the filtrate reached 7. This pretreated
cotton was dried at 50C for 12 h.9
Preparation of SiO2 NPs-treated cotton
A solgel route was used to achieve SiO2 NPs-treated
cotton fibers. A solution containing 2-propanol
(45 mL), tetraethyl orthosilicate (5 mL), and 5 mL of
DI water (volume ratio—9:1:1) was prepared in a
beaker. Then, 5 mL of ammonia solution was added
dropwise to the mixture at room temperature under
magnetic stirring for 45 min. The pretreated cotton
( 10 g) was immersed in the solution and kept for 1 h.
After the hydrolysis and condensation reactions were
over, the SiO2-treated cotton was removed and washed
with 2-propanol and dried in a vacuum oven at 50C
for 12 h.9
Preparation of MTCS/SiO2-treated cotton
The SiO2-treated cotton was dipped in a cyclohexane
solution of 2% (v/v) MTCS for 1 h and then dried at
80C for 12 h to prepare superhydrophobic MTCS/
SiO2-treated cotton.21 A schematic representation of
the preparation of superhydrophobic MTCS/SiO2treated cotton is presented in Fig. S1.
Absorption capacity
Three different quantities were calculated during the
absorption capacity tests. The quantities are absorption
capacity (AC), collection capacity (CC), and recovery
(%). AC, CC, and recovery (%) are defined as follows:
AC g g1 ¼ ðWf Wi Þ=Wi ;
CC g g1 ¼ ðWr Wi Þ=Wi ;
Recoveryð%Þ ¼ ðWf Wr Þ=ðWr Wi Þ 100;
J. Coat. Technol. Res.
where Wi denotes the initial (dry) weight of the
MTCS/SiO2-treated cotton, Wf denotes the weight of
the MTCS/SiO2-treated cotton after absorption of oil,
and Wr denotes the weight of the MTCS/SiO2-treated
cotton after oil desorption by mechanical squeezing.
To evaluate the AC, an equal weight ( 0.105 g) of
cotton was taken to conduct the experiments. The
weight of the cotton before oil absorption was noted
as the initial weight (Wi). An oil–water mixture
(volume ratio = 1:5) was prepared into which the
modified cotton was immersed for oil absorption, and
the weight was noted as residual weight (Wr).
Further, the oil-loaded MTCS/SiO2-treated cotton
was mechanically squeezed to remove oil, and the
residual weight was noted as Wr.6,23 This absorption/
desorption process was carried out for 10 cycles with
two cottons to get an accurate value of the reported
data.
The flux (Lm2 h1 atm1) of oil using MTCS/SiO2treated cotton was determined by calculating the
volume of oil passed in unit time from the oil–water
mixture using the following equation.24
F¼
VL
A:P:t
where F is the volume flux of oil, VL (liter) is the
volume of oil passing through the MTCS/SiO2-treated
cotton, A is the effective area of cotton (m2), P is the
suction pressure (1 atm), and t is the separation time
(s). Also, Vi and Vf are the volume of the oil before
and after the oil–water separation test, respectively.
Furthermore, the separation efficiency (g) was
calculated according to given equation:
g% ¼
Vi
100
Vf
Results and discussion
Reaction mechanisms
A natural plant wax layer is present on the surface of
the pristine cotton.9 This wax layer renders the surface
of cotton fiber quite smooth.25 Therefore, the SiO2 NPs
cannot attach themselves to the surface of the fiber
owing to its smoothness.26 The natural plant wax layer
was removed by pretreatment with an NaOH solution.
Pretreatment exposes many longitudinal corrugations
and interspaces (–OH groups) on the cotton fiber
surface. Further, a solgel route was adopted to form
stöber SiO2 NPs ( 800 nm) where the reactions
proceeded by the hydrolysis of TEOS followed by
the condensation of silanol [Si–(OH)4] groups. The
condensation reaction occurs with the formation of
either alcohol or water and the condensed siloxane
( ” Si–O–Si ” ) species as the end products.27,28 The
corrugations on the cotton fiber surface help the SiO2
sol to permeate into the cotton. Consequently, the SiO2
NPs get attached with the cotton fiber through a
chemical interaction between the hydroxy groups of
the cotton fiber and hydrolyzed TEOS.9,29 These
hydroxy groups on the cotton fiber play a key role in
the evolution of modified cotton fibers.30,31 Moreover,
the SiO2 NPs treatment (in the form of crosslinked
siloxane ” Si–O–Si ” and HO–Si) on the cotton fiber
generates a rough surface texture.7,10 The SiO2-treated
cotton further modified with MTCS develops superhydrophobicity (see Scheme 1). The alkyl groups present
in MTCS react with the hydroxy groups (siloxane ” Si–O–Si ” and HO–Si) present on the SiO2treated fiber and formed a rough surface with formation of polymethylhydrosiloxane and become water
repellent completely.10
Microstructural analysis
Material characterizations
The raw cotton, SiO2 NPs, SiO2-treated cotton, and
MTCS/SiO2-treated cotton were characterized by Xray diffraction (XRD), Fourier-transform infrared
spectroscopy (FTIR), field emission scanning electron
microscopy (FESEM), and energy-dispersive X-ray
spectroscopy (EDX). XRD was performed using a
Philips Rigaku Ultima IV system with Cu-Ka radiation
(kCu = 1.5418 Å) within the 2h range of 5–65.
FESEM was performed using Nova NanoSEM
FEI450 operating at 10 kV. FTIR analysis was done
with PerkinElmer Spectrum version 10.4.00 in wave
number range of 400–3600 cm1 at room temperature
using KBr disks. A drop shape analyzer (Model DSA
4, Kruss, Germany) was used for contact angle measurement.
The FESEM images of the raw cotton illustrated in
Fig. 1a display the smooth fiber surface of natural
cotton. Figure 1b represents the full-frame EDX analysis of raw cotton showing the presence of only C and
O. The SiO2 NPs, prepared by the solgel process, were
able to attach on the pretreated cotton fiber easily as
shown in Fig. 1c. The SiO2 NPs acted as substrates for
MTCS and allowed the cotton surface modification as
discussed earlier. As seen in the FESEM image of
SiO2-treated cotton, the spherical SiO2 NPs have a
smooth surface. Upon treatment with MTCS, the
cotton surfaces become rough (see Figs. 1d, 1e, and
1f), which makes it superhydrophobic and superoleophilic. High-magnification FESEM image of
MTCS/SiO2-treated cotton represented in Fig. 1f
revealed the presence of SiO2 NPs on the cotton fibers
along with a rough surface. Moreover, magnified
FESEM images in Fig. S2a–d clearly describe the
J. Coat. Technol. Res.
Stable water droplet
OH OH
OH
OH
H
O
H
O
OH
Cotton
Cotton
Cotton
OH
H
O
Pre-treatment
OH
C6H7O2(OH)3
(cotton)
OH
OH
OH OH OH
OH
MTCS
modification
Treated by
SiO2 NPs
SiO2 NPs
Cotton fibre
Cotton fibre
Cotton fibre
OH
OH OH OH OH OH
H
C
O
CH2OH
H
OH
C
H
C
C
OH
H
O
OH
H
C
C
H
OH
O
C
C
H
H
MTCS coating
OH
H
OH
Si
OH
O
CH3-Si
H
H
O
H
O
O
CH2OH
OH
Si
OH
O
O
H
n
n
Abundant hydroxyl groups
present on cotton fibres
Hydrophilic
cotton
NaOH solution
SiO2 on hydroxyl groups of cotton fibres
Hydrophilic
cotton
O
OH
OH
C
C
H
O
O
CH3-Si
Si
O
O
Si
O
OH
O
H
H
O
O
O
H
n
MTCS reacts with hydroxyl
groups of SiO2 treated cotton
Superhydrophobic
cotton
Scheme 1: The proposed mechanism for the synthesis of superhydrophobic MTCS/SiO2-treated cotton
surface roughness difference between SiO2-treated
cotton and MTCS/SiO2-treated cotton.
Figure S3 (electronic supplementary information)
represents the full-frame EDX analysis of SiO2-treated
cotton where a high percentage of oxygen and silicon is
present. This is in agreement with the fact that SiO2
NPs were attached to the cotton fibers. In addition, a
small amount of carbon was also evident from EDS
analysis. This could be explained by the presence of
cellulose in the cotton fiber.32 Figure 1g represents the
full-frame EDX analysis of MTCS/SiO2-treated cotton.
It was observed that silicon, oxygen, and carbon
percentages increased by a considerable amount, which
can be explained by the formation of crosslinked
polymethylhydrosiloxane on the SiO2-treated cotton
fibers from the reaction between MTCS and the SiO2
NPs.10
Phase and structural analysis
The XRD pattern (see Fig. 2a) of the raw cotton
demonstrated four Bragg’s diffraction peaks at
14.82, 16.58, 22.9, and 34.5 due to the
monoclinic structure of cellulose [(C6H10O5)x]
phase.6,33 The XRD pattern of SiO2 NPs showed
a single broad peak centered at 22.9 2h value
suggesting the formation of pseudo-amorphous or
nanocrystalline colloidal SiO2 phase.34,35 The single
amorphous peak is due to the smaller particle size
effect and incomplete inner structure of the spherical
SiO2 NPs.34,36,37 The XRD profile of the SiO2 NPstreated cotton showed no additional peaks except the
XRD pattern of the raw cotton (see Fig. 2a). The XRD
profile of MTCS/SiO2-treated cotton did not divulge
any unwanted peaks. The absence of any unwanted
peaks in the SiO2-treated cotton and MTCS/SiO2treated cotton indicated no new crystalline phase
formed during chemical modification of the cotton
surface. The monoclinic structure of cellulose is retained after MTCS surface modification.
FTIR analysis was performed to understand the
surface evolution of raw cotton, SiO2-treated cotton,
and MTCS/SiO2 NPs-treated cotton, respectively (see
Fig. 2b). FTIR identified the chemical bonding in the
raw cotton and SiO2 NPs. The absorption bands
at 461, 799, 950, and 1090 cm1 were observed due to characteristic vibrations of Si–O out-ofplane deformation, Si–OH stretching, Si–O bending,
and Si–O–Si stretching, respectively.9 It was also
observed that a significant absorption band at 1632
cm1 was present corresponding to C–O bending
vibration.7,10 This can be explained by the complete
reaction between TEOS and ammonia solution and
corroborates the presence of siloxane ( ” Si–O–Si ” )
coatings.32 FTIR analysis shows additional absorption
bands at 786 and 1273 cm1 due to MTCS treatment attributed to the characteristic vibrations of
Si–O–Si and C–Si asymmetric stretching present in
the C–Si–O units.3,38,39 The absorption peak around
J. Coat. Technol. Res.
Fig. 1: FESEM images of (a) raw cotton, (b) EDS spectra showing the elemental distribution of raw cotton, (c) SiO2-treated
cotton, (d–f) MTCS/SiO2-treated cotton, and (g) EDS spectra showing the elemental distribution of MTCS/SiO2-treated
cotton
Intensity (a.u.)
MTCS@SiO2-coated cotton
SiO2-coated cotton
SiO2 nanoparticles
Transmittance (a.u.)
(b)
(a)
MTCS/SiO2-treated cotton
786 cm-1
1273 cm-1
SiO2-treated cotton
2852 cm-1
461 cm-1
1090 cm-1
2923 cm-1
SiO2 nanoparticles
1632 cm-1
3367 cm-1
799 cm-1
950 cm-1
Raw cotton
Raw cotton
10
15
20
25
30
35
40
2θ (deg.)
45
50
55
60
65
3500
3000
2500
2000
1500
1000
500
−1
Wavenumber (cm )
Fig. 2: (a) XRD patterns and (b) FTIR spectra of raw cotton, SiO2-treated cotton, and MTCS/SiO2-treated cotton
2923 and 2852 cm1 corresponds to stretching
and bending vibration of –CH2 of the cotton surface,
whose intensity becomes sharp after successful grafting
of MTCS compared to that of the pristine cotton, likely
due to attachment of –CH3 groups of MTCS.4,7
However, the peaks were absent in the FTIR spectrum
J. Coat. Technol. Res.
of the bare SiO2 NPs. Also, the absorption peak
at 3367 cm1 was present in FTIR spectra for entire
cotton samples due to –OH groups.4,7
Superhydrophobicity and wettability
Wettability of any sorbents is an essential parameter to
determine the hydrophobicity and oleophilicity toward
the oil–water separation. The digital images of raw
cotton and MTCS/SiO2-treated cotton illustrated the
physical change in color after MTCS modification (see
Fig. 3a). Superhydrophobicity of the as-synthesized
MTCS/SiO2-treated cotton was determined by a simple immersion test as shown in Figs. 3b and 3c. It was
found that the raw cotton was hydrophilic and
remained completely immersed in water due to the
large liquid contact area of solid–liquid interface and
abundant hydrophilic surface groups. However,
MTCS/SiO2-treated cotton remained floating on water
and absorbed oil quickly due to the formation of
the Cassie state with low surface energy and high
surface roughness.40 The general perception of
hydrophobicity or hydrophilicity was examined by the
extent to which a sessile water droplet remains static or
spreads quickly upon addition on the surface of the
material (see Fig. 3c).
Moreover, SiO2-treated cotton shows hydrophilic
nature upon immersion in water and shows immediate
absorption of both oil–water as displayed in Fig. S4a–b.
The spreading action takes place due to the difference
in the surface tension of the liquid–solid interface. The
extent to which a static water droplet spreads on a
surface is measured by a parameter known as contact
angle (CA) as shown in Fig. 3d. Since the surfaces are
rarely homogeneous, we employed here the Cassie
equation applicable to the heterogeneous rough surface with low surface energy exhibiting superhydrophobicity.40
cos hc ¼ f ðcos h þ 1Þ 1
where f is the fraction of the solid surface in contact
with liquid, h and hc represent the water CA on smooth
and rough surfaces, respectively. Fabrication of a
proper rough surface with a low surface energy can
render a surface superhydrophobic/superoleophilic
since the surface tension of oil are generally much
lower than that of water.41 Microtextured or micropatterned surfaces with hydrophobic tendencies can
exhibit apparent CA greater than 150 (Cassie state)
and are associated with superhydrophobicity or the
‘‘lotus effect.’’42 The CA of a static droplet of water
( 8 lL) on the MTCS/SiO2-treated cotton was measured 173.3 ± 2 at room temperature. Also, the
interfacial surface tension of MTCS/SiO2-treated cotton was calculated 23.5 mN/m, less than 35 mN/m
threshold limit showing superhydrophobicity.20 Furthermore, CA of a water droplet-in air on the SiO2treated cotton was measured 75.1 at room temperature, which shows the hydrophilic nature of SiO2treated cotton as shown in Fig. S4c.
Fig. 3: Optical images of (a) raw cotton and MTCS/SiO2-treated cotton, (b) hydrophilicity, hydrophobicity, and oleophilicity
test of MTCS/SiO2-treated cotton, (c) digital image showing stable water droplet and oil absorbed in superhydrophobic
MTCS/SiO2-treated cotton, and (d) CA measurement of stable water droplet on the surface of MTCS/SiO2-treated cotton
J. Coat. Technol. Res.
Selectivity tests
The oil–water selectivity of the MTCS/SiO2-treated
cotton was examined using two simple tests. First,
colorless aqueous solutions of silver nitrate (AgNO3)
and sodium chloride (NaCl) were taken in two different beakers. AgNO3 (Ag+ ions) solution has a high
sensitivity toward chloride ions (Cl) as shown in
Fig. S5 (electronic supplementary information). If a
small amount of Cl ions comes into contact with Ag+
ions, a white precipitate of silver chloride (AgCl) forms
and changes the color of the solution to milky gray.43
To verify whether MTCS/SiO2-treated cotton is indeed
superhydrophobic or not, an AgCl test was performed
and it was expected that white precipitate would not
form when absorbed oil squeezed into an NaCl
solution using MTCS/SiO2-treated cotton. Since
MTCS/SiO2-treated cotton is superhydrophobic, it did
not consume any water along with soluble AgNO3 salt
which may turn the colorless NaCl solution into milky
gray by the formation of AgCl (see Fig. S5a–d).43
However, a similar experiment with raw cotton
demonstrated that NaCl solution immediately forms a
white precipitate due to the formation of AgCl. In the
second test, a suction pump was used to check the oil–
water selectivity toward MTCS/SiO2-treated cotton
under negative pressure ( 100 Torr) condition. The
experiment was performed using an MTCS/SiO2treated cotton attached to one end of a tube and the
other end connected with a pump while a conical flask
attached with septum/stopper was placed in between
acting as a trap such that anything pumped in through
the cotton would fall into the flask. The MTCS/SiO2treated cotton was placed directly on the surface of an
oil–water mixture. It was observed that only oil
appears into the conical flask after few seconds while
the water despite being pumped-in under negative
pressure did not pass through the cotton surface (see
Fig. S6 electronic supplementary information).43
Oil/water separation tests
The superoleophilicity of MTCS/SiO2-treated cotton
arose from the open pores that exist and allowed oil to
permeate into the channels by virtue of capillary force.
Hence, the oil selectivity of the MTCS/SiO2-treated
cotton increased with time due to capillary action. This
superoleophilic property of the MTCS/SiO2-treated
cotton was quantified by determining the AC in the
presence of oil–water mixtures. The cotton exhibited
excellent oil selectivity and water repellent behavior
when exposed to the oil–water mixtures (see Video 1 in
electronic supplementary material accessible via the
link: https://vimeo.com/279800203). The modified cotton exhibited AC of 30–42 times of its weight for
different oils (see Fig. 4a). The AC of MTCS/SiO2treated cotton was reasonably better than other cottonbased absorbent materials reported in the literature
(see Table 1). Thus, it can be inferred that as-synthe-
sized MTCS/SiO2-treated cotton may serve as an
alternative sorbent for oil spill cleanup. Further, the
AC of the cotton was determined at 20, 40, and 60 min
holding time after reaching the saturation value. Four
sets of oil-absorbed MTCS/SiO2-treated cotton were
placed separately in petri dish allowing oil to permeate
out due to gravitational force. It was observed that a
low amount of oil was released from the MTCS/SiO2treated cotton when kept aside in a petri dish with a
maximum fall of 22%, 14%, 10%, 18%,
14%, 12%, for pump oil, engine oil, diesel,
hydraulic oil, vegetable oil, and kerosene, after
60 min, respectively (see Fig. 4b). This behavior could
be explained by the surface tension that develops
between the oil and the petri dish which facilitates oil
coming out from the sorbent with time.
Reusability of MTCS/SiO2-treated cotton
The MTCS/SiO2-treated cotton was subjected to 10
cycles of continuous absorption–desorption test with
various oils. AC and CC were calculated for MTCS/
SiO2-treated cotton as shown in Fig. 4c. The MTCS/
SiO2-treated cotton was found to possess first cycle AC
of 34.55, 41.64, 31.31, 35.56, 35.76,
and
31.7 g g1 for hydraulic oil, engine oil, kerosene,
pump oil, vegetable oil, and diesel, respectively. For
each cycle, the AC and CC were calculated. However,
there was a gradual drop in AC in the subsequent
cycles due to the presence of oil residue after
mechanical squeezing the MTCS/SiO2-treated cotton
which cannot be desorbed completely without drying.
It was observed that at the tenth cycle AC value did
not fall below 75% of the first cycle AC value of
corresponding oil (see Table 2). It should also be
borne in mind that CC values did not vary much
between the cycles and the maximum deviation
between the fifth and tenth cycle CC was 0.15 g g1.
Hence, it can be inferred that CC remains quite
stable between the cycles. The average recovery of the
modified cotton from the first to ten cycles was
observed between 86% and 89% demonstrating that
MTCS/SiO2-treated cotton could be reused effectively
for ten cycles without compromising its sorption
behavior (see Fig. 4d).
In the real world, the MTCS/SiO2-treated cotton
would be subjected to different pH of water or saline
water. Therefore, AC was determined under the harsh
conditions. Oil–water mixtures were prepared with
varying pH (2, 5, 7, 9, and 12) of water. It was observed
that AC remains stable ( 29.02 to 28.77 g g1)
with pH as shown in Fig. 5a. Further, AC of the
MTCS/SiO2-treated cotton was evaluated with different salts and alkali added kerosene–water mixture (1:5
v:v ratio) as a model oil–water mixture. The concentration of the salts and alkali solution was taken as 1
molar in each experiment. The oil–water separation
results showed AC deviation of 9.71%, 11.29%,
7.92%, 4.48%, 8.87%, 6.39% between first
J. Coat. Technol. Res.
20
31.7
31.31
35.56
30
35.76
41.64
40
10
Hydraulic
oil
40
20
0
Engine
oil
Kerosene Pump oil Vegetable
oil
Diesel
Diesel
AC
60
AC0
AC40
AC
40
20
0
AC
40
20
0
AC
40
20
0
AC
40
20
0
AC
Vegetable oil
CC
Pump oil
CC
Kerosene
CC
Engine oil
CC
AC60
40
30
20
10
0
Pump oil
(d)
CC
40
20
0
AC20
50
Oil recovery(%)
Sorption capacity (g g−1)
(c)
0
Absorption capacity (g g−1)
(b)
50
34.55
Absorption capacity (g g−1)
(a)
Hydraulic oil
CC
90
60
30
0
90
60
30
0
90
60
30
0
90
60
30
0
90
60
30
0
90
60
30
0
1
2
3
4
5
6
7
8
9
10
Hydraulic Vegetable Kerosene
oil
oil
Diesel
Diesel (Average = 89.9%)
Vegetable oil (Average = 87.95%)
Pump oil (Average = 88.54%)
Kerosene (Average = 89.08%)
Engine oil (Average = 89.55%)
Hydraulic oil (Average = 86.21%)
0
0
Engine
oil
1
2
3
4
5
6
7
8
9
10
No. of cycles
No. of cycles
Fig. 4: (a) AC of different oils after the first cycle, (b) AC retention with time (min) using superhydrophobic MTCS/SiO2treated cotton, (c) AC and CC for different oil after 10 cycles, and (d) percent oil recovery after 10 cycles
Table 1: Comparison of selective absorption behavior of cotton-based superhydrophobic sorbents
Absorbent materials
Synthesis route
Absorption type
Performance
Reduced graphene oxidecoated cotton
Graphene-coated cotton
Dip coating of
polydimethylsiloxane
Hydrothermal method
Organic solvents
and oils
Oil
Reduced graphene oxide
cotton
Superhydrophobic cotton
fabrics
Acylated cotton fibers with a
fatty acid
Polydimethylsiloxane
(PDMS)-coated cotton
MTCS/SiO2 cotton
Thermal reduction
Oil
Polydimethylsiloxane and ZnO
coating
Cellulose acylation using
microwave radiation
Chemical vapor deposition
Oil
Oil absorption 11 to 25
times its weight
Oil absorption 30 times its
weight
Oil absorption 22 to 45
times its weight
Higher water rejection
Dip coating of
methyltrichlorosilane
and third cycle for NaCl, MgCl2, NaOH, Na2SO4, hot
water, and cold water solution in kerosene–water
mixture (see Fig. 5b).
Further, gravity-driven oil–water separation test was
performed using MTCS/SiO2-treated cotton.24 The
purpose of the gravity-driven oil–water separation test
is to understand the filtration behavior of MTCS/SiO2treated cotton as an oleophilic membrane. The MTCS/
Oil
Oil
Oil
Oil absorption 11 to 20
times its weight
Oil absorption 30 to 65
times its weight
Oil absorption 30 to 42
times its weight
References
44
43
33
45
46
47
Present
work
SiO2-treated cotton was pressed in a circular shape
with 50 mm diameter and fixed between two glass
tubes of filtration unit as shown in Figs. 6a, 6b, and 6c.
Kerosene–water mixture-filled glass tube filtration unit
was attached at the tilted position so that kerosene–oil
mixture remained in contact with the MTCS/SiO2treated cotton and oil could pass quickly through the
open pores in the cotton at atmospheric pressure (see
J. Coat. Technol. Res.
Table 2: AC and CC of MTCS/SiO2-treated cotton with various oils over multiple cycles
Hydraulic oil
Engine oil
Kerosene
Pump oil
Vegetable oil
Diesel
CC at 1st
cycle
(g g1)
AC at 1st
cycle
(g g1)
34.55
41.64
31.31
35.56
35.76
31.70
2.4
3.2
2.8
2.1
2
2.3
±
±
±
±
±
±
3.95
3.96
2.74
3.82
3.28
2.77
±
±
±
±
±
±
AC retention
after 5th cycle
(%)
CC variation after
5th cycle (g g1)
AC retention after
10th cycle (%)
CC variation after
10th cycle (g g1)
85.79
93.25
84.40
95.78
92.36
89.38
0.08
0.02
0.18
0.01
0.16
0.20
75.58
86.82
74.84
88.15
79.97
85.24
0.05
0.03
0.12
0.06
0.05
0.05
0.44
0.21
0.17
0.20
0.33
0.17
(b)
40
Kerosene-water mixture
35
28.77
20
31.86
30.51
25
32.76
30
29.02
Absorption capacity (g g−1)
(a)
15
10
5
0
1
2
3
4
5
6
7
8
9
10 11
12 13
Absorption capacity (g g−1)
Oils
45
Cycle 1
Cycle 2
Cycle 3
Kerosene-water mixture
40
35
30
25
20
15
10
5
0
Sodium Magnesium Sodium Sodium Hot Water
Chloride Chloride Hydroxide Sulphate
pH
Water
Fig. 5: AC with (a) variable pH and (b) different salts/alkali added kerosene–water mixture using superhydrophobic MTCS/
SiO2-treated cotton
(a)
(b)
(c)
Water
Kerosenewater mixture
Kerosene
100
1000
80
800
60
600
40
400
200
20
Volume flux
Separation efficiency
0
0
0
50
100
150
Time (S)
200
250
Absorption capacity (g g−1)
(e)
1200
Separation efficiency (%)
Volume flux (Lm−2h−1atm−1)
(d)
40
30
Hydraulic oil
Engine oil
Kerosene
Pump oil
Vegetable oil
Diesel
20
10
0
0
5
10
15
20
25
30
Time (S)
Fig. 6: (a–c) Gravity-driven oil–water separation setup, (d) variation of kerosene oil volume flux and separation efficiency
measured with time, and (e) absorption kinetics behavior of MTCS/SiO2-treated cotton with time for different oils
J. Coat. Technol. Res.
Video 2 in electronic supplementary material accessible via the link: https://vimeo.com/279800581). Figure 6d shows the volumetric flux and separation
efficiency of kerosene with time.48 It can be observed
that within 270 s, 94% kerosene can be recovered
from the kerosene–water mixture using MTCS/SiO2treated cotton. Further, the reduction in volume flux
and increase in separation efficiency (%) of kerosene
with time confirmed that hydrocarbon could permeate
easily through the superhydrophobic MTCS/SiO2treated cotton under gravity.
Absorption kinetics for sorption mechanism
A highly interconnected porous structure that exists in
the MTCS/SiO2-treated cotton helps oils become
absorbed quickly with time. However, the absorption
kinetics found that the sorption characteristics also
depend on the oil viscosity. While the MTCS/SiO2treated cotton achieved its saturation AC with low
viscous oils (diesel and kerosene) within 2 s or less, the
saturation AC for oils with higher viscous oils took
almost 10 s (see Fig. 6d and Table 3). The low viscous
oils have a relatively faster diffusion which enables
them to rapidly enter the interconnected porous
structure of the MTCS/SiO2-treated cotton. In the
case of the higher viscous oils, the oils tend to remain
on the surface before permeating into the pores of the
cotton. To understand the kinetic behavior, a pseudosecond-order kinetic model was employed2:
for highly viscous oils (see Table 3). Therefore, it can
be concluded that sorption behavior of various oils in
the MTCS/SiO2-treated cotton depends not only on the
quality of sorbent but also on intrinsic viscosity.
Conclusions
A superhydrophobic/superoleophilic MTCS/SiO2-treated cotton was fabricated using a simple dip coating
method. A low concentration of MTCS solution [2%
(v/v)] was used for surface modification and gave rise
to superior superhydrophobic properties. Since cotton
is reasonably cheap and readily available, the cost of
fabrication becomes low. FESEM, EDX, FTIR, and
XRD analyses confirmed the formation of –C–Si–O–
units on the cotton fiber surface. The –C–Si–O units
grew layer by layer onto the MTCS/SiO2-treated
cotton surface and developed superhydrophobicity on
the cotton surface. This modified cotton exhibited CA
of 173 ± 2 which satisfies the conditions for a
Cassie state. It showed AC of 30–40 g g1 for
various oils and was found to be reusable for ten
cycles with minimal loss in its sorption behavior.
Acknowledgment Dr.
Partha
Saha
gratefully
acknowledges the financial support received from
Department of Science and Technology, Science and
Engineering Research Board (DST-SERB) (Grant
Number ECR/2016/000959).
1
1
¼ K:t
Qs Qt Qs
where t denotes the sorption time, Qs denotes the
saturation AC, Qt indicates the amount of oil absorbed
at time t, and K is a viscosity-dependent sorption
constant. Qs, K, and t were calculated using the
pseudo-second-order kinetic model. The experimental
data were fitted in this model, and an excellent
agreement was achieved with the regression coefficient
(R2) value greater than 0.95.49–52 The K value for less
viscous oils like diesel and kerosene was found to
be 0.652 and 0.627 s1, respectively. However, K
value was found to be between 0.025 and 0.035 s1
Table 3: Saturation absorption time, capacity, K, and R2
values for MTCS/SiO2-treated cotton for various oils
Oil
Hydraulic oil
Engine oil
Kerosene
Pump oil
Vegetable oil
Diesel
ts (s)
Qs (g g1)
K (s1)
R2
8
10
2
8
8
2
31.85
41.44
31.63
35.81
34.97
30.77
0.03
0.02
0.62
0.02
0.02
0.65
0.98
0.97
0.99
0.97
0.97
0.99
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