Janus Membranes with Opposing Surface Wettability Enabling Oil-to

Letter www.acsami.org

Janus Membranes with Opposing Surface Wettability Enabling Oilto-Water and Water-to-Oil Emulsification Ming-Bang Wu,†,‡ Hao-Cheng Yang,†,‡ Jing-Jing Wang,†,‡ Guang-Peng Wu,*,†,‡ and Zhi-Kang Xu*,†,‡ †

MOE Key Laboratory of Macromolecular Synthesis and Functionalization and ‡Key Laboratory of Adsorption and Separation Materials & Technologies of Zhejiang Province, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China S Supporting Information *

ABSTRACT: A Janus membrane with opposing wettability was first reported with both function of water-to-oil and oil-to-water emulsification. This membrane is conveniently fabricated by single-surface deposition of polydopamine/polyethylenimine (PDA/PEI). The asymmetric wettability can also reduce the transmembrane resistance during the process, indicating an economical and promising strategy to prepare various emulsions. This research opens a novel avenue for exploring and understanding the Janus membrane, and provides a perspective to design the asymmetric membrane structures with promoted performance in conventional membrane processes.

KEYWORDS: Janus membrane, surface wettability, surface modification, membrane emulsification, transmembrane pressure

E

Janus membranes with opposing wettability by depositing polydopamine/polyethylenimine (PDA/PEI) on one surface of flat membrane or hollow fiber membrane (Figure 1a).19,20 These membranes have shown great potentials in fine bubble aeration and membrane distillation.19−21 Herein, we first employ the Janus membrane in membrane emulsification to prepare both oil-in-water and water-in-oil emulsions (Figure 1b). In addition, the Janus structure can efficiently reduce the emulsification pressure without significant changes in emulsion size, leading to low energy consumption. Our Janus membrane was facilely prepared by floating a hydrophobic polypropylene membrane on a dopamine/ polyethylenimine (DA/PEI) solution under ambient temperature.19,22 The nascent membrane was prewetted by ethanol before deposition to forbid the air film formation at the membrane/solution interface. In this case, PDA and PEI forms a hydrophilic cross-linked coating on the contacted membrane surface via Michael addition or Schiff-base reactions (Scheme S1).22 The asymmetric structure is identified by the different physical and chemical properties on each membrane surface. The hydrophilic surface shows much higher content of N and O (14.04 and 15.12%, respectively) than the hydrophobic surface (1.93 and 3.85%, correspondingly) because of the asymmetric deposition of PDA/PEI (Table S1 and Figure S1). The slight deposition of PDA/PEI on the hydrophobic surface is caused by the capillary effect during the fabrication process. These results can be also identified by the FT-IR/ATR spectra

mulsions are extensively employed in industry, agriculture, and our daily life. Various emulsification technologies, such as rotor-stator systems,1 ultrasonic dispersion2 and highpressure homogenization,3 have been developed to meet the growing demands for different emulsions. Membrane emulsification stands out from these technologies due to its advantages in tunable emulsion size distribution and low energy consumption.4−8 In a typical membrane emulsification process, one phase (named dispersed phase) is forced to pass through membrane pores and dispersed in another immiscible phase (named continuous phase). To keep the emulsion stable and uniform, the porous membrane should be generally nonwettable to the dispersed phase for the droplets detachment at small size.9,10 Hence, the hydrophilic membranes are only applicable in oil-in-water emulsion preparation, while the hydrophobic ones are only available for water-to-oil emulsification. In fact, only the membrane surface facing to the continuous phase is required to be phobic to the dispersed phase, rather than the whole membrane. Therefore, we propose to combine the oil-to-water and water-to-oil emulsifications by using a membrane with opposing wettability on each surface. In this way, we can both prepare water-in-oil and oil-in-water emulsions just by switching the emulsifying direction via using the Janus membrane. Janus membrane is referred as a membrane with opposing properties, in most cases, opposing wettability on each surface.11−13 It can be prepared by either asymmetric membrane fabrication, such as sequential filtration14 and electrospinning,11 or asymmetric surface modification, such as single-surface photoreaction and coating.15−18 Recently, we reported a facile single-surface modification strategy to fabricate © XXXX American Chemical Society

Received: January 1, 2017 Accepted: January 31, 2017 Published: January 31, 2017 A

DOI: 10.1021/acsami.7b00017 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Letter

ACS Applied Materials & Interfaces

Figure 1. (a) Illustration of the fabrication process for Janus membrane by single-surface deposition of PDA/PEI. (b) Illustration of the water-to-oil and the oil-to-water emulsification processes.

Figure 2. (a) Dynamic water contact angles on the hydrophobic and hydrophilic membranes. (b) Dynamic water contact angles on each surface of the Janus membrane. (c) Oil contact angles under water and water contact angles under oil on each surface of the hydrophobic, Janus, and hydrophilic membranes. The oil is toluene.

(Figure S2).19 The surface wettability is crucial for the permeation and separation performance of Janus membrane. Therefore, we measured the water contact angles (WCAs) on each surface of Janus membrane, and compared them with the

WCAs of hydrophobic one and hydrophilic one prepared by symmetric codeposition of PDA/PEI. In contrast to the hydrophilic membrane, the hydrophilic surface of Janus membrane shows a similar initial WCA of about 34°, which B


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Figure 3. Digital photos and optical micrographs of the oil-in-water emulsion prepared by (a) hydrophobic, (b) Janus, and (c) hydrophilic membranes, and water-in-oil emulsions prepared by (d) hydrophobic, (e) Janus, and (f) hydrophilic membrane. The oil is toluene and the scale bars are 100 μm.

Table 1. Mean Droplet Size and Coefficient Variation of Water-in-Oil and Oil-in-Water Emulsions Prepared by Different Membranesa oil-in-water emulsion

water-in-oil emulsion

parameters

hydrophobic membrane

Janus Membrane

hydrophilic membrane

hydrophobic membrane

Janus membrane

hydrophilic membrane

mean particle size (μm) coefficient variation (%)

N. A. N. A.

19.11 35.8

17.74 42.8

35.43 50.98

34.40 53.82

N. A. N. A.

a

The oil is toluene.

continuously decreases to apparent 0° within 4 s (Figure 2a, b). On the other hand, the hydrophobic surface of Janus membrane presents different wetting behaviors from the hydrophobic membrane. The initial WCA is nearly 130° for both hydrophobic membrane and the hydrophobic surface of Janus membrane, but the water droplet on Janus membrane gradually permeates through the membrane pores without obvious spreading on the membrane surface (Figure 2b and Figure S3). This phenomenon is referred as the directional transport in many related results.14,23,24 Considering the water/ oil/membrane triple-phase in emulsification, we also compared the oil contact angles (OCAs) in water and the WCAs in oil on the hydrophobic, the hydrophilic and the Janus membranes (Figure 2c). The WCA in oil and OCA in water are about 15 and 156° for both surfaces of the hydrophobic membrane, whereas they are 121 and 51° for the hydrophilic one. For the Janus membrane, the hydrophobic surface shows similar WCA and OCA to the hydrophobic membrane, whereas the hydrophilic surface exhibits higher OCA and lower WCA than the hydrophilic membrane. It can be interpreted by the different PDA/PEI contents between the Janus membrane and the hydrophilic one. The dopamine oxidation dramatically

depends on the dissolved oxygen concentration in the solution. Therefore, the hydrophilic surface of Janus membrane shows higher PDA/PEI content than the hydrophilic one because the oxygen concentration at the air/water interface is much higher than that in the bulk solution.25 To demonstrate the advantages of Janus membrane, we employed it in both water-to-oil and oil-to-water emulsification, and both the hydrophilic and the hydrophobic membranes with the same porous structure are also involved for comparison (Figure 3). The membrane was settled in a homemade emulsification device (Figure S4), and the oil (toluene) was dyed to enhance the visibility of the experiment. The hydrophilic surface was faced to the water phase while the hydrophobic surface to the oil phase. In the case of oil-to-water emulsification, both the hydrophilic (Figure 3c) and the Janus membranes (Figure 3b) can generate uniform and stable emulsions, while the hydrophobic membrane fails in emulsification (Figure 3a). On the other hand, only the hydrophobic (Figure 3d) and the Janus membranes (Figure 3e) are available in water-to-oil emulsification. These results can be rationalized by the surface wettability of the membranes and its role in emulsion formation. Take the oil-to-water emulsification C


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Figure 4. Minimum transmembrane pressure of the hydrophobic, Janus, and hydrophilic membranes.

In our cases, the Janus membrane shows no significant difference on ΔPc compared to the hydrophobic membrane because both of them have similar surface wettability and pore structure (Figure 2b, c). Therefore, the transmembrane pressure differences are mainly caused by the different pressure losses. The hydrophobic surafce shows much higher transmembrane resistance than the hydrophilic surface, which is in accordance with the ΔPtrans of the hydrophobic and the hydrophilic membranes. On the other hand, the ΔPloss increases with the membrane thickness. Therefore, with the same total thickness, the Janus membrane exhibits much lower ΔPtrans than the hydrophobic membrane resulting from thinner hydrophobic layer. It should be mentioned that the low ΔPtrans can not only reduce the energy consumption in membrane emulsification, but also benefit to the application of polymer membranes in this process. Most of the reported membranes in membrane emulsification are inorganic membranes owing to their high mechanical strength. Although the polymer membranes have the advantages of tunable pore structure and low producing cost, their low mechanical strength limits their practical application in membrane emulsification. The transmembrane resistance, as well as the membrane strength will be increased simultaneously by the membrane thickness. The Janus membrane shows a great potential to increase the membrane thickness without obvious increase of transmembrane resistance. Lastly, we prepared different oil-in-water and water-in-oil emulsions by the Janus membrane, from petrol, toluene, dichloromethane and bean oil (Table S2 and Figures S6 and S7). All emulsions were successfully prepared by the Janus membrane except the water-in-bean-oil emulsion, which could be attributed to the high viscosity of bean oil. The average size of oil droplets in water is ranked as following: petrol < toluene < dichloroethane < bean oil, corresponding to their viscosity (Table S3). The water-in-oil emulsions show a similar order in average size. In conclusion, we facilely fabricate a Janus membrane with opposing surface wettability by single-sided PDA/PEI codeposition, and first evaluate it in membrane emulsification. Both oil-in-water and water-in-oil emulsions can be successfully prepared by this Janus membrane. In addition, the Janus membrane shows much lower energy consumption as well as no significant deterioration in emulsification performance compared to the conventional membranes with single wettability because of lower transmembrane resistance.

as an example, when the oil is forced to permeate through the hydrophilic or the Janus membrane, it forms a droplet on the outlet surface due to its excellent underwater oleophobicity. The contact area is relatively small between the outlet membrane surface and the oil droplet. It is well-known that the retaining force decreases with the reduced triple-phase contact area. Therefore, the oil droplet is prone to detaching from the membrane surface at small size, facilitating the formation of fine emulsion particles. On the other hand, when the oil is forced out the pores of the hydrophobic membrane, it prefers to form a liquid film on the membrane surface to lower the system free energy. The retaining force increases dramatically and the large oil droplets trend to coalesce to form a layered oil phase. This process was recorded by a digital camera to support the proposed mechanism (Videos S1−S3). Moreover, the oil-in-water emulsion generated by the Janus membrane shows similar size distribution with the emulsion from the hydrophilic membrane (Table 1 and Figure S5). We also found a desirable decrease in minimum transmembrane pressure (ΔPtrans) during the emulsification process with Janus membrane. The ΔPtrans of the hydrophobic, the hydrophilic and the Janus membranes during the water-to-oil emulsification were comparatively studied, and the results are shown in Figure 4a. No emulsion forms in the cases of the hydrophilic membrane despite of the lowest ΔPtrans as mentioned above. On the other hand, the Janus membrane shows much lower ΔPtrans than the hydrophobic one (reduced by above 60%) with competitive emulsification performance. The mechanism is revealed by analyzing the factors affecting the transmembrane pressure. ΔPtrans is composed of two parts (eq 1): ideal critical pressure (ΔPc)5 and pressure losses (ΔPloss).26 ΔPc can be calculated by Laplace eq (eq 2) and ΔPloss can be estimated by (eq 3), respectively. ΔPtrans = ΔPc + ΔPloss

(1)

ΔPc = 4γ cos θ /d p

(2)

ΔPloss = LmemC1QC2

(3)

where γ is the oil/water interfacial tension, θ is dispersed phase contact angle on the membrane surface, and dp is the average pore diameter. In eq 3, Lmem is the thickness of resistance parts. C1 is defined as the pore structure coefficient which is determined by pore size distribution, porosity and other parameters, and C2 is a liquid property coefficient correlated to the flow rate, viscosity, and other parameters. D


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(11) Wu, J.; Wang, N.; Wang, L.; Dong, H.; Zhao, Y.; Jiang, L. Unidirectional Water Penetration Composite Fibrous Film via Electrospinning. Soft Matter 2012, 8, 5996−5999. (12) Zhao, Y.; Wang, H.; Zhou, H.; Lin, T. Directional Fluid Transport in Thin Porous Materials and its Functional Applications. Small 2017, 13, 1601070. (13) Yang, H. C.; Hou, J.; Chen, V.; Xu, Z. K. Janus Membranes: Exploring Duality for Advanced Separation. Angew. Chem., Int. Ed. 2016, 55 (43), 13398−13407. (14) Hu, L.; Gao, S.; Zhu, Y.; Zhang, F.; Jiang, L.; Jin, J. An Ultrathin Bilayer Membrane with Asymmetric Wettability for Pressure Responsive Oil/Water Emulsion Separation. J. Mater. Chem. A 2015, 3, 23477−23482. (15) Wang, Z.; Wang, Y.; Liu, G. Rapid and Efficient Separation of Oil from Oil-in-Water Emulsions Using a Janus Cotton Fabric. Angew. Chem., Int. Ed. 2016, 55 (4), 1291−1294. (16) Wang, Z.; Liu, G.; Huang, s. In Situ Generated Janus Fabrics for the Rapid and Efficient Separation of Oil from Oil-in-Water Emulsions. Angew. Chem., Int. Ed. 2016, 55, 14610−14613. (17) Wang, H.; Ding, J.; Dai, L.; Wang, X.; Lin, T. Directional WaterTransfer through Fabrics Induced by Asymmetric Wettability. J. Mater. Chem. 2010, 20, 7938−7940. (18) Chen, J.; Liu, Y.; Guo, D.; Cao, M.; Jiang, L. Under-Water Unidirectional Air Penetration via a Janus Mesh. Chem. Commun. 2015, 51, 11872−11875. (19) Yang, H. C.; Hou, J.; Wan, L. S.; Chen, V.; Xu, Z. K. Janus Membranes with Asymmetric Wettability for Fine Bubble Aeration. Adv. Mater. Interfaces 2016, 3, 1500774. (20) Yang, H.-C.; Zhong, W.; Hou, J.; Chen, V.; Xu, Z.-K. Janus Hollow Fiber Membrane with a Mussel-Inspired Coating on the Lumen Surface for Direct Contact Membrane Distillation. J. Membr. Sci. 2017, 523, 1−7. (21) Lee, H.; Dellatore, S. M.; Miller, W. M.; Messersmith, P. B. Mussel-Inspired Surface Chemistry for Multifunctional Coatings. Science 2007, 318, 426−430. (22) Yang, H. C.; Liao, K. J.; Huang, H.; Wu, Q. Y.; Wan, L. S.; Xu, Z. K. Mussel-Inspired Modification of a Polymer Membrane for UltraHigh Water Permeability and Oil-in-Water Emulsion Separation. J. Mater. Chem. A 2014, 2 (26), 10225−10230. (23) Chen, Q.; Meng, L.; Li, Q.; Wang, D.; Guo, W.; Shuai, Z.; Jiang, L. Water Transport and Purification in Nanochannels Controlled by Asymmetric Wettability. Small 2011, 7 (15), 2225−2231. (24) Zhou, H.; Wang, H.; Niu, H.; Lin, T. Superphobicity/philicity Janus Fabrics with Switchable, Spontaneous, Directional Transport Ability to Water and Oil Fluids. Sci. Rep. 2013, 3, 2964. (25) Yang, H.-C.; Wu, Q.-Y.; Wan, L.-S.; Xu, Z.-K. Polydopamine Gradients by Oxygen Diffusion Controlled Autoxidation. Chem. Commun. 2013, 49 (89), 10522−10524. (26) Hornig, N.; Fritsching, U. Liquid Dispersion in Premix Emulsification within Porous Membrane Structures. J. Membr. Sci. 2016, 514, 574−585.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b00017. Experimental procedures, XPS and FT-IR/ATR spectra, drop conjugate diameters, emulsification performances (PDF) Video S1, oil-to-water emulsification through the Janus membrane (AVI) Video S2, oil-to-water emulsification through the hydrophilic membrane (AVI) Video S3, oil-to-water emulsification through the hydrophobic membrane (AVI)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Guang-Peng Wu: 0000-0001-8935-964X Zhi-Kang Xu: 0000-0002-2261-7162 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support is acknowledged to the National Natural Science Foundation of China (Grant 21534009). REFERENCES

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Janus Membranes with Opposing Surface Wettability Enabling Oil-to

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