Preparation of Superwetting Porous Materials for Ultrafast Separation

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Preparation of Superwetting Porous Materials for Ultrafast Separation of Water-in-Oil Emulsions Chih-Feng Wang, and Liang-Ting Chen Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b04344 • Publication Date (Web): 01 Feb 2017 Downloaded from http://pubs.acs.org on February 3, 2017

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Preparation of Superwetting Porous Materials for Ultrafast Separation of Water-in-Oil Emulsions

Chih-Feng Wang*, Liang-Ting Chen

Department of Materials Science and Engineering, I-Shou University, Kaohsiung, 840, Taiwan.

*To whom all correspondence should be addressed E-mail: [email protected] Tel: 886-7-6577711-3129 Fax: 886-7-6578444

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Table of contents

Preparation of Superwetting Porous Materials for Ultrafast Separation of Water-in-Oil Emulsions Chih-Feng Wang*, Liang-Ting Chen Department of Materials Science and Engineering, I-Shou University, Kaohsiung, 840, Taiwan.


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Abstract Functional materials with a superwetting surface property have been extensively explored to achieve emulsions separation. In this paper, we report a simple and an inexpensive method for

fabricating superhydrophobic/superoleophilic

porous

materials from polymeric sponges. The microstructured porous materials, which do not contain any fluorinated compounds, maintain their superhydrophobicity and superoleophilicity

after

long-term

organic

solvent

immersion

and

display

environmental stability. These superhydrophobic porous materials could effective separate a wide range of water-in-oil emulsions including surfactant-free and -stabilized water-in-oil emulsions with high efficiency (>99.98%) and high flux (up to 155,000 L m–2 h–1 bar–1). Meanwhile, this superhydrophobic porous material exhibited excellent pH-resistance and antifouling properties. The high performance of our superhydrophobic porous materials and their efficient, low-energy, cost-effective preparation suggest that they have great potential for practical application.

Key words: superhydrophobic, superoleophilic, porous materials, polymer, emulsion separation


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Introduction Nowadays, tremendous threats both on the environment and on the human health were caused by large quantities of oily wastewater expelled from daily life and industrial processes. With the expansion of oil production and transportation, there is increasing potential for oil spills from industrial accidents or the sinking of oil tankers or ships.1,

2

The International Tanker Owners Pollution Federation (ITOPE) has

reported that approximately 5.72 million tons of oil was lost as a result of over 1800 large oil tanker accidents from 1970 to 2015. In addition, oily wastewaters containing emulsified oil/water mixtures can cause severe environmental and ecological problems, and can also threaten human life. Accordingly, there remains a need to develop new materials for the separation of immiscible oil/water mixtures and oil/water emulsions. Recently, sponge-like porous materials possessing both superhydrophobic and superoleophilic properties have become attractive for their capacity to efficiently separate oil/water mixtures.3-20 Wang et al. described microfibrillated cellulose fiber sponges possessing superhydrophobicity and superoleophilicity for the efficient separation of oil from water.3 Gao and coworkers prepared superhydrophobic and superoleophilic carbon soot sponges that absorb a broad range of organic solvents efficiently and with high selectivity.4 Wang et al. employed

a

thermally-induced

phase

separation


technique

to

prepare

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superhydrophobic polypropylene sponges, which were applicable for water/oil separation.5 Although most of these materials can facilitate immiscible oil/water separation, they are not effective at removing water droplets of small diameter from water-in-oil emulsions, especially for surfactant-stabilized emulsions having droplet sizes of less than 20 µm. Therefore, the need remains to develop efficient, cost-effective, and mass-producible materials for the separations of surfactant free and stabilized emulsions. In this paper, we present a facile two step method for the fabrication of superhydrophobic and superoleophilic porous materials. First, the melamine sponge was compressed into a compact form. Afterward we anchored hydrophobic polymer coatings onto the frames of the sponges to change their wettability from hydrophilic to superhydrophobic. The as-prepared superhydrophobic porous materials possessed excellent repellency not only toward pure water but also toward corrosive aqueous liquids. Interestingly, we found that the superhydrophobic porous materials could separate both surfactant-free and -stabilized water-in-oil emulsions with fluxes of up to 155,000 L m–2 h–1 bar–1—two to three orders of magnitude higher than that of commercial filtration membranes—and with high separation efficiencies (>99.98 wt% in terms of oil purity in the filtrate). The excellent performance of our superhydrophobic porous materials in water-in-oil emulsions separation and their


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simple preparation through an industrially feasible process suggest that they have great potential applicability in both academic and industrial settings.

Experimental Section Materials: Commercial melamine sponges were purchased from BASF. Span 80 was obtained from Acros. SE 1700, a hydrophobic adhesive, was supplied by Dow Corning. Superhydrophobic porous material: The superhydrophobic porous material was prepared using a two step method. First, the melamine sponge was compressed into a compact form having a volume approximately one-tenth that of the pristine sample. The compressed melamine sponge (thickness: 48mm) was immersed into the hydrophobic polymer solution (containing 2.50 g SE 1700, 0.25 g curing agent, and 50 mL EtOAc) and then cured in an oven (80 °C, 16 h). Water-in-Oil Emulsions: Surfactant-free water-in-oil emulsions were prepared by mixing water with an oil (n-hexane, n-hexadecane, n-octane, isooctane, or toluene; 1:9, v:v) and then sonicating for 1 h to produce a white solution. To prepare surfactant-stabilized water-in-oil emulsions, span 80 (0.08 g) was dissolved in oil (n-hexane, n-hexadecane, n-octane, or isooctane; 200 mL), and water (2.0 mL) was added, and then the mixture was stirred for 3 h. For surfactant-stabilized


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water-in-toluene emulsions, span 80 (1.30 g) was added into toluene (200 mL), then water (2.0 mL) also added. The mixture was stirred for 3 h. Instruments and Characterization: The microstructure of the superhydrophobic melamine sponge was characterized using a HITACHI S-4700 scanning electron microscope (acceleration voltage: 15.0 kV). Static contact angles and sliding angles of droplets (5 µL) were measured using an FDSA MagicDroplet-100 contact angle goniometer; each reported contact angle represents the average of six measurements. The water contents in the original emulsions and corresponding collected filtrates were determined using a MKC-500 Coulometric Karl Fischer moisture titrator. Optical microscopy images were recorded using an Olympus BX51M instrument after placing a drop of an emulsion solution onto a biological counting board.


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Results and Discussion Generally, the wettability of a solid surface is controlled by its topographical microstructure and surface chemical composition. Combining hydrophobic polymer coatings with the rough surfaces of compressed melamine sponges provided us with the desired superhydrophobic porous materials. The as-prepared superhydrophobic porous materials in Figure 1a possessed a high water contact angle (160°) and superoleophilicity (contact angles of n-hexane, n-octane, n-hexadecane, isooctane, and toluene: all close to 0°). On this superhydrophobic surface, water droplets possessed near-spherical shapes and rolled off easily. Figures 1b and 1c present top-view scanning electron microscopy (SEM) images of the sponge before and after modification, respectively. Compared with the pristine sponge (Figure 1b), the surface morphology of the modified sponge changed such that the diameters of the pores decreased and the skeleton of the sponge was packed more compactly. Beside, the smooth skeleton of the original sponge was covered with the hydrophobic polymer after the modification. The higher-magnification image (Figure 1c) of the hydrophobic polymer coating reveals hierarchical structures that existed in the form of crater-like nanostructures. Such a morphology raised the surface roughness dramatically and provided a composite interface in which air became trapped within the grooves beneath the liquid, thereby inducing superhydrophobicity. Typically,


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superhydrophobicity is lost under harsh environments, such as those containing corrosive acids, bases, and organic solvents. We found that our as-prepared superhydrophobic porous material possessed excellent repellency toward corrosive aqueous liquids, including acidic (1.0 M HCl) and basic (1.0 M NaOH) (Figure 1a). Droplets of these solutions stood spherically on the superhydrophobic sponge (contact angles of acidic, alkali, and salt solutions: all greater than 150°) and rolled off readily. Moreover, the superhydrophobic porous material was very stable against long-term (150 h) immersion in organic solvents. Samples were removed every six hours and dried prior to performing water contact angle measurements and sliding angle tests. Figure 2 reveals the negligible effect of long-term immersion in the organic solvent on the superhydrophobicity of the porous material. Wastewater containing emulsified oil/water mixtures is a major environmental issue affecting a range of industries. Direct discharge of such wastewater is extremely harmful, not only to the environment but also to human health. Therefore, there remains a need to develop efficient, low-energy, and cost-effective processes to meet the stringent standards of emulsion separation. Interestingly, we found that our superhydrophobic porous material could be used for the successful separation of emulsified oil/water. To test the separation ability, we passed a series of water-in-oil emulsions, including surfactant-free and -stabilized emulsions, through the


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superhydrophobic porous material under a suction vacuum pressure of 10 kPa. The oils immediately absorbed and permeated through the sponge, causing the emulsion droplets to demulsify and leaving behind the water, similar to previous reports.21

All

of the emulsified oil/water mixtures were well separated and collected through a single step. Figure 3a presents the results of separating the surfactant-free water-in-isooctane emulsion through the superhydrophobic porous material. The collected filtrate was transparent, whereas the original feed emulsion was milky white. To confirm the effective separation, we used optical microscopy to observe the droplets in the feed and in the collected filtrate. We did not observe any droplets in the collected filtrate over the whole view, implying that the water had been removed from the surfactant-free water-in-isooctane emulsion. The fluxes of the n-hexane, n-hexadecane, n-octane, isooctane, and toluene emulsions permeating though the superhydrophobic porous material were surprisingly high: 156,700, 27,630, 137,800, 138,100, and 136,600 L m–2 h–1 bar–1, respectively (Figure 4a). The superhydrophobic porous material also displayed high efficiency when separating surfactant-stabilized water-in-oil

emulsions.

Figure

3b

reveals

that

the

components

of

a

surfactant-stabilized water-in-oil emulsion could be separated well and collected in a single step. Similar to the results obtained when separating surfactant-free water-in-oil emulsions, we observed no oil droplets in the image of the filtrate, confirming that the


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superhydrophobic porous material was effective for the separation of the surfactant-stabilized water-in-oil emulsion. The fluxes of all surfactant-stabilized water-in-oil emulsion decreased slightly (155,400, 131,200, 130,900, and 124,400 L m−2 h−1 bar emulsions,

−1

for n-hexane, n-octane, isooctane, and toluene surfactant-stabilized

respectively),

except

for

that

of

the

surfactant-stabilized

water–in–n-hexadecane emulsion (9715 L m–2 h–1 bar–1) (Figure 3a). The fluxes of all emulsions are extremely high compared to those of commercial filtration membranes. As summarized in Figure 4b, the oil purities of all the separated emulsions were greater than 99.98%, with some even reaching up to 99.99 wt%, indicating extremely high separation efficiency. The purities of the original oils used in the experiments were also tested (Table 1); the purities of filtrates nearly matched those of the pure reagents. Recently, some pioneer researchers have successfully developed superwetting materials for fast separation of water-in-oil emulsions (Table 2).21-33 Shi and coworkers developed ultrathin free-standing single-walled carbon nanotube (SWCNT) network films displaying hydrophobicity and superoleophilicity for the ultrafast separation of emulsified oil/water mixtures.21 Zhang et al. used an inert solvent-induced

phase-inversion

process

to

prepare

superhydrophobic

and

superoleophilic poly(vinylidene fluoride) (PVDF) membranes that were applicable in the separations of both a surfactant-free emulsion and a surfactant-stabilized


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water-in-oil

emulsions.22

Recently,

Gu

and

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co-workers,

who

synthesized

superhydrophobic polymer/CNTs hybrid membranes for highly effective separation of various surfactant-stabilized water-in-oil emulsions.23 Chu et al. reported that Silicone nanofilaments

coated

porous

glass

materials

can

effectively

separate

surfactant-stabilized water-in-oil emulsions with high flux and high separation efficiency.30 Compared with many other kinds of special wettable materials, our superhydrophobic porous material possess extremely high fluxes and display outstanding separation efficiencies during separations of water-in-oil emulsions. (Table 2) The method we described herein possesses the advantages of being simple and inexpensive while being able to utilize non-fluorine-containing compounds. Oil fouling during oil/water separation processes is a common and tough issue for many filtration materials. An ideal material for oil/water separation should have good antifouling properties. We tested the antifouling performance of our superhydrophobic porous material by performing a cyclic experiment for the treatment of the surfactant-stabilized water-in-isooctane emulsion. For each cycle, 200 mL of the surfactant-stabilized emulsion was permeated through the sponge and then the sponge was simply washed with acetone. The variation of the flux during this process was also tested. Figure 5 reveals that the flux did not decrease obviously upon increasing the emulsion volume that permeated through the sponge within one cycle,


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and it could be returned completely to the starting permeation flux after cleaning. The oil purity in every cycle remained greater than 99.97 wt%; thus, the separation efficiency was not sacrificed during these cycles. These results reveal the excellent antifouling properties of the superhydrophobic porous material during long-term use in the treatment of water-in-oil emulsions.

Conclusions Separating water from emulsified oil/water mixtures has become a worldwide subject that is tough and challenging. The mixing of water and oil is a common problem in the petroleum industry. Herein, we have developed a facile, inexpensive method for the fabrication of microstructured porous materials exhibiting superhydrophobicity and superoleophilicity; these materials have practical use in the ultrafast separation of surfactant-free and -stabilized water-in-oil emulsions. The superhydrophobic porous materials could separate surfactant-free and -stabilized water-in-oil emulsions with ultrahigh permeation fluxes of up to 156,700 and 155,400 L m–2 h–1 bar–1, respectively. The oil purities of all the separated emulsions were greater than 99.98% indicating extremely high separation efficiency. Meanwhile, this superhydrophobic porous material exhibited excellent antifouling properties. We believe that our special wettable materials are promising candidates for practical use


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in the treatment of wastewater produced industrially and in daily life, providing high-quality water as a result.

Acknowledgment This study was supported financially by the Ministry of Science and Technology, Taiwan, Republic of China, under contract MOST 104-2221-E-214 -048 -MY2.

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Figure Captions Figure 1.

(a) Photograph of water and oil droplets on the superhydrophobic porous

material. (b, c) SEM images of (b) the pristine sponge and (c) the superhydrophobic porous material.

Figure 2. Variations of water contact angle on the superhydrophobic porous material with immersion time in isooctane at ambient temperature.

Figure 3. Photographs of (a) surfactant-free and (b) -stabilized water-in-isooctane emulsions before and after separation using a superhydrophobic porous material.


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Figure 4. (a) Fluxes and (b) oil purities of filtrates after separating various surfactant-free and -stabilized water-in-oil emulsions.

Figure 5. Real-time monitoring of the separation flux and oil purity in the filtrate during the cycles of a surfactant-stabilized water-in-isooctane emulsion separation test using a superhydrophobic porous material.


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Langmuir

Table 1. The purities of the original oils used in the experiments

Table

2.

Oils

Oil purity (wt %)

n-hexane

99.99

n-hexadecane n-octane

99.99 99.99

isooctane toluene

99.99 99.98

Comparison

of

various

special

wettable

materials

used

for

surfactant-stabilized water-in-oil emulsion separation. Materials

Flux of surfactant stabilized Oil purity Ref. water-in-oil emulsions (wt %)

Single-walled carbon nanotube (SWCNT) Up to 17000 L m−2 h−1 bar −1 network films Poly(vinylidene fluoride) (PVDF) Up to 1000 L m−2 h−1 membrane Polystyrene/CNT hybrid membranes SiO2 nanoparticles coated membranes Janus polymer/CNT hybrid membranes

Up to 7500 L m−2 h−1 bar −1 none Up to 9000 L m−2 h−1 bar −1 coated Up to 16810 L m−2 h−1 bar −1

Fluorinated silica nanoparticles papers Polydivinylbenzene (PDVB) coated PVDF Up to 1500 L m−2 h−1 bar −1 membrane SWCNT based bilayer membrane Perfluorodecyltriethoxysilane/CNT

Up to 54000 L m−2 h−1 bar −1 hybrid Up to 42500 L m−2 h−1 bar −1

membrane Silicone nanofilaments coated porous glass Up to 120000 L m−2 h−1 bar −1 substrates Hydrophobic polymer coated melamine Up to 155000 L m−2 h−1 bar −1 sponge


> 99.95

21

> 99.95

22

> 99.94 > 99.96 > 99.98 > 99.9

23 24 25 26

> 99.98

27

> 99.95 > 99.98

28 29

> 99.98

30

> 99.98

This work

Langmuir

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Figure 1


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Langmuir

Figure 2


Langmuir

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Figure 3


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Langmuir

Figure 4


Langmuir

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Figure 5


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Preparation of Superwetting Porous Materials for Ultrafast Separation

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