Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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A Nanometer-Thick, Mechanically Robust, and Easy-to-Fabricate Simultaneously Oleophobic/Hydrophilic Polymer Coating for Oil− Water Separation Yongjin Wang,† Christopher You,† Cliff Kowall,‡ and Lei Li*,† †
Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States Lubrizol Corporation, 29400 Lakeland Boulevard, Wickliffe, Ohio 44092, United States
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ABSTRACT: The simultaneously oleophobic/hydrophilic membranes are highly desirable in oil−water separation since both gravity and wettability promote the separation, which renders the separation process energy-efficient. Although simultaneously oleophobic/hydrophilic coatings have been reported before, the fabrication process is complicated and often involves chemical synthesis and multiple steps. Moreover, the tribological performance, which is critical to long-term application, has been rarely investigated. In the current paper, we report a nanometerthick simultaneously oleophobic/hydrophilic polymer coating for oil−water separation. The coating is fabricated using a commerically available perfluoropolyether (PFPE) via a single-step dip-coating process. The tribology testing results suggest that the PFPE coating is mechanically robust. The contact angle and oil−water separation tests indicate that such coating is efficient in oil−water separation. The finding here has important implications in real-life application of simultaneouly oleophobic/hydrophilic coatings in oil−water separation.
1. INTRODUCTION Currently, oily wastewater is being produced in large quantity in several industries, including gas, oil, chemical, and pharmaceutical.1,2 The treatment of oily wastewater is a serious challenge since it has a massively negative impact on agriculture and aquatic environments.1,2 Meanwhile, with the growth of the population, there is an urgent need for more clean water. Clearly, the successful purification of the oily wastewater will address the above-mentioned two issues simultaneously.1,2 Water−oil separation is the key step in wastewater treatment. Conventional approaches used for oil− water separation include gravity separation and skimming, flocculation, de-emulsification, dissolved air flotation, and coagulation.1 However, the above-mentioned techniques are not efficient enough and cannot meet the increasing requirements.1 Membrane technology has been shown to be a promising solution for a more energy-effective, cost-effective, and efficient method for water−oil separation. In separation, the membrane serves as a semipermeable “filter” between two immiscible liquids, i.e., oil and water, which controls the movement of the two liquids. The advantages of membrane separation include high oil-removing efficiency, straightforward operational processes, and low energy requirements. Polymeric materials, such as polysulfone, poly(vinylidene fluoride), and polyacrylonitrile, are frequently utilized as the membrane material due to their low cost and ready availability. However, oil easily adsorbs on the polymer membrane surface, which is usually referred to as “fouling”.1 Once fouling occurs, the oil− © XXXX American Chemical Society
water separation will no longer be effective. To solve this problem, the approach has been taken to use hydrophilic polymer additives to the membrane to decrease the amount of oil adsorbing onto the membrane. However, even hydrophilic membranes, e.g., glass membranes, like oil better than water since the surface tension of water is higher than that of oil.3 In the long run, fouling still occurs.3 Recently, simultaneously hydrophilic/oleophobic coatings have been developed to mitigate the fouling issue.4 Such material likes water and hates oil. As a result, oily moiety does not adsorb on the membrane surface, and therefore fouling is prevented. This emerging technique also enables gravity/ capillary-driven oil−water separation, which does not require external pressure and is energy-efficient. Though the underlying mechanism of the simultaneous hydrophilicity/oleophobicity is still under debate, the efficient water−oil separation has been successfully demonstrated by several groups. Howarter and Youngblood5 covalently attached poly(ethylene glycol) with fluorinated end groups to glass membranes and demonstrated that the simultaneously oleophobic/hydrophilic membrane separates water and hexadecane efficiently. Xue et al.6 fabricated a simultaneously superhydrophilic/(underwater) superoleophobic hydrogel-coated mesh and showed that the Received: Revised: Accepted: Published: A
August 23, 2018 October 25, 2018 October 29, 2018 October 29, 2018 DOI: 10.1021/acs.iecr.8b04071 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
which was purchased from Silicon Quest International, Inc., and 25 × 7 × 51 mm plain glass microscope slides obtained from Fisher Scientific. Ztetraol was applied on the substrate via a simple dip-coating process using a 0.8 g/L Ztetraol/2,3dihydrodecafluoropentane solution. The detailed dip-coating procedure was described elsewhere.10 2.2. Characterizations. 2.2.1. Contact Angle Testing. The contact angle tests were conducted with a VCA optima XE (AST Production Inc.) system in the ambient lab environment. Two probe liquids were utilized here: (1) deionized (DI) water, which is produced from a Millipore Academic A10 system (total organic carbon lower than 40 ppb) and (2) hexadecane (anhydrous, ≥99%), which is obtained from Sigma-Aldrich. In a static contact angle test, the image of the liquid−substrate interface was taken right after a sessile drop (∼2 μL) was placed on the sample. All the reported static contact angles are the mean of at least three repeats at different locations of the same sample. 2.2.2. Ellipsometry. The thickness of Ztetraol on the substrate was determined with a J.A. Wollam alpha-SETM Ellipsometer. In each measurement, the uncoated silica was first characterized to determine the accurate thickness of the native oxide. Afterward, the thickness of the Ztetraol nanofilm was determined by curve-fitting using the Cauchy model, with the refractive constants of bulk Ztetraol as described in details in our previous report.10 2.2.3. Coefficient of Friction (COF). Friction measurements were conducted with a CSM Instruments NanoTribometer (NTR2), which is placed on a Kinetic Systems antivibration platform. The NTR2 utilizes a dual-beam cantilever and highresolution capacitive sensors, which enable accurate microscale measurements. In all the friction experiments, a stainless steel ball, with a diameter of 2 mm and roughness (Ra) of 50 nm, was used as conterface. Between tests, the ball was carefully cleaned using isopropyl alcohol. All of the friction tests were conducted, with relative humidity ranging from 30 to 50% during the tests, at room temperature. In all of the tests, the sample was fixed on a glass slide using double-side tape. The stainless steel ball was contacted with the test sample at a desired normal load, and, in each test, the sample reciprocated for a desired number of cycles at a linear velocity of 0.20 cm/s. 2.2.4. Oil/Water Separation. In a simple separation setup, the glass membrane was placed between two funnels as shown in Figure 5. A mixture of 20 mL of water and 20 mL of hexadecane was added through the top funnel; at certain time intervals, the initial and final passings of oil and water were observed. The times at which oil and water start and end passings were recorded. The time intervals in which the passing of oil or water was observed were instant after pouring the mixture, at 5, 10, 20, and 30 min. Regardless of whether all the liquids pass the membrane, the experiment ended after 30 min.
mesh efficiently (>99%) separates water from various water− oil mixtures, including gasoline, vegetable oil, diesel, and crude oil, with no external power. Yang et al.7 synthesized the simultaneously oleophobic/hydrophilic polymer by reacting poly(diallyldimethylammonium chloride) (PDDA) with sodium perfluorooctanoate (PFO) and then fabricated simultaneously superoleophobic/superhydrophilic nanocomposite coatings by spray casting polymer−nanoparticle suspensions on the desired substrate. They successfully demonstrated the oil−water separation using such coatings applied on the stainless steel mesh.7 Kota et al.4 fabricated the superoleophobic/superhydrophilic membrane by dip-coating the polyester fabric or stainless steel mesh with the mixture of UV cross-linked poly(ethylene glycol) diacrylate (x-PEGDA) and fluorodecyl polyhedral oligomeric silsesquioxane (POSS). They4 have shown that such membranes can separate various water−oil mixtures efficiently (>99.9% separation efficiency). Brown et al.8 synthesized oleophobic/hydrophilic polyelectrolyte−surfactant complex by reacting maleic anhydride-based copolymers with cationic fluorosurfactant and achieved >98% oil−water separation efficiency, using the complex-coated stainless steel mesh. Brown and Bhushan9 fabricated a simultaneously oleophobic/hydrophilic coating with SiO2 nanoparticles and a polyelectrolyte−fluorosurfactant complex. They took advantage of a layer-by-layer technique to ensure high adhesion with the substrate and good coverage of the fluorosurfactant, which results in the improved mechanical durability. They9 also demonstrated that a stainless steel mesh with such coating efficiently separates oil and water. Although the water−oil separation has been successfully demonstrated with the membranes coated with simultaneouly hydrophilic/oleophobic materials, the fabrication process is complicated and often involves chemical synthesis and multistep processing. Moreover, the tribological performance, which is critical to long-term applications, has been rarely investigated.9 Previously, we have demosntrated that the nanometer-thick simultaneously hydrophilic/oleophobic coatings can be fabricated with a simple one-step dip-coating process using commercially available perfluoropolyethers (PFPEs). In the current paper, we report that the membrane with such coating, i.e., commercially known as Ztetraol, is effective in oil−water separation. Moreover, such coatings have excellent tribological performance. The findings here have important implications in large-scale application of simultaneouly hydrophilic/oleophobic materials in oil−water separation.
2. EXPERIMENTAL SECTION 2.1. Materials and Sample Fabrication. The perfluoropolyether (PFPE) polymer with four hydroxyl end groups, commercially referred to as Ztetraol (Mn = 2000 g/mol), was purchased from Solvay Solexis Inc. The chemical structure of Ztetraol polymer is shown below:2,3-Dihydrodecafluoropen-
3. RESULTS AND DISCUSSION 3.1. Friction. The thickness of Ztetraol is ∼2 nm as determined by ellipsometry. The coefficient of friction (COF) of the Ztetraol/silicon sample, with respect to the testing time at various normal loads, is shown in Figure 1. The normal load ranges from 1 to 50 mN in the tribology tests. At all loads, COFs are ∼0.12−0.15, which is significantly lower than that of bare silicon wafer, i.e., ∼ 0.7.11 The results indicate that Ztetraol serves as an excellent lubricant on the silica surface. Our finding here is in line with previous reports12 on the good
tane, obtained from Miller Stephenson Chemical Co., was utilized as the solvent for Ztetraol polymer.10 The abovementioned chemicals were used as received. The solid substrates utilized include silica (silicon wafer with 2 nm native oxide; P/B ⟨100⟩ 1−10 OHM-CM, 279 ± 25 μm), B
DOI: 10.1021/acs.iecr.8b04071 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
Ztetraol nanofilm while large oil molecules do not. Therefore, the HCA is higher than the WCA on the surface.3,16,17 Interestingly, both WCA and HCA are 0° for the bare glass membrane. Indeed, both water and hexadecane are “adsorbed” by the membrane instantaneously after the liquid droplet is in contact with the membrane. This can be rationalized by the porous structure of the glass membrane. When a liquid is in contact with a porous membrane, the membrane can be simplified as a series of cylinder tubes. Here the key question is how easily the liquid enters the tube. As schematically shown in Figure 2, assuming no external pressure is applied, the physics can be described by the Laplace−Young equation. driving force = ρgh + 2γ cos θ/r
(1)
Figure 1. Coefficient of friction (COF) of 2 nm Ztetraol on a silicon wafer.
tribology performance of nanometer-thick Ztetraol. Moreover, during the tribology tests, the COFs are stable and there is no sign of the increase in COF, which indicates that there is no loss of Ztetraol during the tribology contact. Since it is unlikely to have continuous solid−solid contact under high normal load in water−oil separation using a membrane, our tribology testing results, though conducted in a brief time period, suggest that Ztetraol is mechanically durable for long-term application as a membrane coating in oil−water separation. This idea is also supported by the fact that Ztetraol is being utilized as the nanometer-thick media lubricant in hard disk drive industry.1314 3.2. Simultaneous Oleophobicity/Hydrophilicity. The static hexadecane contact angle (HCA) and water contact angle (WCA) for glass slides and glass membranes, with and without Ztetraol coatings, are shown in Table 1. The WCA of a
Figure 2. Schematic show of a liquid entering a cylinder tube. (Driving force is described in eq 1.)
Here, both gravity (ρgh) and Laplace pressure (PL = 2γ cos θ/r) play a role. The liquid always passes the tube as long as it wets the tube surface, i.e., θ < 90°, because both gravity and Laplace pressure drive the liquid to go through the tube. Since both WCA and HCA on the flat glass slide are low, i.e., glass likes both water and hexadecane, water as well as hexadecane enter the tube easily. As a result, both WCA and HCA are 0°. However, when the liquid does not wet the surface of the tube, i.e., θ > 90°, the Laplace pressure and gravity are in the opposite direction. As a result, it is possible that the liquid cannot enter the tube, depending on the liquid (density, surface tension, height) and the size of the tube. Based on the above discussion, it seems that the liquid will definitely enter the tube when the contact angle is lower than 90°. As a result, it is expected that HCA of Zetraol/glass membrane is ∼0° since the HCA of Ztetraol/glass slide is ∼70.6 ± 0.3°. However, the HCA of Ztetraol/glass membrane is ∼93.2 ± 12.4°. Why? Previously, Cao et al.18 and Tuteja et al.19 showed that the topography of the solid surface could have a significant impact and change this conclusion. As illustrated in Figure 3, when the surface has a “re-entry” feature, the sign of the Laplace pressure could change so that the gravity and the Laplace pressure are in the opposite direction. As a result, it is possible that the liquid cannot enter the tube even if it wets the solid surface, i.e., θ < 90°. Here, the key is that the re-entry angle must be lower than the liquid contact angle on the flat solid surface (see Figure 3.) If the liquid cannot enter the tube, air will be trapped at the liquid− solid interface, and the liquid is in contact with a composite surface of the solid and air. Since the liquid contact angle on air
Table 1. Static C03ontact Angles on the PFPE/Glass Samples glass slide Ztetraol/glass slide glass membrane Ztetraol/glass membrane
WCA (deg)
HCA (deg)
4.0 ± 1.1 34.5 ± 0.9 0 0
30.3 ± 2.1 70.6 ± 0.3 0 93.2 ± 12.4
bare glass slide is only ∼4.0 ± 1.1°, and the HCA is ∼30.3 ± 2.2°. The results can be explained by the fact that, under ambient conditions, water adsorbs on the glass surface so that HCA is significantly higher than WCA. Indeed, it has been reported previously that the WCA of the glass surface with adsorbed water is ∼0° while the oil contact angle on the same glass surface is ∼37°.15 For the Ztetraol/Glass slide sample, the WCA is ∼34.5 ± 0.9°, indicating the coated surface is still hydrophilic. Interestingly, the HCA of Ztetraol/Glass slide is ∼70.6 ± 0.3° and much higher than the WCA, which shows that the coated sample surface likes water more than oil.16 The detailed discussion on the underlying mechanisms can be found elsewhere.3,16,17 In brief, the hydroxyl end group of the Ztetraol in the nanofilm is the key to the desired oleophobicity/hydrophilicity. Ztetraol polymer, which has four hydroxyl end groups, interacts with the polar groups on the glass substrate possibly via hydrogen bonding and results in ordered packing. Therefore, the between-chain distance is appropriately small so that small water molecules penetrate the C
DOI: 10.1021/acs.iecr.8b04071 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research
Figure 3. Effect of the re-entry surface.18,19
Figure 5. Oil−water separation with glass membrane (left) and Ztetraol/glass membrane (right). (Water is dyed blue and hexadecane is dyed red.)
is ∼180°, the liquid contact angle on the re-entry surface should be larger than that on the flat surface according to Cassie’s law.20 To understand whether or not the re-entry topography exists in our glass membrane and thus impacts the contact angle, SEM characterization was conducted. As shown in Figure 4,
instantly while hexadecane passes the membrane very slowly as shown in Figure 5b. The results suggest that the Ztetraol coating is effective in oil−water separation. The oil−water separation results correlate well with the wettability of the glass membrane as shown in Table 1. The bare glass membrane is oleophilic/hydrophilic, and it is not efficient in oil−water separation. The Ztetraol/glass membrane is simultaneously oleophobic/hydrophilic, and it is efficient in oil−water separation, indicating that the simultaneous oleophobicity/ hydrophilicity is the key. The detailed water/oil pass time is shown in Table 2, in which the results of three repeats of both bare and ZtetraolTable 2. Oil/Water Separation Resultsa glass membrane
Ztetraol/glass membrane
instant
5 min
W W W W W W
H
10 min
30 min
H H END H END
a
W = all water has passed; H = all hexadecane has passed; END = the experiment ended without all liquids passing.
coated glass membranes are shown. As expected, for both bare and Ztetraol-coated glass membranes, water penetrates the membrane instantaneously. For the Ztetraol-coated glass membrane, hexadecane does not pass the membrane initially as shown in Figure 5b. Very slowly, hexadecane penetrates the membrane, and it takes more than 30 min for all the liquid to pass the membrane. This slow penetration indicates that, though re-entry topography of the membrane results in the oleophobicity of the surface as illustrated in the hexadecane contact angle (see Figure 3), the gravity force is still higher than the Laplace pressure when large amount of liquids (i.e., larger “h” as shown in Figure 2) are involved. Nevertheless, for the Ztetraol-coated glass membrane, the passing times of water and oil are so different that it will result in an efficient oil/water separation in real-life applications. Interestingly, for the bare glass membrane, though hexadecane starts passing the membrane almost instantaneously as shown in Figure 5a, it took ∼5−10 min for all the hexadecane to go through the membrane. This result is
Figure 4. SEM images of the glass membrane.
the “fiberlike” feature of the membrane arranges in such a way that above-mentioned re-entry topography is clearly visible. Therefore, the higher HCA of Ztetraol/glass membrane, i.e., ∼93.2 ± 12.4°, compared to that of the Ztetraol/glass slide, i.e., ∼70.6 ± 0.3°, can be explained by this mechanism. 3.3. Oil−Water Separation. As shown in Figure 5a, for a bare glass membrane, water passed the membrane instantly and hexadecane started passing the membrane also instantly, indicating an inefficient oil−water separation. However, for the Ztetraol-coated glass membrane, water passes the membrane D
DOI: 10.1021/acs.iecr.8b04071 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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(4) Kota, A. K.; Kwon, G.; Choi, W.; Mabry, J. M.; Tuteja, A. Hygroresponsive membranes for effective oil−water separation. Nat. Commun. 2012, 3, 1025. (5) Howarter, J. A.; Youngblood, J. P. Amphiphile grafted membranes for the separation of oil-in-water dispersions. J. Colloid Interface Sci. 2009, 329 (1), 127−132. (6) Xue, Z.; Wang, S.; Lin, L.; Chen, L.; Liu, M.; Feng, L.; Jiang, L. A novel superhydrophilic and underwater superoleophobic hydrogelcoated mesh for oil/water separation. Adv. Mater. 2011, 23 (37), 4270−4273. (7) Yang, J.; Zhang, Z.; Xu, X.; Zhu, X.; Men, X.; Zhou, X. Superhydrophilic−superoleophobic coatings. J. Mater. Chem. 2012, 22 (7), 2834−2837. (8) Brown, P.; Atkinson, O.; Badyal, J. Ultrafast oleophobic− hydrophilic switching surfaces for antifogging, self-cleaning, and oil− water separation. ACS Appl. Mater. Interfaces 2014, 6 (10), 7504− 7511. (9) Brown, P. S.; Bhushan, B. Mechanically durable, superoleophobic coatings prepared by layer-by-layer technique for antismudge and oil-water separation. Sci. Rep. 2015, 5, 8701. (10) Wang, Y.; Sun, J.; Li, L. What is the role of the interfacial interaction in the slow relaxation of nanometer-thick polymer melts on a solid surface? Langmuir 2012, 28 (14), 6151−6156. (11) Gong, X.; Vahdat, V.; Frankert, S.; Anderson, E.; Chen, S.; Hsia, Y.-T.; Li, L. Study on the Interaction between Talc and Perfluoropolyethers under Tribological Contact. Tribol. Trans. 2015, 58 (4), 679−685. (12) Palacio, M.; Bhushan, B. Ultrathin Wear-Resistant Ionic Liquid Films for Novel MEMS/NEMS Applications. Adv. Mater. 2008, 20 (6), 1194−1198. (13) Tani, H.; Lu, R.; Koganezawa, S.; Tagawa, N. Adsorption properties of an ultrathin PFPE lubricant with ionic end-groups for DLC surfaces. IEEE Trans. Magn. 2018, 54 (2), 1−6. (14) Li, L.; Jones, P.; Hsia, Y.-T. Effect of chemical structure and molecular weight on high-temperature stability of some fomblin Ztype lubricants. Tribol. Lett. 2004, 16 (1), 21−27. (15) Shafrin, E. G.; Zisman, W. A. Effect of Adsorbed Water on the Spreading of Organic Liquids on Soda-Lime Glass. J. Am. Ceram. Soc. 1967, 50 (9), 478−484. (16) Wang, Y.; Knapp, J.; Legere, A.; Raney, J.; Li, L. Effect of endgroups on simultaneous oleophobicity/hydrophilicity and anti-fogging performance of nanometer-thick perfluoropolyethers (PFPEs). RSC Adv. 2015, 5 (39), 30570−30576. (17) Wang, Y.; Dugan, M.; Urbaniak, B.; Li, L. Fabricating Nanometer-Thick Simultaneously Oleophobic/Hydrophilic Polymer Coatings via a Photochemical Approach. Langmuir 2016, 32 (26), 6723−6729. (18) Cao, L.; Hu, H.-H.; Gao, D. Design and fabrication of microtextures for inducing a superhydrophobic behavior on hydrophilic materials. Langmuir 2007, 23 (8), 4310−4314. (19) Tuteja, A.; Choi, W.; Ma, M.; Mabry, J. M.; Mazzella, S. A.; Rutledge, G. C.; McKinley, G. H.; Cohen, R. E. Designing superoleophobic surfaces. Science 2007, 318 (5856), 1618−1622. (20) Cassie, A.; Baxter, S. Wettability of porous surfaces. Trans. Faraday Soc. 1944, 40, 546−551.
unexpected since the hexadecane contact angle on the bare glass membrane is 0°, indicating hexadecane should penetrate the membrane in a very short time. The surprising finding can be rationalized by the fact that water is heavier than hexadecane and the glass membrane adsorbs water readily. Because the density of water is higher, in the oil−water mixture, water always penetrates the membrane first, which is consistent with our observation. After water goes through the membrane, it is expected that water adsorbs on the inner surface of the glass membrane. As a result, oil is in contact with a water-covered glass surface, which is more oleophobic. Therefore, oil passes the bare glass membrane more slowly than expected. Our finding here is in line with a previous report6 that the underwater−oil contact angle on a hydrophilic surface is significantly higher.
4. CONCLUSIONS In conclusion, we have developed a nanometer-thick, mechanically robust, and easy-to-fabricate simultaneously oleophobic/hydrophilic polymer coating for oil−water separation. The coating is fabricated using a commercially available perfluoropolyether (PFPE), i.e., Ztetraol, via a single-step dipcoating process. The tribology testing results showed that the COF of the PFPE coating is low and that there was no loss of the coating during the test duration, which suggests that the coating is mechanically robust. The contact angle tests confirmed that the Ztetraol-coated glass membrane is simultaneously oleophobic/hydrophilic. An oil−water separation test indicated that such a coating is efficient in oil−water separation. The simultaneously oleophobic/hydrophilic membranes are highly desirable in oil−water separation since both gravity and wettability promotes the separation, which renders the process energy-efficient. However, for long-term applications, it is critical to develop easy-to-fabricate and mechanically robust coatings. Therefore, the current finding has important implications in real-life application of simultaneouly oleophobic/hydrophilic coatings in oil−water separation.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Lei Li: 0000-0002-8679-9575 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Y.W., C.Y., and L.L. gratefully acknowledge the financial support from Lubrizol Corporation. We also thank Dr. Qian Guo from Seagate Technology LLC for providing the PFPE Ztetraol sample.
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REFERENCES
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DOI: 10.1021/acs.iecr.8b04071 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
