science journal

1. Functionalized Superwettable Fabric with Switchable Wettability for
Efficient Oily Wastewater Purification, in Situ Chemical Reaction
System Separation, and Photocatalysis Degradation
Lili Ma, Jinmei He,* Jiaxin Wang, Yichen Zhou, Yu Zhao, Yuangang Li, Xiangrong Liu, Lei Peng,
and Mengnan Qu*
College of Chemistry and Chemical Engineering, Xi’an University of Science and Technology, Xi’an 710054, Shaanxi, China
*S Supporting Information
ABSTRACT: In view of the increasing serious water environmental and human health issues caused by oily wastewater,
functional superwetting materials with controllable wettability, high durability, and scale preparation methods are highly desired
for efficient oil/water separation. In this respect, a pH-responsive multifunctional fabric with switchable surface wettability,
favorable mechanical durability, and self-repairing property has been developed via decorating the modified TiO2 nanoparticles
of special surface compositions onto the fabric surface. By virtue of the intelligent surface wettability, the resulted superwettable
fabric can be used for controllable separation of multiple oil/water mixtures, particularly the complicated oil/water/oil ternary
mixtures, showing excellent separation efficiency and high filtration flux even under extreme pH conditions, which is comparable
to most of the commercial and currently reported functionalized membranes. Simultaneously, the negative pressure-driven,
continuous, high-speed, and highly efficient in situ purification of large volumes of oily wastewater is successfully achieved based
on the resulted superwettable fabric. More importantly, with the as-prepared superwettable fabric as the filtration membrane,
the continuous in situ separation of the synthetic oily product from the corresponding chemical reaction systems is well
performed without interruption of the reaction, demonstrating outstanding merits of simplifying procedures, saving operation
time, and increasing product yield. In addition, it is worth noting that the alkali-treated superhydrophilic fabric presents superior
photocatalysis self-cleaning performance for various water-soluble organic pollutants. These unique advantages of the
functionalized smart superwettable fabric ensure that it can be competent in multifarious relevant challenging settings,
indicating a broad prospect for diverse practical applications, especially the oily wastewater treatment and multiple industrial
operation optimizations.
KEYWORDS: superwettable, pH-responsive, controllable oil/water separation, chemical reaction system, photo-catalysis degradation
1. INTRODUCTION
Nowadays, oil/water separation is becoming more and more
the focus of attention owing to the increasing demands in
multiple applications, particular the oil spill cleanup, organic
wastewater treatment, and chemical manufacturing.1−3
Tradi-
tional separation methods,4−6
including gravity settlement,
adsorption technique, chemical demulsification, and so on,
inevitably suffer from the disadvantages of low efficiency, high
material cost and energy consumption, and cumbersome
operation. Therefore, it is of great importance to develop
advanced functional separation materials that are cost-effective
with controllable selectivity and convenient properties to
realize the effective separation of oil/water mixtures.
In the recent years, bioinspired superwettable materials with
special wettability have been widely researched and demon-
strated to be of great interest for highly efficient oil/water
separation.7,8
On the basis of the difference in surface
wettability, these materials can be commonly classified into
three types, including the superhydrophobic−superoleophilic
Received: September 4, 2019
Accepted: October 29, 2019
Published: October 29, 2019
Research Article
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2. materials,9,10
superhydrophilic and underwater superoleopho-
bic materials,11,12
and smart superwettable materials with
controllable surface wettability.13,14
Among them, the super-
hydrophobic materials with great water repellency and oil
affinity can be applied to efficiently remove oil from the
aqueous systems. However, during the separation process, oil
contamination of the material surfaces is inevitable.15
With
respect to this problem and inspired by fish scales, the
superhydrophilic and underwater superoleophobic materials
have been studied and fabricated. This type of superwettable
materials can effectively separate water from oil and
simultaneously protect their surfaces from the pollution of
oils, whereas prewetting of water is an essential prerequisite
which results in the limitations of the superwetting materials in
practical application.16
To overcome these drawbacks, smart
superwettable materials with controllable wettability to oil and
water have gained much attention, which can intelligently
switch their surface wetting property between two completely
different states in response to proper external stimuli, such as
light,17,18
pH,19,20
temperature,21,22
gas,23,24
electric field,25,26
or magnetic field,27,28
showing great research and application
values in separating oil/water mixtures for different demands.
Generally, smart separation materials are designed associat-
ing the synergistic effect of well-defined surface morphology
and special chemical composition.29
In this regard, great efforts
have been made to develop smart superwettable surfaces with
different responsiveness via various methods. Xin et al.30
developed a smart membrane containing polyvinylidene
fluoride and TiO2 particles, which can switch its surface
wettability from superhydrophobicity to superhydrophilicity
via ultraviolet (UV) irradiation. Wang et al.31
reported a
temperature-responsive intelligent melamine sponge by graft-
ing octadecyltrichlorosilane of low surface energy and poly(N-
isopropyl acrylamide) of excellent thermosensitivity onto the
sponge surface. Among the above stimuli/responsive systems,
pH-responsive superwettable materials recently have been
brought into the focus of the interesting and emerging areas of
scientific research by virtue of the rapid responsibility,
favorable reversibility, convenient operation, and widespread
applicability.32,33
To obtain pH-responsive superwettable
materials, there are two main ways:34,35
(1) introducing the
elaborate chemicals containing the pH-sensitive groups onto
the material surface; such a strategy is widely adopted for
constructing the pH-responsive surface, but also has the
disadvantages of high cost, poor durability, and time-
consuming; (2) fabricating the polymer-based surfaces intrinsi-
cally embracing the specific functional groups susceptible to
pH stimulus; in this way, the special wettability of the resulted
material will be more durable. However, the design and
selection of suitable materials are difficult. For example, Wen et
al.32
prepared an intelligent fabric with pH-controlled switch-
able surface wettability by grafting stearyl methacrylate and
pH-sensitive undecylenic acid onto the thiol-functionalized
fabric surface. Manna et al.36
developed a smart multifunc-
tional material from biodegradable chitosan polymer via
Michael addition reaction. The as-prepared material could
present two distinct extreme wettabilities by governing the
dominance of two orthogonal functional groups on the
material surface under different pH conditions. Despite the
significant progress, the limited wettability convertibility, poor
durability, low separation efficiency, and complex preparation
procedures have been the biggest disadvantages restricting the
development of smart separation materials.
Besides, in addition to the above-mentioned typical oil/
water separation, the removal of immiscible oily products from
chemical reaction systems is also considered as an important
“oil/water separation” process in chemical manufacturing and
has become the hot topic, attracting extensive concern in the
present research.37,38
Typically, chemical reaction and product
separation are two independent processes, which usually
means the abundant cost of time and energy. In view of the
selective liquid-wetting behaviors of the superwettable
membrane materials, integrating the special membranes with
reactors can provide a continuous in situ separation process to
directly remove the oily products from the reaction systems
and without interrupting the chemical reactions. However,
seldom works have been reported till now to apply the
superwettable membranes in chemical reaction systems.39
The
major reason lies in the fact that the existing membranes lack
excellent durability to maintain their surface superwettability in
the complex and harsh environments for a long time.40
On the basis of the present research situation and prospect, a
multifunctional smart superwettable fabric with favorable pH
responsiveness, tunable surface wettability, superior durability,
and self-healing ability has been developed via a facile dip-
coating method. TiO2 nanoparticles were selected to construct
appropriate hierarchical roughness on the fabric surface, and
delicate fluorochemicals were adopted to endow the
membrane with low surface energy and simultaneously
introduce the pH-sensitive chemistry onto the membrane
surface. As a result, the obtained superwetting fabric possesses
a special pH-controlled surface wettability, which can
reversibly transit between the water-repellent and underwater
oil-repellent states dozens of times, exhibiting good repeat-
ability. Moreover, it is noteworthy that the as-prepared fabric
membrane exhibits excellent degradation property for various
water-soluble organic pollutants owing to the brilliant
photocatalytic property of TiO2, which holds potential
applications in high-quality wastewater purification and self-
cleaning. By virtue of the attractive advantages, the resulted
intelligent fabric can be applied to selectively separate
multitypes of oil/water mixtures (especially the complicated
oil/water/oil mixture) on demand, showing the high
separation flux and efficiency even under extreme pH
conditions. In terms of the excellent oil/water separation
performance and high durability, the superwettable fabric-
based, continuous, high-speed in situ purification of a large
amount of oily wastewater is able to be realized under the drive
of negative pressure. More importantly, with the resulted
superwettable fabric as the filtration membrane, the continuous
in situ separation of the immiscible oily products from reaction
systems can be achieved, which presents potential merits of
simplifying separation procedures, saving operation time, and
increasing product yield in comparison with the traditional
separation methods.4−6
Therefore, we anticipate that the as-
prepared smart fabric will be competent in multifarious
relevant challenging settings and possess a broad application
prospect in oil spill cleanup, wastewater treatment, and
optimizing various operations in the industrial field.
2. EXPERIMENTAL SECTION
2.1. Materials. The polyester fabric was obtained from the local
market, which was rinsed with acetone and anhydrous ethanol several
times and dried in a vacuum oven before use. Anatase titanium
dioxide (TiO2, 99.8%, 5−10 nm) was provided by Shanghai Yi’en
Chemical Technology Co., Ltd. Tetrabutyl titanate [Ti(OBu)4] and
ACS Applied Materials & Interfaces Research Article
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3. tert-butyl alcohol (tert-C4H10O) were purchased from Tianjin Fuchen
Chemical Reagents Factory. Perfluorooctanoic acid (PFOA, 98%) and
trichloro(1H,1H,1H-perfluorooctyl)silane (PFOTS, 97%) were ob-
tained from J&K Chemical Ltd. Sodium hydroxide (NaOH) was got
from Tianjin Tianli Chemical Reagent Co., Ltd. Hydrochloric acid
(HCl, 36%) was purchased from Taicang Hushi Reagent Co., Ltd.
Anhydrous ethanol was acquired from Shanxi Tongjie Chemical
Reagent Co. Ltd. All of the chemicals were used as received without
further purification.
2.2. Preparation of the Smart Superwettable Fabric. First,
0.20 g of PFOA was mixed with 1.50 g of Ti(OBu)4, and then 5 mL of
deionized water and 5 mL of ethanol were added to the mixture.
Subsequently, the resulted mixed solution was ultrasonically dispersed
for 20 min and stirred at 60 °C for 3 h. Finally, the PFOA−Ti(OBu)4
emulsion was prepared.
Second, 0.20 g of TiO2 was dispersed in 25 mL of ethanol, and
then 0.1 mL of PFOTS was added into the obtained mixture and
ultrasound dispersed for 10 min. Later on, the precleaned polyester
fabrics were immersed into the PFOTS−TiO2 ethanol dispersion and
continuously sonicated for 20 min. Then, the obtained homogenous
mixture was stirred at 65 °C for 3 h. After that, 10 mL of the prepared
PFOA−Ti(OBu)4 emulsion was added into the resulted mixture and
further stirred at 65 °C for 3 h. Finally, the obtained fabrics were
rinsed with ethanol to remove the redundant TiO2 particles
undecorated on the fabric surfaces and dried at 90 °C for 2 h.
More experimental details and instrumentations have been
described in the Experimental Section of the Supporting Information.
3. RESULTS AND DISCUSSION
3.1. Fabrication of the pH-Responsive Intelligent
Superwettable Fabric. To obtain a smart pH-switched
superwetting material, the elaborate surface morphology and
pH-responsive chemical groups are two indispensable pre-
requisites.41
With respect to the specific design principle, a
multiple superwettable fabric with smart pH-responsive surface
wettability has been fabricated, as schematically shown in
Figure 1. First, the precleaned fabric was immersed in a mixed
solution of PFOTS−TiO2 to increase its surface roughness and
simultaneously decrease its surface energy to a certain extent.
As a result, the resulted fabric exhibited favorable hydro-
phobicity, whereas it possessed poor mechanical durability
owing to the weak binding force between the nanoscaled TiO2
aggregates and the obtained fabric surface. To further enhance
the durability and improve the water repellency of the fabric,
the as-prepared PFOA−Ti(OBu)4 sol solution was then added
into the mixture of PFOTS−TiO2 and fabric, after which
PFOA−TiO2 containing both titanium carboxylate coordina-
tion complexes and Ti−O−Ti networks were in situ formed on
the fabric surface, as shown in Figure S1. Because of the
synergistic effect of PFOTS−TiO2 and PFOA−TiO2, plentiful
micro/nanoscaled and well interconnected TiO2 rough
structures in conjunction with abundant fluorochemicals
were constructed on the fabric surface, which endowed the
as-prepared fabric with excellent superhydrophobicity and
superior durability. Moreover, it is worth noting that in the
presence of the titanium−carboxylate complexes, the surface
wettability of the resultant superwettable fabric could be
switched according to the pH condition of an aqueous
solution. It was mainly attributed to the pH sensitivity of the
titanium−carboxylate bonding, which was easily cleaved under
a strong alkaline condition and formed the ionic PFOA of great
water affinity on the fabric surface,24
inducing the wettability of
the superwettable fabric transited from superhydrophobicity−
superoleophilicity to superhydrophilicity−underwater super-
oleophobicity. Furthermore, the superhydrophobicity of the
alkali-treated superhydrophilic fabric could easily regain by
treating the fabric in an acidic aqueous solution for several
minutes, and such a special transition was able to be reversibly
cycled by tuning the pH value of an aqueous solution.
Benefitting from the special and highly accessible pH
responsiveness, the as-prepared superwettable fabric is
promising for selective and controllable separation of various
oil/water mixtures on different demands.
The hierarchical surface rough structure is important for the
fabrication of the superwettable materials with special surface
wettability.42
Herein, the surface microcosmic morphologies of
the pristine and surface-functionalized superwettable fabric are
characterized by scanning electron microscopy (SEM). As
presented in Figure 2a,d, the pristine fabric possessed a quite
smooth surface. After the modification of PFOTS and TiO2
nanoparticles, a layer of rough PFOTS−TiO2 coating was
observed on the surface of the treated fabric, but the fabric
structure did not show any obvious change as a whole from its
low-magnification SEM image (Figure 2b). Further observing
the high-magnification morphology (Figure 2e) of the treated
fabric, a large amount of TiO2 granular aggregates unevenly
distributed on the treated fabric surface greatly increased its
surface roughness, endowing the treated fabric with favorable
hydrophobicity and a water contact angle (WCA) about 148°.
However, the TiO2 granule structures were seemed loosely
covered on the fabric surface and exhibited poor durability. As
a consequence, the sol solution of PFOA−Ti(OBu)4 was
further added into the mixture of PFOTS−TiO2 and the fabric,
resulting in a rougher surface morphology and higher WCA
(∼165°) of the obtained fabric (Figure 2c). Figure 2f shows
the enlarged view of the PFOTS−TiO2/PFOA−TiO2 coated
superwettable fabric surface, where massive TiO2 nanoparticles
could be observed overlapped densely on the fabric surface and
from a large number of micro/nanoaggregates, with
interconnected networks even deeply embedded inside the
fabric surface. Moreover, from the corresponding energy-
dispersive X-ray spectrometry (EDS) mapping images of the
PFOTS−TiO2/PFOA−TiO2 coated superwettable fabric sur-
Figure 1. Schematic illustration of the fabrication of the pH-
responsive smart superwettable fabric for selective and controllable
oil/water separation.
ACS Applied Materials & Interfaces Research Article
DOI: 10.1021/acsami.9b15952
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4. face in Figure 2g−j, it can be found that C, O, F, and Ti
elements were evenly distributed on the modified fabric
surface, indicating the full and uniform coverage of TiO2
nanoparticles grafted with fluorine polymers on the function-
alized fabric. Consequently, the abundant rough structures
combined with pretty low surface energy could provide the
modified fabric surface with a large area to trap air but repel
water, imparting the modified fabric with an excellent
superhydrophobicity.
3.2. pH-Responsive Superwetting Performance of
the Intelligent Superwettable Fabric. For a study of the
surface wettability variation of the obtained superwettable
fabric under different pH conditions, the WCAs of the
superwettable surfaces after treatment of different pH aqueous
solutions were measured. As shown in Figure S2, when the
aqueous solution shows a pH value less than or equal to 9, the
treated fabric surface exhibited a stable superhydrophobicity
with a WCA about 150°, and water droplets could maintain a
spherical shape on the fabric surface without any obvious
deformation in 270 s. For the superwettable fabric treated by
pH ≥ 11 solutions, the WCAs of the treated surfaces decreased
visibly with the increase of time, and such a change was
seemed to be more rapid after the pH value of the treated
solution reached 13. More specifically, when the fabric surface
was treated by pH 11 solution, the WCA of the treated fabric
surface gradually decreased from 146 ± 1.1° to 124 ± 1.5° in
270 s, whereas on continuously increasing the solution pH
value to 13, the water droplets placed on the treated fabric
surface quickly penetrated and spread within 3 s, achieving a
rapid pH-responsive wettability transition from superhydro-
phobicity to superhydrophilicity. The remarkable wettability
transition was mainly ascribed to the cleavage of titanium
carboxylate coordination bonds under strong alkaline con-
dition, which further formed the perfluorocarboxylate ions with
great water affinity on the treated fabric surface.
The surface-wetting behavior and pH responsiveness of the
superwettable fabric have been investigated in Figure 3. For the
pH 1 acidic-treated superwettable fabric surface, all water
droplets could exhibit a stable and near spherical shape with a
WCA about 161°. The superior superhydrophobicity was
ascribed to the plentiful perfluorooctyl chains of low surface
energy that grafted with the micro-/nanoscaled rough TiO2
aggregates distributed on the acidic-treated fabric surface,
which jointly decreased the surface energy and enhanced the
three-dimensional capillary effect of the treated fabric surface,
endowing the resulted fabric with excellent water repellency
(Figure 3b). In this case, both water and other daily use
liquids, such as orange juice, cola, tea, milk, and cafe, could
maintain stability on the acidic-treated fabric surface (Figure
3f), and their contact angles were all greater than 150° (Figure
S3a). Conversely, when the superwettable fabric was treated by
pH 13 alkali solution, water droplets that contacted the fabric
surface could quickly spread and penetrate (Figure 3c). The
main reason lies in the formation of sodium perfluorooctanoate
in the presence of NaOH based on the cleavage of titanium
carboxylate coordination bonding (Figure 3d), which could
generate a strong hydrogen bonding force with water
molecules that was far greater than the hydrophobic
interaction produced between residual PFOTS−TiO2 and
water. As a result, the alkali-treated superwettable fabric surface
presented superior superhydrophilicity with a WCA about 0°.
Interestingly, when submerging the alkali-treated super-
hydrophilic fabric in water, the surface showed extreme
repellency to oils (Figure S3b), and the dichloromethane
droplets placed onto the fabric surface could present an
elliptical sphere (Figure 3g) with an underwater oil contact
angle (OCA) of about 152°. The special wetting behavior was
ascribed to the formation of a water film on the fabric surface,
which could serve as a “barrier” to prevent oil from contacting
the fabric surface, resulting in preeminent underwater
superoleophobicity. Of particular note is that the super-
hydrophobicity of the alkali-treated superhydrophilic fabric
surface could regain after further treating the fabric with pH 1
acidic solution, on account of the deprotonation of
perfluorooctyl carboxylates. As illustrated in Figure 3e, the
smart and controllable transition between the two extreme
wettabilities could be reversibly cycled for nearly 20 times with
a minor fluctuation in the responsiveness, indicating favorable
pH-sensitivity and stable switchable ability of the superwet-
table fabric.
For an investigation of the influence of chemical
compositions on the fabric surface wettability, X-ray photo-
electron spectroscopy (XPS) has been utilized to characterize
the changes in the surface composition of the as-prepared
superwettable fabric before and after alkali treatment, as
exhibited in Figure 4. For the pristine fabric, only C and O
elements were detected on its surface. After modification, the
typical peaks of Si 2p, Ti 2p, F 1s, and F KLL were observed
appearing at around 110.2, 456.7, 688.3, and 835.1 eV in the
XPS spectrum of the as-prepared fabric surface, respectively,
which were mainly ascribed to the PFOTS−TiO2 and PFOA−
TiO2 (Figure 4a). Moreover, in the high-resolution C 1s
spectrum of the as-prepared fabric (Figure 4b), it can be found
Figure 2. (a−f) Low- and high-resolution SEM images of the (a,d)
pristine fabric, (b,e) PFOTS−TiO2 coated fabric, and (c,f) PFOTS−
TiO2/PFOA−TiO2-coated superwettable fabric. (g−j) EDS mapping
images of the PFOTS−TiO2/PFOA−TiO2-coated superwettable
fabric.
ACS Applied Materials & Interfaces Research Article
DOI: 10.1021/acsami.9b15952
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5. that there are three signals at 284.6, 287.9, and 292.8 eV, which
are mainly assigned to the C−C, −CF2−, and −CF3 groups in
PFOTS and PFOA. All these results fully proved that the TiO2
nanoparticles grafted with long perfluorooctyl chains were
successfully coated on the as-prepared fabric surface, which
was identical with the EDS results in Figure 2, contributing to
the excellent superhydrophobicity. For the resulted superwet-
table fabric treated by alkali (pH 13) solution, an additional
Figure 3. (a,c) Water-wetting behaviors of the superwettable fabric after separately treated by (a) acidic solution and (c) alkali solution. The
inserted images with red dotted borders in (a,c) are the profiles of water droplets on the corresponding superwetting fabric surface. (b,d) Surface-
wetting mechanism of the superhydrophobicity of the acidic-treated fabric surface and the superhydrophilicity of the alkali-treated fabric surface,
respectively. (e) WCA transition of the smart superwettable fabric surface with a cyclic treatment of acidic and alkali solutions. (f) Various daily use
liquids on the acidic-treated fabric surface. (g) Underwater dichloromethane droplets (red) on the alkali-treated fabric surface; the image with blue
border shows the side view of the oil droplets on the fabric surface.
Figure 4. (a) XPS spectra of the pristine fabric, the as-prepared superwettable fabric, and the pH 13 alkali-treated superwettable fabric. (b) High-
resolution C 1s spectrum of the superwettable fabric. (c,d) High-resolution Ti 2p and Na 1s spectra of the alkali-treated superwettable fabric.
ACS Applied Materials & Interfaces Research Article
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6. peak of Na 1s emerged in its corresponding XPS spectrum,
which was around 1072.8 eV (Figure 4d). Further magnifying
the Ti 2p peak in the spectrum of the alkali-treated fabric
(Figure 4c), two adsorption peaks were found at 457.9 and
464.9 eV, which were identified as Ti−OH and TiO2,
respectively. Moreover, the concentration of Ti−OH was
higher than that of TiO2 on the alkali-treated fabric surface,
demonstrating the cleavage of titanium carboxylate coordina-
tion bonds between PFOA and TiO2, which further formed
Ti−OH and sodium perfluorooctanoate in the presence of
NaOH alkali solution. Therefore, the alkali-treated fabric
presented favorable superhydrophilicity because of the strong
water affinity of sodium perfluorooctanoate.
3.3. pH-Controlled Selective Oil/Water Separation. In
comparison with the traditional oil/water separation materials
with single fixed surface wettability, the as-prepared smart
superwettable fabric with pH-controlled switchable surface
wettability is the candidate for selectively separating the
desired liquid from the relevant oil/water mixtures. As a proof
of concept, two types of lab-assembled separation devices were
developed for selective separation of oil phase or aqueous
phase from the corresponding oil/water mixtures, with the
intelligent superwettable fabrics as the filtration membranes.
Of particular note, few works have referred to the smart
superwettable material that is able to simultaneously separate
oil/water mixtures with oil density higher (high-density oil) or
less (low-density oil) than that of water’s via either of its two
different surface wettabilities,43
which shows a prominent
advantage in dealing with the problems of oil fouling and water
barrier during the separation process.
For “oil removing” type oil/water separation (Figure 5), the
superwettable fabric with a superior superhydrophobic−
superoleophilic property was utilized as a separation
membrane to remove the oil phase from the oil/water mixtures
without any pretreatment. In this case, when pouring a mixture
of water (100 mL, blue) and dichloromethane (100 mL,
orange) into the separation device, dichloromethane quickly
formed the bottom layer owing to its high density and
immediately filtrated through the fabric with a flux of about
7300 L·m−2
·h−1
(Figure 5a−c), whereas the water layer was
blocked on the fabric surface on account of the favorable water
repellency of the obtained fabric (Figure 5d). In addition, the
superwettable fabric with efficient “oil removing” performance
also could be employed to separate the low-density oil/water
mixture via the lab-made inclined separation device. As
exhibited in Figure 5e−g, it can be observed that n-hexane
could first expose to the fabric surface once the mixture of n-
hexane (30 mL, yellow) and water (30 mL, blue) is poured
into the inclined separation device and then rapidly penetrate
with a flux of about 6400 L·m−2
·h−1
. After fast separation, n-
hexane was collected in the below beaker, and water was
retained on the fabric (Figure 5h).
In view of the above separation results, multitypes of model
oil/water mixtures derived from various high-density oils
(trichloromethane and tetrachloromethane) and low-density
oils (toluene, xylene, cyclohexane, and hexadecane) were
introduced into the above two forms of lab-made separation
devices to further evaluate the separation capability of the
resulted superwettable fabric. The separation results of all these
oil/water mixtures are presented in Figure 5i,;it can be found
Figure 5. “Oil removing” type oil/water separation performance of the smart superwettable fabric. (a−d) Separation process of the high-density oil
(dichloromethane, orange) and water (blue) mixture. (e−h) Separation process of the low-density oil (n-hexane, yellow) and water (blue) mixture.
(i) Filtration flux and the corresponding separation efficiency of the superwettable fabric for multiple types of oil/water mixtures. (j) Cyclic
separation performance of the dichloromethane/water mixture.
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7. that the selective separation efficiency of the superwettable
fabric for these oil/water mixtures was mostly above 98.8%,
and the corresponding filtration flux was higher than 6250 L·
m−2
·h−1
, all of which indicated that the oil/water mixtures
were effectively separated with the merit of excellent selective
separation performance of the superwettable fabric. It is
noteworthy that the filtration flux for low-density oil/water
mixtures was lower than that for high-density oil/water
mixtures, which was possible ascribed to the larger surface
tension and viscosity of the low-density oils in comparison with
the high-density oils as well as the smaller effective filtration
area of the inclined separation device. In addition, the
reusability of the superwettable fabric was also carried out by
a cyclic oil/water separation experiment. After each separation
cycle, the superwettable fabric was rinsed with deionized water
to remove the oil droplets absorbed on the fabric surface. As
exhibited in Figure 5j, the cyclic separation of the dichloro-
methane/water mixture could be conducted multiple times
without any compromise in the separation efficiency. Even
after 20 cycles of separation, the separation efficiency of the
oil/water mixture was still higher than 98.9% and the
corresponding filtration flux remained about 7020 L·m−2
·h−1
,
which were slightly lower than that of the first separation cycle
(99.9%, 7244 L·m−2
·h−1
), demonstrating superior separation
performance and favorable durability of the superwettable
fabric.
In addition, the as-prepared superwettable fabric also could
be applied to “water-removing” type oil/water separation
merited from its superior pH-responsive surface wettability. As
shown in Figure S4, the superwettable fabrics that are used for
separating aqueous solution phases from oil/water mixtures
were pretreated by pH 13 alkali solution, so as to switch their
surface wettability to superhydrophilicity−underwater super-
oleophobicity. Following two kinds of oil/water mixtures,
involving n-hexane (100 mL, yellow)/water (100 mL, sky-
blue) and dichloromethane (30 mL, orange)/water (30 mL,
blue), were separately introduced into the above two types of
lab-made separation devices (Figure S4a,b,e,f). Benefitting
from the excellent water affinity and underwater oil repellency
of the alkali-treated superwettable fabric, the water phase
contacted to the fabric surface could quickly spread and filtrate
and meanwhile form a layer of water film on the surface that
could effectively prevent the oil phase from penetrating
through the fabric. As a consequence, the oil/water mixtures
were separated successfully (Figure S4c,d,g,h). To further
investigate the “water removing” performance of the alkali-
treated superwettable fabric, the effective separations of the
treated fabric for multiple oil/water mixtures were carried out,
as shown in Figure S4 Supporting Information. For all of these
oil/water mixtures, the flux was ranging from 11 758 to 9243
L·m−2
·h−1
and the corresponding separation efficiency was
kept above than 99.2%, comparable to a series of superwettable
membranes, with the wetting property similar to that reported
in previous works.44,45
In addition, the cyclic separation of n-
hexane/water mixtures was performed to investigate the
reusability of the alkali-treated fabric. Similarly, the superwet-
table fabric was cleaned with alkaline water solution and dried
up after each separation experiment to remove the water film
on the fabric surface. As exhibited in Figure S4j, the
permeation flux and separation efficiency of the alkali-treated
fabric for the first cycle separation were 13758 L·m−2
·h−1
and
99.8%, respectively, and remained basically stable for the
following separation cycles without any obvious fluctuant. As a
result, the permeation flux was still higher than 12 936 L·m−2
·
h−1
and the corresponding separation efficiency was main-
tained above than 99.2% even after 20 cycles of separation,
showing an excellent durability of the separation performance.
3.4. pH-Controlled Continuous in Situ Separation of
Oil/Water/Oil Ternary Mixtures. Conventional oil/water
separation systems generally require multiple steps to achieve
Figure 6. (a−h) As-prepared smart superwettable fabric-based continuous separation process of the oil/water/oil ternary mixture: dichloromethane
(high-density oil, 15 mL, red), water (pH = 7, 15 mL, colorless), and n-hexane (low-density oil, 15 mL, yellow). During the separation process, the
wettability of the smart superwettable fabric switched from superhydrophobicity−superoleophilicity to superhydrophilicity−underwater
superoleophobicity by in situ alkaline treatment, thus leading to the complete and continuous separation of each liquid phase from the
corresponding oil/water/oil mixture.
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8. the efficient separation of complex oil/water mixtures
consisting of multiphases of liquids. Therefore, developing a
novel separation system that can continuously separate each
phase of liquid from the complex oil/water mixture is highly
desired but remains challenging. Herein, the as-prepared
superwettable fabric with intelligent surface wettability and
selective oil/water wetting performance has been applied in the
continuous and controllable separation of the oil/water/oil
ternary mixture. The detailed separation process is shown in
Figure 6, where the obtained superwettable fabric was fixed
between two glass tubes as the filter membrane without any
pretreatment (Figure 6a), and then a mixture of dichloro-
methane (high-density oil, 15 mL, red), water (pH = 7, 15 mL,
colorless), and n-hexane (low-density oil, 15 mL, yellow) was
introduced into the upper glass tube in sequence, which
quickly formed three immiscible oil/water/oil layers. In this
case, the bottom high-density oil rapidly permeated through
the smart fabric, whereas the neutral water and low-density oil
were retained in the upper glass tube (Figure 6b,c) on account
of the superoleophilicity and superhydrophobicity of the fabric
surface. Subsequently, an alkali aqueous solution (pH = 13, 15
mL, purple, dyed with phenolphthalein) was poured into the
upper glass tube (Figure 6d) and immediately mixed with the
bottom water layer. As shown in Figure 6e, after the addition
of the alkali aqueous solution, the color of the obtained
aqueous solution turned into purple, demonstrating an
alkalinity with a pH value of about 12.7. After a few minutes
of in situ alkalizations, the surface wettability of the
superwettable fabric expectedly switched to superhydrophilic-
ity and underwater superoleophobicity, thus leading to the
filtration of the bottom aqueous solution through the fabric
(Figure 6f,g). Ultimately, the light oil was blocked and retained
in the separation device (Figure 6h). In this way, the
continuous separation and collection of all three liquids in
the corresponding oil/water/oil mixture were achieved, which
indicated the great potential of the resulted smart superwet-
table fabric for complete separation of complicated oil/water
mixtures. For a good verification of this proof-of-concept, the
smart superwettable fabric was also utilized to continuously
separate the oil/water/oil mixtures of trichloromethane (high-
density oil, 15 mL, red), aqueous solution (pH = 7, 15 mL,
blue), and cyclo-hexane (low-density oil, 15 mL, yellow), and
as expected, the sequential separation of all three components
in the tested oil/water/oil mixture was realized by in situ
switching the wettability of the smart superwettable fabric-
(Figure S5a−h).
3.5. Continuous in Situ Separation of Oily Waste-
water and Chemical Reaction Systems. Though a variety
of functionalized superwettable membranes have been
developed for oil/water separation, the current applications
of these membranes are still confined to laboratory environ-
ments. The main limitation is that most of these superwettable
membranes can only be utilized to separate oil/water mixture
via the conventional pour and gravity-driven way, and hence a
precollection of the oily wastewater is required.46
However, for
the large scale of oily wastewater, especially the oil spills, the
collection of these oil/water mixtures is cumbersome and
energy-consuming.47
In light of this thorny issue, a novel oil-
recovery device has been designed herein based on the as-
prepared superwettable fabric for continuous in situ
purification of the oily wastewater. As shown in Figure 7, the
oil-recovery device consists of two parts, the filtration system
and the collection system, and the specific separation process
was conducted as follows. First, start the vacuum pump and
adjust the suction pressure-regulating valve to provide a
suitable negative pressure for the entire system. Subsequently,
insert the filter head into the mixture of n-hexane (oil, 150 mL,
red) and water (150 mL, blue) at a tilt angle of about 45°
(Figure 7a,b) to ensure that the superwettable membrane is
fixed at the opening of the filter head and can be exposed to
both oil and water layers. Then, it can be observed that under
the driving of negative pressure, n-hexane in the oil/water
mixture started to selectively and quickly penetrate into the
filtration system and flowed into the collection bottle, whereas
the water phase was repelled because of the superoleophilicity
and superhydrophobicity of the membrane surface (Figure 7c).
This separation process could be conducted continuously and
effectively, and after a while, n-hexane was completely removed
from the water and contributed to the high purification of the
water phase (Figure 7d, see the Movie S1). The continuous in
situ purification of the oily wastewater was desirable for oil spill
cleanup and bulk wastewater treatment.
Generally, the “oil/water separation” process widely exists in
the chemical industry, especially the products separation and
liquid-phase extraction.48
The conventional separating meth-
ods for these “oil/water mixtures” usually involve multiple
procedures, which are always subject to the limitations of long-
time cost, low efficiency, and high energy consumption. In this
respect, the superwettable membranes with special surface
wettability are expected to be applied in the chemical
separation process. However, there are still a few works
referred to the functional membranes that can be useful for the
chemical reaction systems owing to their insufficient durability
for long-term work in harsh environments.39
Herein, in terms
of the superior durability of the as-prepared superwettable
fabric, the in situ separation performance of the functionalized
fabric for chemical reaction systems has been investigated, as
shown in Figure 8. Taking the reaction to produce the tert-
butyl chloride (tert-C4H9Cl) from tert-butyl alcohol (tert-
C4H10O) and high-concentrated hydrochloric acid (HCl) as an
example (Figure 8a), the superwettable fabric with excellent
superhydrophobicity−superoleophilicity was utilized to con-
Figure 7. Continuous in situ removal of the oil phase (n-hexane) from
oily wastewater via the superwettable fabric-based separation device.
(a,b) The filter head with the resulted superhydrophobic−super-
oleophilic fabric as the filtration membrane was inserted into the oil/
water mixture of n-hexane (150 mL, red, dyed with Sudan III) and
water (150 mL, blue, dyed with methyl blue) with a tilt angle about
45°. The inset shows the front view of the filter head. (c,d) Under the
drive of the negative pressure, the oil (n-hexane) quickly permeated
through the filtration membrane and flowed into the collector,
resulting in highly purified water.
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9. tinuously in situ separate the immiscible oily product (tert-
C4H9Cl) from the aqueous reaction system. Moreover, the
contact angles (CAs) of the resulted superwettable fabric
surface to aqueous reactants and oily product were
characterized, which were about 148.5° and 11.5° (Figure
8b), respectively, demonstrating great water repellency and oil
affinity of the fabric.
To achieve the above separation process, a specific in situ
separation device was created based on the superwettable
fabric (Figure 8c), which consisted of four units: continuous
sample introduction system, reactor, separator, and collector,
from the right to the left. It was noteworthy that the functional
superwettable fabric was firmly assembled between the reactor
and the collector by a stainless steel clamp and regarded as the
separator (Figure 8d). After assembling, the separation device
was applied to in situ separation of the above chemical reaction
system, and the concrete separation process is shown in Figure
8d−h. First, tert-C4H10O (10.5 mL, red, dyed by Sudan III)
and HCl (15.0 mL, blue, dyed by Sudan III) were
simultaneously introduced into the reactor by a syringe
pump with an injection speed of 600 mL/h (Figure 8d).
Under magnetic stirring, these two feeds were quickly and
thoroughly mixed and formed a blue mixture (Figure 8e),
indicating that the whole reaction system was acidic, which was
beneficial to increase the nucleophilic substitution reaction rate
between the two reactants. After a while, the reaction occurred,
and an oily product (tert-C4H9Cl) was generated. Meanwhile,
the dyestuff in mixed solution was extracted into the oily
product, exhibiting an orange-red color (Figure 8f). Then, over
time, the resulting oily product (orange-red) gradually floated
to the upper layer of the mixed solution and started to outflow
the superwettable fabric under the drive of gravity (Figure 8g).
When the reaction ended and the stirring stopped, as shown in
Figure 8h, the product was completely removed from the
reaction system and collected in the beaker, just leaving the
transparent aqueous solution in the reactor. Of particular note
was that the above separation process effectively integrated the
two independent procedures (synthetic reaction and product
filtration) together, realizing the successful and continuous in
situ separation of the oily product from the corresponding
chemical reaction system (Movie S2), which was superior to
most of the previously reported research works that only
focused on a simple separation process.15,17,32
Moreover, to
further confirm the purity and structure of the separated
product, the 1
H nuclear magnetic resonance (NMR) and 13
C
NMR spectra of the product have been recorded, as shown in
Figure S6. It can be found that almost no water and other
byproducts were detected in the separated product; the strong
and sharp signal at 1.60 ppm (Figure S6a) and the noticeable
peaks at 34.4 and 66.9 ppm (Figure S6b) were all ascribed to
tert-C4H9Cl, indicating the high purity of the separated
product. To evaluate the durability of the superwettable fabric,
a cyclic in situ separation experiment of the above-mentioned
synthetic reaction systems was performed. After each
separation cycle, the WCA of the fabric surface was measured,
as exhibited in Figure S6c. From the results of the WCA
measurements, it can be found that the superwettable fabric
can maintain its superhydrophobicity even after 20 cycles of
separation tests, showing high durability for the strong acidic
and strong exothermic harsh chemical environments. There-
fore, it was a special wettability-based high-efficiency, novel, in
situ synthetic reaction separation process, in which the
synthetic reaction and product separation could be conducted
concurrently without any interruption, exhibiting promising
potential in the chemical industry field. We anticipate that the
as-prepared superwettable membrane with superior separation
performance can be useful for treating diverse complex
reaction systems to achieve the goal of in situ immiscible
product separation, which will have important significance for
optimizing various operations in the chemical industry.
3.6. Photocatalysis Self-Cleaning Performance. In
recent years, dye wastewater with complex compositions,
high chroma, excessive emission, and toxicity has become one
of the major sources of water pollution and is difficult to be
degraded by organisms. In consideration of the intrinsic
photosensitivity of the dyes, the use of a photocatalyst for
degradation of the organic dyes has been proven to be effective
in dye wastewater treatment and has received extensive
Figure 8. In situ separation of the chemical reaction systems based on the as-prepared superwettable fabric. (a) Chemical equation of the chosen
reaction. (b) CA measurements of the superwettable fabric to the aqueous reactants and oily product of the chemical reaction systems. (c−h) In
situ separation process of the superwettable fabric-based separation device to the corresponding chemical reaction systems. (c) Assembled
separation unit. (d) Continuous injection process of the two reactants [tert-C4H10O (10.5 mL, red, dyed by Sudan III) and HCl (15.0 mL, blue,
dyed by Sudan III)] into the reactor. (d,e) Mixing of the two reactants under magnetic stirring. (f) Generation of the oily product (tert-C4H9Cl,
orange-red). (g) Oily product started to separate from the reaction systems. (h) End of separation and collection of the product.
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10. attention.49
It is well-known that TiO2 is a common
photocatalytic material with advantages of safe, nontoxic,
favorable chemical stability, and high catalytic activity, which
has important application value in photodegradation in the
organic pollution field.50
Therefore, integrating TiO2 with
superhydrophilic separation materials will be a desirable way
for wastewater purification, which can simultaneously remove
oil pollution from water and decompose the organic
contaminants in water. From this base, herein, the photo-
catalytic capability of the alkali-treated superhydrophilic fabric
was evaluated by separately immersing the obtained super-
hydrophilic fabric into methylene blue (MB) aqueous solution
(pH = 7, 30 mg/L) and congo red (CR) aqueous solution (pH
= 7, 20 mg/L), and the corresponding results are exhibited in
Figure 9. It can be observed that with the increase of the UV
irradiation time, the intensities of the characteristic peaks of
both MB aqueous (664 nm) and CR aqueous (497 nm) were
gradually decreased (Figure 9a,c). To further evaluate the
degradation capacity of the superhydrophilic fabric, the
degradation performance of the pristine fabric was also studied
under the same condition. As shown in Figure 9b,d, the
degradation rate of the pristine fabrics and the super-
hydrophilic fabrics for MB aqueous were 18.2 and 97.3%,
respectively (Figure 9b), and for CR aqueous, they were 16.9
and 83.62%, respectively (Figure 9d), demonstrating an
excellent degradation performance of the resulted super-
hydrophilic fabric. Moreover, after being irradiated by UV
light for 80 min, the MB aqueous solution was almost
degraded by the superhydrophilic fabric and became colorless,
indicating the excellent degradation property of the super-
hydrophilic fabric to MB. It was noted that the CR aqueous
solution was lightly orange even after 8 h of UV irradiation,
which was not similar to the results of MB and showed a lower
degradation rate than that of MB. This result demonstrated
that the degradation efficiency of the superhydrophilic fabric
for MB aqueous solution was better than that for CR aqueous
solution owing to the different degradation ability of the
superhydrophilic fabric for the cationic dye and anionic dye. In
addition, the surface morphologies of the superhydrophilic
fabric before and after the UV degradation of MB and CR are
shown in Figure 9e. The original superhydrophilic fabric was
white, meanwhile, plentiful macro-/nanosized TiO2 aggregates
were observed distributed on the microscopic surface of the
fabric. After the UV degradation for MB aqueous solution and
CR aqueous solution, a large number of dyes were adsorbed on
the surfaces of the superhydrophilic fabrics and turned their
surface colors into blue and pink separately. Further
magnifying their surface morphologies, it can be seen that
the superhydrophilic fabric adsorbed with MB presented a
rougher surface, and lots of MB particles agglomerated on the
fabric surface, whereas for the superhydrophilic fabric adsorbed
with CR, its surface roughness was smaller than that of the
MB-adsorbed fabric surface, and the CR granular structures
with uneven grain sizes were sparsely covered on the fabric
surface, which further indicated that the superhydrophilic
fabric had a better absorption capability to MB.
To investigate the different degradation efficiency of the
superhydrophilic fabric for MB aqueous solution and CR
Figure 9. Photocatalysis self-cleaning properties of the alkali-treated superhydrophilic fabric. (a) Absorbance of MB solution with the increase of
the UV degradation time. (b) Degradation rate of the pristine fabric and superhydrophilic fabric to MB solution. (c) Absorbance of CR solution
with the increase of the UV degradation time. (d) Degradation rate of the pristine fabric and superhydrophilic fabric to CR solution. (e)
Photographs and surface morphologies of the superhydrophilic fabrics before and after UV degradation. (f) Schematic illustration of the
photocatalytic degradation mechanism of the obtained superhydrophilic fabric to the aqueous dye solution.
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11. aqueous solution, the zeta potential of the superhydrophilic
fabric was measured, as shown in Table S1. It can be found
that the zeta potential of the superhydrophilic fabric in neutral
(pH = 7) aqueous solution was negative (−99.539 ± 1.508
mV), indicating that the fabric surface has a negative charge,
which might be ascribed to the massive hydroxyl ions
absorbing on the superhydrophilic fabric surface after alkali
treatment. In this case, the superhydrophilic surface would
have a better adsorption capacity to cationic dyes (MB) with a
positive charge than anionic dyes (CR) with a negative charge,
thus contributing to more complete degradation of MB.
In accordance with the above results, the plausible
mechanisms for the degradation property of the super-
hydrophilic fabric can be related to the following factors.51
The most important one is that the TiO2 anatase nanoparticles
have excellent photodegradation property (Figure 9f), which
will produce the photogenerated electron−hole pairs of high
activity after irradiated by UV light. Then, these generated
electron hole pairs will migrate to the surfaces of TiO2 particles
and react with O2 and H2O that were absorbed on the surfaces,
producing a plenty of reactive radicals with a strong oxidizing
activity. Once these reactive radicals contact with organic
matters in the aqueous solution, the oxidation−reduction
between them will take place, which thoroughly decomposes
the organic dyes into H2O, CO2, and other small molecular
substances. Besides, the superior superhydrophilicity of the
superwettable fabric significantly increased the effective contact
area of TiO2 nanoparticles on the fabric surface with water-
soluble organic dyes, which is helpful to promote the
photocatalytic reaction. The brilliant UV degradation perform-
ance and self-cleaning property of the superhydrophilic fabric
can effectively degrade the organic pollutants in wastewater
and simultaneously benefit the regeneration of the correspond-
ing separation property, thus providing a viable method to
achieve the efficient oily wastewater treatment by reducing the
cost of recycling and simplifying the separation process.
3.7. Self-Repairing Property of the Superwettable
Fabric Surface for UV Damage. Despite various superwet-
Figure 10. (a) WCAs of the as-prepared superwettable fabric after the cyclic alternation of UV irradiation and heating treatment. (b,c) Water-
wetting behaviors of the resulted superwettable fabric surface after the cyclic alternation of (b) heating treatment and (c) UV irradiation. Water
droplets (blue) were dyed by MB. (d) XPS spectrum of the superwettable fabric before and after UV irradiation as well as the superwettable fabric
after a cycle of UV irradiation and heating treatment. (e−g) High resolution of the O 1s spectrum of the different superwettable fabric surfaces: (e)
the as-prepared fabric surface before UV irradiation, (f) the as-prepared fabric surface after UV irradiation for 16 h, and (g) the as-prepared fabric
surface after a cycle of UV irradiation and heating treatment.
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12. table materials with special surface wettability having been
developed, the real widespread application of these functional
materials is still unimplemented owing to their poor durability
for long service time in an outdoor environment.52,53
Therefore, it is expected to propose an effective strategy to
enhance the environmental durability of the superwettable
materials. For this purpose, endowing a superwettable material
with favourable self-repairing property has been considered as
an attractive way to achieve its long-term durability in actual
applications.54,55
Herein, the as-prepared superwettable fabric
has been investigated to have a good self-healing property for
UV damage. To be specific, the superhydrophobicity of the
fabric surface would be gradually getting worse with the
increase of the UV irradiation time and switch to super-
hydrophilicity with a WCA about 0° after being irradiated for
16 h (Figure 10c), thus leading to rapid permeation of the
water droplets on the fabric surface. Fortunately, the
superhydrophobicity of the UV-damaged fabric surface could
be easily recovered from 0° to over 150° via a heating
treatment at 120 °C for about 60 min (Figure 10b), and water
droplets could maintain a stable spherical shape on the heat-
treated fabric surface. Moreover, as shown in Figure 10a, the
damage/self-repairing cycle process of the fabric surface
wettability induced by UV irradiation/heating treatment was
able to reversibly repeat for a dozen times, which was
significant for the long-term application of the superwettable
surface.
In order to explore the mechanism responsible for the
damage/self-repairing process of the superwettable surface, the
chemical variations on the obtained fabric surface were
detected by XPS, as exhibited in Figure 10d−g. The typical
peaks of Si, C, Ti, O, and F all could be observed in the XPS
spectra of the superwettable fabric surface under different
conditions (Figure 10d). The high-resolution O 1s spectra of
the three different samples are exhibited in Figure 10e−g. It
can be found that before UV irradiation, the O 1s of the
superwettable fabric surface mainly consisted of Ti−O (530.8
eV) and H−O−H/O−CO (534.1 eV) bondings (Figure
10e), which indicated that a certain amount of water molecules
had been attracted by the residual TiO2 particles on the fabric
surface that were exposed to air. After UV irradiation for 16 h,
the O 1s of the irradiated superwettable surface was primarily
composed of Ti−O (530.8 eV) and Ti−O−H/O−CO
(532.2 eV) (Figure 10f). In response to this result, the possible
reason was that when the TiO2 surface was irradiated by UV
light, lots of electron−hole pairs with high activity were
produced and then migrated to the TiO2 surface and reacted
with the H2O and O2 absorbed on the surface, resulting in
plentiful peroxide intermediates that could further react with
the TiO2 surface to form Ti−O−H bonds.56
Therefore, the
intensity of the H−O−H peak (534.1 eV) was decreased,
whereas that of the Ti−O−H peak at 532.2 eV was obviously
increased, ultimately contributing to the superhydrophilicity of
the fabric surface owing to the great water affinity of the Ti−
O−H groups. Moreover, after the UV-damaged fabric was
heated at 120 °C for about 60 min, it can be found that the
high-resolution O 1s spectra of the obtained fabric surface still
consisted of Ti−O and Ti−O−H bonds (Figure 10g).
However, the intensity of Ti−O−H/O−CO (532.2 eV)
peak was significantly decreased and that of the Ti−O peak
was intensified. This result indicates that the Ti−O−H groups
on the TiO2 particles surface could effectively dehydrate at
high temperature, leading to a reduced concentration of the
Ti−O−H groups on the fabric, and thus the super-
hydrophobicity of the fabric surface was regained.
3.8. Antifouling Property and Mechanical Durability
of the Superwettable Fabric. The artificial superwettable
separation materials are inclined to lose their inherent wetting
properties because of the contamination of oil or organic
pollutants during the separation process. Herein, the
antifouling property of the as-prepared smart superwettable
fabric was tested and discussed (Figure S7a−f). As presented
in Figure S7a, a piece of the resultant superhydrophobic fabric
was stuck to the inclined glass slide, and then the CuSO4·6H2O
particles were utilized as contaminants and evenly sprinkled on
the fabric surface. When dropping the water droplets onto the
polluted fabric surface, the contaminant particles could quickly
dissolve in water and rolled down with water droplets (Figure
S7b), finally resulting in a quite clean fabric surface (Figure
S7c). The favorable anti-contaminants-fouling property was
mainly benefited from the superior water repellency and the
small WCA hysteresis of the superhydrophobic fabric surface.
For an investigation of the anti-oil-fouling property of the
superwettable fabric surface, a piece of alkali-treated super-
hydrophilic fabric was immersed in water (Figure S7d), and
then dichloromethane oil droplets (red) were ejected onto the
fabric surface (Figure S7e). As shown in Figure S7f, the oil jet
contacting with the fabric surface quickly bounced off without
leaving any adhesion trace because of the excellent underwater
superoleophobicity of the alkali-treated fabric surface.
Furthermore, the mechanical stability of the superwettable
surfaces plays an important role in their practical applications,
and hence, the mechanical tests of the resulted fabric have
been carried out as shown in Figure S7g,h. First, an ethanol-
washing test was performed to estimate the launderability of
the superwettable fabric. Briefly, a few pieces of the
superhyrophobic fabrics (2.5 cm × 2.5 cm) were violently
rinsed in ethanol with a magnetic stirring speed of 1000 rpm.
After that, the WCA and the water sliding angle (WSA) of the
water droplet were utilized to assess the hydrophobicity of the
rinsed fabric, and the corresponding result is shown in Figure
S7g. It can be found that with the increase of the rinsing time,
the WCA and WSA of water droplet were slightly fluctuated;
after rinsing for 3 h, the WCA of the superwettable fabric was
still above 150° and the WSA was maintaining lower than 10°,
indicating that the as-prepared fabric possessed favorable
stability. For further testing the mechanical durability of the
obtained fabric, a sandpaper abrasion test that was reported in
a previous literature was conducted.13
Specifically, a piece of
the superhyrophobic fabric with a load of 200 g weight was
pulled to move on the 600-grits sandpaper straightly for 25 cm
one time at a speed of 2.5 cm/s, and after every 50 cm abrasion
length, the WCA and WSA of the abraded fabric surface were
measured. The concrete result is shown in Figure S7h, from
which it can be observed that the WCA of the abraded fabric
surface was slightly decreased and the corresponding WSA was
gently increased along with the increase of the abrasion length
from a whole perspective. Moreover, even after a total wear
length of 10 m, the abrade fabric surface still maintained
superhydrophobicity, and the WCA of the abraded surface
decreased slightly from 158.0 ± 1.1 to 151.5 ± 1.3°;
meanwhile, the SA remained below 10°. These results again
demonstrated that the as-prepared fabric possessed favorable
mechanical durability, which was advantageous to allow it to be
competent for wider application fields. The desirable durability
of the superwettable fabric could be attributed to the robust
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13. surface structures constructed by TiO2 aggregates with
interconnected Ti−O−Ti networks and the stable hydrogen
interaction between fluoropolymer-modified-TiO2 and the
fabric, both of which endowed the obtained superwettable
fabric with superior durability to resist the rigorous mechanical
tests.
4. CONCLUSIONS
In summary, a multifunctional and smart superwettable fabric
with switchable surface wettability has been fabricated through
decorating the fluorine polymer-modified-TiO2 nanoparticles
onto the fabric surface via a facile dip-coating way. The
resulting fabric surface shows brilliant pH responsiveness,
which is able to be reversibly switched between two distinct
durable wettabilities depending on different pH conditions,
and exhibits favorable antifouling properties. By virtue of the
intelligent surface wettability, good mechanical durability, and
self-repairing property, the as-prepared superwettable fabric
can be confidently exploited to separate various oil/water
mixtures, especially the complicated oil/water/oil ternary
mixtures, exhibiting an excellent separation efficiency and
high filtration flux even under extreme pH situations, which is
comparable to most of the currently reported functionalized
membranes. With the merits of the superior and durable
separation performance, the superwettable fabric-based,
continuous, high-speed, and in situ removal of the oil phase
from the bulk of oily wastewater is achieved effectively under
the drive of negative pressure. It is worth noting that the
obtained superwettable fabric possesses favorable UV degra-
dation performance for multiple water-soluble organic
pollutants, which is mainly ascribed to the joint effect of
photocatalysis of anatase TiO2 and superhydrophilicity of the
functionalized fabric. More importantly, the continuous in situ
separation of chemical reaction systems is well performed with
the superwettable fabric as the filtration membrane, demon-
strating outstanding advantages of simplifying procedures,
saving operation time, and increasing product yield. We
anticipate that the smart superwettable fabric with unique
advantages can be competent in multifarious relevant
challenging settings and have a broad prospect for diverse
practical applications, especially the oily wastewater treatment
and multiple industrial operation optimizations.
■ ASSOCIATED CONTENT
*S Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acsami.9b15952.
More experimental details and instrumentations; mech-
anism diagram for the fabrication of the pH-responsive
smart superwettable fabric; time-dependence WCA
measurements of the as-prepared superwettable fabrics
treated by aqueous solutions of different pH values; the
CAs of the pH 1 acidic-treated superwettable fabrics
toward daily use liquids and underwater OCA of the pH
13 alkali-treated superwettable fabrics toward different
oils; “water removing” type oil/water separation
performance of the alkali-treated superwettable fabric;
superwettable fabric-based continuous separation proc-
ess of the oil/water/oil (trichloromethane/water/n-
hexane) ternary mixture; 1
H NMR and 13
C NMR
spectra of the separated oily product (tert-C4H9Cl) and
WCA measurements for the evaluation of the durability
of the superwettable fabric in the cyclic in situ separation
of chemical reaction systems; and anti-fouling behaviors
and mechanical durability tests of the superwettable
fabric (PDF)
Negative pressure-driven, continuous, and high-speed
removal process of the oil phase (n-hexane) from the
bulk of oily wastewater via the superwettable fabric-
based specially designed oil-recovery device (MP4)
In situ continuous separation process of the superwet-
table fabric-based separation device to chemical reaction
systems (MP4)
■ AUTHOR INFORMATION
Corresponding Authors
*E-mail: jinmhe@gmail.com (J.H.).
*E-mail: mnanqu@gmail.com (M.Q.).
ORCID
Yuangang Li: 0000-0002-9147-9824
Mengnan Qu: 0000-0002-0684-4162
Notes
The authors declare no competing financial interest.
■ ACKNOWLEDGMENTS
The authors gratefully acknowledge the financial support of the
National Natural Science Foundation of China (grant no.
21473132), the Youth Innovation Team of Shaanxi Uni-
versities, the Shaanxi Provincial Science and Technology
Department (grant no. 2019JM-371), the Outstanding Youth
Science Fund of Xi’an University of Science and Technology
(grant no. 2019YQ2-09), and Huyang Scholar Program of
Xi’an University of Science and Technology.
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