Understanding the Antifouling Mechanism of Zwitterionic Monomer

Subscriber access provided by TULANE UNIVERSITY

Surfaces, Interfaces, and Applications

Understanding the Antifouling Mechanism of Zwitterionic Monomer Grafted PVDF Membranes: A Comparative Experimental and Molecular Dynamics Simulation Study Zi-Yu Liu, Qin Jiang, Zhiqiang Jin, Zhenyu Sun, Wang-Jing Ma, and Yanlei Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b22059 • Publication Date (Web): 21 Mar 2019 Downloaded from http://pubs.acs.org on March 26, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Understanding

the

Antifouling

Mechanism

of

Zwitterionic Monomer Grafted PVDF Membranes: A Comparative Experimental and Molecular Dynamics Simulation Study Zi-Yu Liua,*, Qin Jianga, Zhiqiang Jina, Zhenyu Sunc, Wangjing Maa, Yanlei Wangb,* a

Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of

Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, People's Republic of China; b

Beijing Key Laboratory of Ionic Liquids Clean Process, CAS Key Laboratory of Green Process and

Engineering, State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, People's Republic of China. c

State Key Laboratory of Organic-Inorganic Composites, College of Chemical Engineering, Beijing

University of Chemical Technology, Beijing 100029, People's Republic of China.

Keywords: zwitterionic membrane; antifouling mechanism; alginates; molecular dynamics simulation; electrolyte; electrostatic repulsion Corresponding Author: *Email: [email protected] *Email: [email protected]

1 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 31

Abstract: The antifouling process of the membrane is very vital for the highly efficient treatment of industrial wastewater, especially high salinity wastewater containing oil and other pollutants. In the present work, the dynamical antifouling mechanism is explored via molecular dynamics simulations while the corresponding experiments about surface properties of zwitterionic monomer grafted PVDF membrane are designed to verify the simulated mechanism. The fact can be found that water can form a stable hydration layer at the grafted membrane surface, where all the simulated radial distribution function of water-membrane, hydrogen bond number, water diffusivity and experimental oil contact angles keep stable. However, the water flux across the membrane will increase firstly and then decrease as grafting ratio increases, which not only depends on the reduced pore size of the zwitterionic grafted membrane but also results from water diffusion. Furthermore, the dynamical fouling processes of pollutants (take sodium alginate as an example) on the grafted membrane in water and brine solution are investigated, where both the high grafting ratio and electrolyte CaCl2 can enhance the fouling energy barrier of pollutant. The results show that both enhanced hydrophilic property and the electrostatic repulsion can affect the antifouling capability of the grafted membrane. Finally, the ternary synergistic antifouling mechanisms among zwitterionic membrane, electrolyte and pollutant sodium alginates are discussed, which could be helpful to the rational design and preparation of new and highly efficient zwitterionic antifouling membranes.


Page 3 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. Introduction The treatment of wastewater containing oils or other pollutants is of great importance to the rapid growth of industries and the global population. Due to the low-energy consumption and high separation efficiency, the membrane technology has been viewed as one of most effective approaches to remove oil or other pollutants from wastewater comparing with the traditional methods, such as gravity and skimming, coagulation and flotation, etc. Polyvinylidene difluoride (PVDF) has been widely used as the membrane material to treat wastewater because of its high mechanical strength and wonderful chemical stability.1-4 However, PVDF membrane is easy to be fouled by most pollutants as its hydrophobic property.5-7 Membrane fouling will greatly shorten the service life and separation efficiency of PVDF membrane, which restrains its development and application. Hence, improving and modifying the antifouling property of PVDF membrane is thus very vital to the treatment of wastewater containing oil or other pollutants. Many modification methods are investigated to increase the hydrophilicity of membrane surface, such as blending with amphiphilic copolymers8 or nanoparticles,9 surface coating10 or grafting hydrophilic molecules at the surface.11 Beyond that, the characters of modified molecules also become more concerned by researchers. Zwitterionic molecules12-15 have overwhelming advantages as grafting monomers because of their hydrophilicity, chemical stability, and high-salt solubility. Not only that, zwitterionic molecules derive strong universality from their special structure of two opposing electriferous groups in solutions.16-19 Thus it is promising to construct a zwitterionic surface to increase membrane properties. Recently, zwitterionic molecules have been successfully applied on membrane surface by many experts.20-24 Bengani-Lutz and coworkers25 reported the performances of amphiphilic zwitterionic copolymers coating membrane surfaces, which have a strong fouling resistance and high flux in protein fouling process. Han and Liu et al26 prepared anti-fouling membranes using zwitterions 3 ACS Paragon Plus Environment


Page 4 of 31

which can well resistant inorganic/organic fouling. These results confirm that the zwitterionic surface can greatly improve the hydrophilic property and separation performance of the membrane. However, most prior studies concentrate on the membrane actual properties, and little work is devoted to understanding the antifouling mechanism on a molecular scale, which is the basis of highly efficient membrane preparation. In comparison to the experiments, molecular dynamics (MD) simulations have been adopted to investigate the structural, electronic and interfacial properties at a molecular level27-31,32. To date, many membrane surfaces have been studied via theoretical simulation, such as polyvinyl chloride (PVC) surface,33-34 sulfonated polyether sulfone surface,35 PVDF-co-PEGMA surface36 and so on. Xiang at al37 used MD simulations to investigate molecular interactions between sulfobetaine zwitterions or between sulfobetaine brushes in different media. They found that sulfobetaine brush arrays with different grafting densities have different structures and antifouling mechanisms. However, the theoretical study on the structure and property of the zwitterionic membrane surfaces is still little and thus dig deeper into zwitterionic effect on the membrane is needed. Besides simulations of membrane surficial properties, interactions between membrane and foulants29 are also simulated by some researchers. Simulated foulants are focused on bovine serum albumin (BSA),29,

38-39

humic acids38,

40

and

lysozyme.41-42 Nevertheless, simulations about membrane fouling process are still needed a more diverse and thorough exploration, such as alginates37, 43-44 fouling the zwitterionic membrane. Herein, the surface property and dynamical antifouling mechanism of zwitterionic monomer grafted PVDF membranes are explored via the joint MD simulations and experiments. The chemical structure of this random copolymer is shown in Figure 1. The molecular structure of water-membrane interface and diffusion behavior of water were explored firstly. Then the dynamical fouling processes of pollutants (take alginate as an example) on the grafted membrane were revealed via the steered MD simulations. 4 ACS Paragon Plus Environment

Page 5 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Besides MD simulations, zwitterionic surfaces were prepared by UV photochemical grafting45-46 to measure the surface topology structure, contact angle, oil adhesion force and resistance of sodium alginates to confirm simulated results. Finally, the ternary synergistic antifouling mechanisms among zwitterionic surface, electrolyte and pollutant sodium alginates were discussed based on the results from MD simulations and experiments.

2. Models and Methods 2.1 Simulation details and atomic structures All the MD simulations in this paper were carried out by using GROMACS 5.1.47 SPC model was used to describe the water molecules while CHARMM36 force field44 was chosen for the other molecules including polymers, sodium alginates, and ions. The electronic terms were employed via the Coulomb interaction and the long-range electrostatic interaction was calculated by the particle mesh Ewald (PME) method. The van der Waals interaction was described using Lennard-Jones potential with a cut off distance of 10 Å. Periodic boundary conditions (PBCs) were applied in all three directions, where the PBCs along the membrane surface can largely weaken the size-effect of the simulated cell. The structure of a PVDF polymer chain is -[CH2-CF2]n-, where n is the repeat unit number. In this work, each PVDF chain contained 80 repeat units and 50 PVDF chains were built in a slab whose dimension was about 6.5×6.5×6.0 nm3. The grafted degree can be described via the DMAPS grafting ratio (RDMAPS): RDMAPS = mDMAPS/mPVDF, where mDMAPS was the mass of grafted DMAPS to replace the F atoms and mPVDF was the mass of pure PVDF chains. All the grafting membranes consisted of 40 PVDF-g-DMAPS chains and constructed slab size was similar to PVDF slab. In order to form consolidated polymer surfaces, the simulated temperature was set at 500 K and 1000 K successively, and each NPT simulation was carried out for 5 ns. Then polymers were kept on NPT simulation at 300 K for 30 ns. The pressure in all simulations was maintained at 1 bar by coupling to a Berendsen barostat. The density of the consolidated 5 ACS Paragon Plus Environment


Page 6 of 31

PVDF polymer matrix in the simulation is 1.70 g/cm3, close to the experimental value48 of 1.68 g/cm3. A water box of 6.0×6.0×8.0 nm3 was built with 9280 water molecules. An NPT simulation was performed for 10 ns long to guarantee the equilibrium of the system. Then the water box combined with different polymer surfaces were put together to establish different water-membrane systems. Alginates are linear chains consist of α-L-guluronate (G) and β-D-mannuronate (M) residues. In this simulation, the alginate molecule was selected as G-G-G-M-M-M and sodium alginate with 30 sodium ions were distributed in 9280 water molecules to constitute the sodium alginate solution. To make a calcium alginate gel, 360 calcium ions (Ca2+) and 720 chloride ions (Cl−) were also added. The size of sodium alginate box was similar to the water box and the box was also simulated for 10 ns before combined with polymers. An NPT simulation was run for 20 ns to ensure the equilibrium for all water-membrane systems and alginate-membrane systems, while the last 5 ns was used to analysis. The plots for the initial model of the alginates brine solution PVDF-g-DMAPS system and the agglomerated alginate molecules were shown in Figure 1B and Figure S1 respectively. 2.2 Materials and membrane characterization Zwitterionic monomer [2-(Methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl) (DMAPS) was purchased from Aladdin industrial corporation. Photosensitizer benzophenone (BP), N,N’-methylene bis(acrylamide) (MBAA), ammonium ceric nitrate (ACN), sodium alginate (SA) were purchased from Shanghai Macklin Biochemical co., Ltd. PLS-SXE300D/DUV was used to provide ultraviolet(UV) which was obtained from Beijing Perfect Light corporation. PVDF membrane (pore size is about 0.1μm) was purchased from Ande Membrane Separation Technology＆Engineering (Beijing) co., Ltd. Deionized water and anhydrous ethanol were used for all experiments. UV irradiation-induced atom transfer radical polymerization (ATRP) was used to graft hydrophilic zwitterionic monomers onto the PVDF membrane surface (see Supporting 6 ACS Paragon Plus Environment

Page 7 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Information for the details). The grafting ratio in experiments can be described via CDMAPS, which stands for the used grafting monomer concentration. It should be noted that CDMAPS have a positive relationship with RDMAPS defined in the MD simulations. ATR-FTIR was used to verify the success of grafting zwitterionic monomers. Scanning-electron microscopy (SEM) images were used to characterize the morphology and microstructure of the membrane surfaces. Water contact angles49, 50 and underwater oil contact angles on membrane surfaces were measured via the sessile drop method using the Data Physics OCA20 (DataPhysics Company, Germany). Force spectroscopy51 was also applied to measure oil adhesion forces on membrane surfaces. 2.3 Membrane separation performance and alginates fouling experiments A dead-end stirred cell filtration device was used to examine the membrane separation performance. The effective area of the used filtration cup was 0.001m2 and operation pressure was kept stable by using a constant pressure pump. The test pressure was maintained at 0.1 MPa after membrane compacted at 0.15 MPa for 0.5 h. The pure water permeation flux (Jw, L m−2 h−1) was calculated by Equation (1). Then, sodium alginate was selected to investigate the membrane antifouling performance. The foulant solution flux (Jp) and the pure water permeation flux (Jwf) after hydraulically cleaning were also measured by Equation (1). J = V/At

(1)

where V (L) was the permeate volume, A (m2) was the effective membrane area and t (h) was the permeate time. Then the flux properties can be described via four different parameters: Flux recovery ratio (FRR = Jwf/Jw), total flux decline ratio (DRt = (Jw-Jp)/Jw), reversible flux decline ratio (DRr = (Jwf-Jp)/Jw) and irreversible flux decline ratio (DRir= (Jw-Jwf)/Jw).

3. Results and discussion 3.1 Molecular structure and water diffusion at water/membrane interface The membrane antifouling properties depend on the molecular structure of 7 ACS Paragon Plus Environment


Page 8 of 31

water-membrane surface to a great extent. The Z-density distribution of water and polymers for different RDMAPS are displayed firstly in Figure 2A, showing that more water molecules distribute at the membrane surface when RDMAPS increases. The definition of the scope of the membrane surface is according to the 90 criterion, which is explained detailed in the Supporting Information. To further illustrate the interfacial structure clearly, radial distribution function (RDF), g(r) is analyzed. RDF refers to the ratio of local density to bulk density as a function of the distance from a reference point. Figure 2B shows RDF of water at the membrane surface with different RDMAPS. The oxygen atoms in H2O molecules was set as target atoms while the F atoms and zwitterionic groups of membrane surface were selected as reference points for pure PVDF and grafting membranes, respectively. More than two obvious water peaks can be found for grafting membrane surfaces while no peak appeared for PVDF membrane surface. That is to say, water molecules can be enriched at the membrane surface when F atoms were replaced by grafting zwitterionic molecules. Furthermore, grafting degree can also influence the distribution of water molecules at the surface. When a small number of DMAPS are grafted, water molecules are inclined to locate at the second or future back peaks (~ 3 Å) that are far away from the membrane surface. With the increase of grafting degree, more water molecules are located at the first water peak (~ 1.5 Å) instead of keeping at the back peaks. The values of all water peaks obviously increase when RDMAPS exceeds 20%. In other words, the hydration layer is gradually formed as shown in Figure S2. To further reveal the formation process of hydration shell, the number of hydrogen bond (HB) is calculated for the PVDF membrane with different RDMAPS. Figure 2C displays the number of HBs within 0.35 nm along membrane surface for different systems. No HBs are found for pure PVDF membrane that means the interactions between water and membrane completely originate from the grafted zwitterionic molecules as shown in inset plot of Figure 2C. As RDMAPS increases, the number of HB increases gradually, 8 ACS Paragon Plus Environment

Page 9 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


indicating more and more water molecules are enriched at the grafted membrane surface. It is worth noting that there exists an obvious jumping ratio (20%) that makes growth rate of HB number changes sharply. The number of HBs increases dramatically from 67 to 218 (~ three times) when the grafting ratio increases from 15% to 20%, which verifies the formation of stable hydration layer. Beyond 20%, there is no greatly raise of the HBs number, showing that the stable hydration layer has been steady. The enhanced HB strength at water/membrane interface and the existence of the hydration layer will affect the diffusive ability of water at the surface. In consequence, the self-diffusivity (D) is calculated to investigate the movement of water molecules at the surface as shown in Figure 2D. In our simulations, water diffusivity at the horizontal dimension is slightly higher than that at the vertical dimension, which stands for the less impact of membrane on water horizontal movement. However, the varied trends of water diffusivity at different dimensions are similar. The simulated self-diffusivity of water molecules in bulk aqueous phase is about 2.1 × 10-5 cm2/s, which is consistent with experimental value (2.0-2.5×10-5 cm2/s). The diffusivity of water molecules at the PVDF membrane surface is much less than that in aqueous phase, which stands for the restrained movements of water molecules and hydrophobic property of PVDF membrane. Then grafting a small number of zwitterionic molecules (5%), water diffusivity increases surprisingly and the value (5.4×10-5 cm2/s) obviously exceed the bulk value, showing a superfast water diffusion. That is to say, the drastic diffusion-exchange process of water molecules at the membrane surface occurs when slight zwitterionic molecules are grafted. Increasing grafting degree continuously, water diffusivity starts to decrease while the interaction between water and membrane is gradually strengthened. When RDMAPS reaches 20%, water diffusivity decreases to bulk aqueous value and the value keeps stable, which reflects the establishment of steady hydration layer again. Based on the above analysis about density, RDF, HB number and diffusivity, the interactions between water and membrane surface can be revealed clearly. For 9 ACS Paragon Plus Environment


Page 10 of 31

PVDF-g-DMAPS systems, it exists a critical grafting ratio (20%) to form the steady hydration layer around the membrane surface. When grafting ratio is less than critical grafting ratio, water movement at the membrane surface is much faster than water molecules in the bulk aqueous phase. When grafting ratio reaches to the critical grafting ratio,

interactions

between

water

and

membrane

reaches

maximum

while

diffusion-exchange of water molecules is same as that in aqueous phase. The hydrophilic property of membrane surface also reaches a maximum and becomes stable as RDMAPS goes on growing. To verify the above theoretical reasoning, experimental characters of zwitterionic surfaces are necessary. 3.2 Membrane characters of zwitterionic surfaces ATR-FTIR is used to verify the success of grafting zwitterionic monomers, and the results can be found in Figure S3. To probe the morphology of PVDF and grafting membrane surfaces directly, scanning-electron microscopy (SEM) images are obtained as shown in Figure 3(A-D). For PVDF membrane, obvious membrane pores can be found, whose radius is on the scale of ~100 nm. When the concentration of zwitterionic monomers is small (CDMAPS = 0.1 wt%), the distribution of membrane pore and morphology changes very little. As the grafted monomer ratio increases continuously, the membrane surface becomes more smooth while membrane pore number and size decrease greatly as shown in Figure 3C. When the monomer concentration is up to 2 wt%, membrane pores almost disappear and can hardly be seen, which may greatly affect the hydrophilic property and membrane flux property. Furthermore, the hydrophilic properties of membrane surfaces with different monomer concentration (CDMAPS) are characterized by experimental contact angles. Besides the sessile drop contact angles (θw), the oil (Decane) contact angles under water (θoil) are also measured (Figure 3E). θw is about 79.3º for pure PVDF, and the value will rapidly decay as the used zwitterionic monomer concentration increases. When CDMAPS is beyond 1wt%, θw decreases to a convergence of 28º, which is consistent with the formation 10 ACS Paragon Plus Environment

Page 11 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


process of hydration layer discussed in Figure 2. θoil under water has an opposite tendency, where θoil for pure PVDF is about 134.9 º and increases gradually with the increase of CDMAPS. θoil can also approach the constant value of 166º (underwater superoleophobic) when the concentration is up to 1wt%. That is to say, the addition of zwitterionic molecules can make the membrane more hydrophilic and oleophobic, which is meaningful for the membrane to serve as an oil-water separation membrane. Force spectroscopies between a decane droplet and different membrane surfaces are measured as shown in Figure 3F to investigate anti-adhesion performances of zwitterionic membrane surfaces. The moving distance and speed for all experiments are the same. For pure PVDF membrane, the largest oil adhesion forces (69 μN) are found and most decane molecules are left on the membrane surface as shown in the inset plot of Figure 3F. The oil adhesion force decreases greatly as CDMAPS increases which corresponds to the enhanced oleophobic property. Similar to the contact angle, a dramatic decrease also occurs after grafting little DMAPS molecules. And little adhesion force can be found when DMAPS concentration reaches 1wt%. Hence, there exists a critical zwitterionic DMAPS monomer concentration (1 wt%) to keep hydrophilic-lipophilic properties of zwitterionic surfaces steady. The evolution of hydrophilic or oleophobic properties for grafted PVDF membrane well corresponds to the simulated results of RDF and HB number. Because of the evolution of membrane hydrophilicity, both experiment and simulation demonstrate that the grafted membrane will form a stable hydration layer and own convergent hydrophilic-lipophilic properties when the grafting ratio beyond the critical concentration. The water flux of the membrane plays a key role in the final oil-water separation performance, which is measured and summarized as a function of CDMAPS in Figure 3(G). For pure PVDF membrane, the water flux is about 1067 L/(m2 ·h) and the value sharply increases by 67% when traces of DMAPS are grafted (CDMAPS = 0.1 wt%). When CDMAPS increases continuously, the flux will decrease gradually. Interestingly, the 11 ACS Paragon Plus Environment


Page 12 of 31

tendency of water flux (increase firstly and then decrease) is consistent well with simulated diffusivity. The first increase of water flux mainly owes to the increased water diffusivity caused by grafting a small number of hydrophilic monomers. The enhanced water diffusivity will promote the transport of water at the membrane surface, and then improve the possibility of water to enter into the pore of the grafted membrane. However, when DMAPS concentration increases continuously, both the size of the pore of membrane and water diffusive coefficient will decrease, leading to a lower water flux. That’s to say, two factors dominate the water flux across the membrane, one is the pore-clogging of the membrane as shown in Figure 3(A-D), the other one is the restrained diffusive ability of water at the surface which is summarized in Figure 2D. After above discussions, the intrinsic properties of membrane with different CDMAPS including surface morphology, water/oil contact angle, adhesion force of oil droplet, and water flux are explored clearly. However, the interaction and mechanism of membrane resisting pollutants such as sodium alginates are still puzzled. 3.3 Dynamical process of sodium alginates fouling on the membrane surface Sodium alginate (SA) is selected as the model pollutant to evaluate the antifouling properties of the zwitterionic membrane. SA is often applied in the form of a gel (CaCl2 brine solution) for industrial engineering. However, the interactions within the zwitterionic surface, ions and SA gel pollutants are little reported. Therefore, the investigation of ternary synergistic mechanisms is significant for the rational design of membrane. In this part, above problems will be explored by both simulations and experiments. SA is dragged by a spring force to approach the membrane surface to reveal the fouling process via steered molecular dynamics (SMD) simulations (the details are summarized in the Supporting Information). Figure 4A illustrates the dynamical process of SA approaching the membrane surface when the distance is at 3 nm, 2 nm, 1.5 nm and contacting the surface, respectively. Then the potential of mean force (PMF) can be 12 ACS Paragon Plus Environment

Page 13 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


obtained (Figure 4B) through the umbrella sampling method. For pure PVDF and zwitterionic membrane with RDMAPS = 5%, PMF is negative and decreases continuously when SA approaches membrane, which indicates the self-adsorption process of SA at the membrane surface. To the contrary, when RDMAPS is beyond 5%, PMF becomes positive and increases to a constant value as distance decreases. An energy barrier (EB) can be defined as the maximum value of PMF peak to further describe the energy profile of the adsorption process. Figure 4C displays the EB as a function of RDMAPS, showing that EB increases from the negative value to the positive one. That is to say, grafted DMAPS can resist the SA from the membrane effectively. However, there is no obvious critical grafting ratio for the evolution of EB and adsorption site with RDMAPS increases, which is far different from the hydrophilic property of the membrane surface. That is to say, the hydrophilic property is not the only reason for PVDF-g-DMAPS membrane possessing the antifouling ability. Although zwitterionic molecules are considered as neutral one because of equal positive and negative charge located on the same molecule. From above simulations, the grafted zwitterionic molecules not only modify the hydrophilic property of membrane but also may play an important role to affect the ions distribution at the membrane surface actually. 3.4 Effect of zwitterionic molecules on ions distribution at the membrane surface To detect the role of ions in the interfacial structure of membrane surface, RDF for different ions around DMAPS of membrane surface and SA (RDMAPS = 25% system) is calculated in Figure 5(A-B). The results show that positive ion has a strong tendency to enrich the zwitterionic membrane surface. Similarly, SA molecules can also induce positive ions to aggregate around. Hence, it can be found a strong peak at ~2.4 Å from the positive ions to the SA or membrane surface, which indicates the formation of a cationic layer. Besides, we also summarize the density of cations along z axis in Figure S4, three density peaks confirm that the cationic layers have formed around the DMAPS (6.5nm) surface and SA molecules (8.5nm and 11nm). For both cases, divalent cation 13 ACS Paragon Plus Environment


Page 14 of 31

(Ca2+) will have a stronger aggregate degree than the monovalent cation (Na+). The anions scarcely appear around the membrane surface or alginate molecules. RDF for water around different hydrophilic groups are also analyzed (Figure 5C) to clearly show the reason for the formation of a cationic layer. The fact can be seen that the position of RDF peak for water-N+(CH3)2 is at ~0.5 nm while that for SO32- is only about 0.24 nm, which means water gets closer to SO32-. All the ions exist at the surface in the form of a hydrated ion, hence the larger amount of water around SO32- promotes the formation of thicker ion layer at the grafted membrance surface. In other words, the formation of the cationic layer around the grafted zwitterionic surface/SA molecules also plays a key role in the antifouling process of the zwitterionic grafted membrane. Therefore, the antifouling ability of grafted membrane should originate from two different mechanisms: one is the stable hydration layer at the membrane surface that can prevent hydrophobic groups of SA molecules adhering to membrane surface; the other one is the strong electrostatic repulsion between two cationic layers around the membrane surface and SA. The synergistic effect between hydrophilic property and the cationic layer at the surface makes the PVDF-g-DMAPS membrane possess a highly efficient antifouling ability. To further confirm the effect of ionic layers on the zwitterionic membrane, SA dissolved in aqueous solution and 0.2 wt% CaCl2 brine solution are experimentally prepared, respectively. Because the antifouling property of the non-porous membrane surface in the experiment plays a decisive role in the total membrane resistance. It is reasonable to use the porous DMAPS grafted membrane to verify the antifouling results of membrane surface from the MD simulations. The concentration of SA in each solution is 100 ppm, where CDMAPS = 1 wt% DMAPS grafted membrane is taken as an example to compare with the pure PVDF membrane. As we know, alginate molecules can aggregate to form order structures in the condition of cations. For SA in CaCl2 solution, fouling degree is lower than SA without a CaCl2 solution because aggregate alginate is more hydrophilic than individual alginate. 14 ACS Paragon Plus Environment

Page 15 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Therefore, to exclude the effect of alginate structure on membrane antifouling results, a corrected coefficient (γ) is defined as: γ = A𝑝𝑏 𝐴𝑝𝑎

(2)

Where A𝑝𝑏 is antifouling parameter in CaCl2 brine solution for PVDF membrane and 𝐴𝑝𝑎 is antifouling parameter in aqueous solution for PVDF membrane. Then the theoretical antifouling values (𝐴𝑧𝑡) only considering the alginate structures for zwitterionic systems are calculated by: (3)

𝐴𝑧𝑡 = 𝐴𝑧𝑎 × 𝛾

Where 𝐴𝑧𝑎 is the antifouling parameters in aqueous solution for zwitterionic membrane. The antifouling parameters for PVDF and grafted membranes are summarized in Table 1. For 100 ppm SA solution, the total flux decline ratio (DRt) of pure PVDF membrane reaches 91.5% and the flux recovery ratio (FRR) is only 33.3%, which means the membrane fouling is very terrible. Above experimental dynamics fouling process of SA can be improved obviously when zwitterionic molecules are grafted on the membrane because of the enhanced antifouling properties. However, the antifouling results are still not desirable in aqueous solution. On the contrary, the zwitterionic membrane surface shows striking antifouling results in a brine solution. FRR can reach 99.3% which is much greater than the theoretical value. Besides, the other parameters are also superior to the theoretical value. From the above experimental result, we can certainly confirm that electrolyte can dramatically improve the antifouling properties of zwitterionic membrane surface. Although the membrane in the experiment is porous, the non-porous region in the membrane surface is still dominative. Hence, such excellent antifouling performance can be attributed to the intrinsic mechanism of the non-porous PVDF membrane via the MD simulation. 3.5 Molecular mechanism of antifouling property for zwitterionic membranes Based upon the above analysis, the ternary synergistic antifouling mechanisms among zwitterionic surface, electrolyte and pollutant sodium alginates can be summarized 15 ACS Paragon Plus Environment


Page 16 of 31

combining with the natural states of SA at the different membrane surfaces (Figure 6) and experimental results. The agglomerated structure of alginates in CaCl2 solution is illustrated in Figure 6A. It can be found that alginate gel can be freely stretched in all the directions. When alginates are located at pure PVDF membrane surface, almost all alginate molecules flat on the membrane surface. More than that, SA aggregated structure is also dramatically distorted due to the strong interaction between alginates and PVDF (Figure 6B). With the corresponding, experimental highest contact angles, the largest oil adhesion forces and lowest antifouling property can be found. When PVDF membrane is grafted by zwitterionic molecules, the hydrophilicity and electrostatic repulsion between SA and membrane will be enhanced greatly. For membrane with RDMAPS = 5% or 10%, SA starts to depart away from the membrane partially while the other part still adheres to the membrane surface, leading to a distorted vertical shape (Figure 6C-D). In these systems, experimental contact angles and the oil adhesion forces decrease while membrane antifouling property increases. As grafting ratio increases (15% and 20%, Figure 6E-F), alginate molecules totally depart away from the membrane, but the aggregated structure is still affected by the membrane surface. When the grafting ratio exceeds 20% (Figure 6G-H), the hydrophilicity of membrane surface reaches a maximum value and electrostatic interaction continues to increase. SA can depart totally from the membrane, and it can be stretched randomly on the membrane surface just like that in the environment of bulk brine solution. Meanwhile, experimental contact angle reaches a minimum value, no oil adhesion forces can be found and the membrane FRR can reach 99.3%.

4. Conclusions In brief, the antifouling mechanism of zwitterionic monomer (DMAPS) grafted PVDF membranes are explored combining MD simulations and experiments. The molecular structure and diffusivity of water at the surface are analyzed to probe the effect of DMAPS on the hydrophilic property firstly. It’s found that there exists a critical 16 ACS Paragon Plus Environment

Page 17 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


zwitterionic DMAPS grafting ratio (20%) to make the hydrophilic property stable and reach a maximum value. The critical RDMAPS should originate from the formation of a stable hydration layer at the membrane surface, which agrees well with the contact angle, and oil adhesion force from experiments. Then SMD simulations are performed to reveal the antifouling mechanism of PVDF-g-DMAPS membrane, where SA is taken as an example pollutant. The energy barrier for SA fouling on the surface increases continuously with RDMAPS increases, implying the highly efficient antifouling capability. The hydrophilic property of membrane surface and electrostatic repulsion dominate the antifouling capability. Both hydrophilicity and electrostatic interaction contribute to the membrane antifouling process with low RDMAPS, while electrostatic effect becomes the dominated factor when RDMAPS exceeds the critical ratio after forming a steady hydration layer. Due to the electrostatic repulsion, the PVDF-g-DMAPS membrane shows a great advantage to treat wastewater with electrolyte (Ca2+, Mg2+, Na+ etc.), which is verified by our controlled experiments for grafted membrane working in the pure water and CaCl2 solution. Hence, the ternary synergistic antifouling mechanisms among zwitterionic membrane, ions and pollutant sodium alginates is provided combining simulated and experimental study, which could be helpful to the rational design and preparation of new and highly efficient zwitterionic antifouling membranes.

Supporting information The details of experimental process of UV irradiation, SMD simulations, the ATR-FTIR spectrum, molecular structure of alginates in brine solution and water-membrane interfaces，the density profile of the cations are included in the supporting information.

Acknowledgment The authors thank financial support from the National Natural Science Foundation of China through Grant No. 21606245 and 21808220, the National Science and Technology Major Project through Grant No. 2016ZX05011-003, and the Beijing Natural Science Foundation through Grant No. 2184124. 17 ACS Paragon Plus Environment


Page 18 of 31

Figures and Captions:

Figure 1. (A) Initial simulated structure of alginates brine solution for PVDF-g-DMAPS system (ions are not shown in the figure). The colors are as follows: gray is for polymer skeleton; yellow and blue are for S atom and N atom of DMAPS respectively; cyan is for C atom of alginates; red and white are for O atom and H atom in all the molecules, respectively. (B) The structure of one alginate molecule with G-G-G-M-M-M residue. (C) The molecular structure of PVDF-g-DMAPS.


Page 19 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. (A) The density distribution for membrane and water at the interface, where the gray and green region represents the interfacial layer for RDMAPS = 0 and 30%, respectively. (B) The radial distribution function of the water-membrane pair for PVDF grafted with different RDMAPS. (C) The number of hydrogen bond between water and membrane as a function of the DMAPS grafting ratio, where the inset plot shows the different type of HBs including R-SO3---H, R-N---H, and R-COO---H. (D) The self-diffusive coefficient of water at the membrane surfaces along parallel and perpendicular directions, respectively.



Page 20 of 31

Figure 3. (A-D) The SEM image of the surface structure for PVDF membrane grafted with different DMAPS ratio. (E) Water contact angles and underwater oil contact angles on membrane surfaces with different DMAPS ratio. (F) The adhesion force for oil as a function of the DMAPS grafting ratio, where the inset plot shows the adsorption and desorption processes in the experiments. (G) The water flux across the membrane with different DMAPS grafting ratio.


Page 21 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. (A) The dynamical process of sodium alginate approaching the membrane surface with RDMAPS = 20 %. (B) The potential of mean force for different membrane as a function of the distance between the membrane surface and pollutant. (C) The energy barrier (EB) as a function of RDMAPS.



Page 22 of 31

Figure 5. (A-B) RDF of ions/water around DMAPS and SA for 25% DMAPS grafted membrane. (C) The RDF for water around different functional groups. (D) The illustration of the effect of cationic layers around the SA and membrane surface on the antifouling capability of the grafted membrane.


Page 23 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. Natural states of alginate molecules in the brine solution (A) and at the PVDF membrane surfaces with different RDMAPS (B-H) from MD simulations. The colors are as follows: gray is for polymer skeleton; yellow and blue are for S atom and N atom of DMAPS respectively; cyan is for C atom of alginates; red and white are for O atom and H atom in all the molecules.



Page 24 of 31

Table 1. Antifouling parameters of the PVDF-g-DMAPS (1 wt%) and PVDF membranes polluted by SA and SA brine solution PVDF

Antifouling

PVDF-g-DMAPS

parameters

SA

SA + CaCl2

γ

SA

FRR

33.3

65.5

2.0

DRt

91.5

70.5

DRr

24.8

DRir

66.7

SA + CaCl2 theoretical

actual

41.8

82.2

99.3

0.8

88.6

70.9

51.6

36.0

1.4

30.4

42.6

50.9

34.5

0.5

58.2

29.1

0.7


Page 25 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


References 1.

Ma, W. Z.; Rajabzadeh, S.; Shaikh, A. R.; Kakihana, Y.; Sun, Y. C.; Matsuyama, H. Effect of Type of Poly(Ethylene Glycol) (PEG) Based Amphiphilic Copolymer on Antifouling Properties of Copolymer/Poly(Vinylidene Fluoride) (PVDF) Blend Membranes. J. Membr. Sci. 2016, 514, 429-439.

2.

Venault, A.; Hsu, C. H.; Ishihara, K.; Chang, Y. Zwitterionic Bi-Continuous Membranes from a Phosphobetaine Copolymer/Poly(Vinylidene Fluoride) Blend via VIPS for Biofouling Mitigation. J. Membr. Sci. 2018, 550, 377-388.

3.

Chen, F. Z.; Lu, Y.; Liu, X.; Song, J. L.; He, G. J.; Tiwari, M. K.; Carmalt, C. J.; Parkin, I. P. Table Salt as a Template to Prepare Reusable Porous PVDF-MWCNT Foam for Separation of Immiscible Oils/Organic Solvents and Corrosive Aqueous Solutions. Adv. Funct. Mater. 2017, 27 (41), 1702926.

4.

Zhi, S. H.; Wan, L. S.; Xu, Z. K. Poly(Vinylidene Fluoride)/Poly(Acrylic Acid)/Calcium Carbonate Composite Membranes via Mineralization. J. Membr. Sci. 2014, 454, 144-154.

5.

Zhu, Y. B.; Yu, S. L.; Zhang, B.; Li, J. F.; Zhao, D. S.; Gu, Z. Y.; Gong, C.; Liu, G. C. Antifouling

Performance

Ultrafiltration

of

Membranes

Polytetrafluoroethylene

during

and

Polyvinylidene

Alkali/Surfactant/Polymer

Flooding

Fluoride

Wastewater

Treatment: Distinctions and Mechanisms. Sci. Total Environ. 2018, 642, 988-998. 6.

Damtie, M. M.; Kim, B.; Woo, Y. C.; Choi, J. S. Membrane Distillation for Industrial Wastewater Treatment: Studying the Effects of Membrane Parameters on the Wetting Performance. Chemosphere 2018, 206, 793-801.

7.

Sun, M.; Zucker, I.; Davenport, D. M.; Zhou, X.; Qu, J.; Elimelech, M. Reactive, Self-Cleaning

Ultrafiltration

Membrane

Functionalized

with

Iron

Oxychloride

Nanocatalysts. Environmental science & technology 2018, 52 (15), 8674-8683. 8.

Zhang, W. B.; Zhu, Y. Z.; Liu, X.; Wang, D.; Li, J. Y.; Jiang, L.; Jin, J. Salt-Induced Fabrication

of

Superhydrophilic

and

Underwater

Superoleophobic

PAA-g-PVDF

Membranes for Effective Separation of Oil-in-Water Emulsions. Angew. Chem.-Int. Edit. 25 ACS Paragon Plus Environment


Page 26 of 31

2014, 53 (3), 856-860. 9.

Mohammadali, B. D. R.; Christopher, Q. L.; Takeshi, M. Effects of Hydrophilic Silica Nanoparticles and Backing Material in Improving the Structure and Performance of VMD PVDF Membranes. Sep. Purif. Technol. 2016, 157, 60-71.

10. Yuan, T.; Meng, J. Q.; Hao, T. Y.; Wang, Z. H.; Zhang, Y. F. A Scalable Method toward Superhydrophilic and Underwater Superoleophobic PVDF Membranes for Effective Oil/Water Emulsion Separation. ACS Appl. Mater. Interfaces 2015, 7 (27), 14896-14904. 11. Akamatsu, K.; Noto, W.; Fukuzawa, H.; Hara, A.; Nakao, S. Grafting of Carboxybetaine Polymers to Polyethylene Membranes via Plasma Graft Polymerization to Improve Low-Fouling Properties and to Tune the Molecular Weight Cut-Off. Sep. Purif. Technol. 2018, 204, 298-303. 12. Piatkovsky, M.; Acar, H.; Marciel, A. B.; Tirrell, M.; Herzberg, M. A Zwitterionic Block-Copolymer, Based on Glutamic Acid and Lysine, Reduces the Biofouling of UF and RO Membranes. J. Membr. Sci. 2018, 549, 507-514. 13. Dizon, G. V.; Venault, A. Direct in-Situ Modification of PVDF Membranes with a Zwitterionic Copolymer to form Bi-Continuous and Fouling Resistant Membranes. J. Membr. Sci. 2018, 550, 45-58. 14. Liu, Z. Y.; Xu, Z. C.; Zhou, H.; Wang, Y. L.; Liao, Q.; Zhang, L.; Zhao, S. Interfacial Behaviors of Betaine and Binary Betaine/Carboxylic Acid Mixtures in Molecular Dynamics Simulation. J. Mol. Liq. 2017, 240, 412-419. 15. Song, W. L.; Li, Z. P.; Li, Y. Z.; You, H.; Qi, P. S.; Liu, F.; Loy, D. A. Facile Sol-Gel Coating Process for Anti-Biofouling Modification of Poly (Vinylidene Fluoride) Microfiltration Membrane Based on Novel Zwitterionic Organosilica. J. Membr. Sci. 2018, 550, 266-277. 16. Kitano, H.; Kawasaki, A.; Kawasaki, H.; Morokoshi, S. Resistance of Zwitterionic Telomers Accumulated on Metal Surfaces against Nonspecific Adsorption of Proteins. J. Colloid Interface Sci. 2005, 282 (2), 340-348. 26 ACS Paragon Plus Environment

Page 27 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


17. Shamsayei, M.; Yamini, Y.; Asiabi, H. Fabrication of Zwitterionic Histidine/Layered Double Hydroxide Hybrid Nanosheets for Highly Efficient and Fast Removal of Anionic Dyes. J. Colloid Interface Sci. 2018, 529, 255-264. 18. Lim, C. M.; Seo, J.; Jang, H.; Seo, J. H. Optimizing Grafting Thickness of Zwitterionic Sulfobetaine Polymer on Cross-Linked Polyethylene Surface to Reduce Friction Coefficient. Appl. Surf. Sci. 2018, 452, 102-112. 19. Li, Z. G.; Hu, W. H.; Zhao, Y. H.; Ren, L. X.; Yuan, X. Y. Integrated Antibacterial and Antifouling Surfaces via Cross-Linking Chitosan-G-Eugenol/Zwitterionic Copolymer on Electrospun Membranes. Colloid Surf. B-Biointerfaces 2018, 169, 151-159. 20. Ozcan, S.; Kaner, P.; Thomas, D.; Cebe, P.; Asatekin, A. Hydrophobic Antifouling Electrospun Mats from Zwitterionic Amphiphilic Copolymers. ACS Appl. Mater. Interfaces 2018, 10 (21), 18300-18309. 21. Bengani-Lutz, P.; Zaf, R. D.; Culfaz-Emecen, P. Z.; Asatekin, A. Extremely Fouling Resistant Zwitterionic Copolymer Membranes with Similar to 1 nm Pore Size for Treating Municipal, Oily and Textile Wastewater Streams. J. Membr. Sci. 2017, 543, 184-194. 22. Zhang, W.; Yang, Z.; Kaufman, Y.; Bernstein, R. Surface and Anti-Fouling Properties of a Polyampholyte Hydrogel Grafted onto a Polyethersulfone Membrane. J. Colloid Interface Sci. 2018, 517, 155-165. 23. Duong, P. H. H.; Daumann, K.; Hong, P. Y.; Ulbricht, M.; Nunes, S. P. Interfacial Polymerization of Zwitterionic Building Blocks for High-Flux Nanofiltration Membranes. Langmuir 2018, 10.1021/acs.langmuir.8b00960. 24. Wang, J.; Xiao, T. T.; Bao, R. Y.; Li, T.; Wang, Y. Q.; Li, D. X.; Li, X. M.; He, T. Zwitterionic Surface Modification of Forward Osmosis Membranes using N-Aminoethyl Piperazine Propane Sulfonate for Grey Water Treatment. Process Saf. Environ. Protect. 2018, 116, 632-639. 25. Bengani-Lutz, P.; Converse, E.; Cebe, P.; Asatekin, A. Self-Assembling Zwitterionic Copolymers as Membrane Selective Layers with Excellent Fouling Resistance: Effect of 27 ACS Paragon Plus Environment


Page 28 of 31

Zwitterion Chemistry. ACS Appl. Mater. Interfaces 2017, 9 (24), 20859-20872. 26. Han, G.; Liu, J. T.; Lu, K. J.; Chung, T. S. Advanced Anti-Fouling Membranes for Osmotic Power Generation from Wastewater via Pressure Retarded Osmosis (PRO). Environ. Sci. Tech. 2018, 52 (11), 6686-6694. 27. Ridgway, H. F.; Orbell, J.; Gray, S. Molecular Simulations of Polyamide Membrane Materials Used in Desalination and Water Reuse Applications: Recent Developments and Future Prospects. J. Membr. Sci. 2017, 524, 436-448. 28. Shen, M.; Keten, S.; Lueptow, R. M. Dynamics of Water and Solute Transport in Polymeric Reverse Osmosis Membranes via Molecular Dynamics Simulations. J. Membr. Sci. 2016, 506, 95-108. 29. Takizawa, Y.; Inukai, S.; Araki, T.; Cruz-Silva, R.; Uemura, N.; Morelos-Gomez, A.; Ortiz-Medina, J.; Tejima, S.; Takeuchi, K.; Kawaguchi, T.; Noguchi, T.; Hayashi, T.; Terrones,

M.;

Endo,

M.

Antiorganic

Fouling

and

Low-Protein

Adhesion

on

Reverse-Osmosis Membranes Made of Carbon Nanotubes and Polyamide Nanocomposite. ACS Appl. Mater. Interfaces 2017, 9 (37), 32192-32201. 30. Usta, M.; Morabito, M.; Anqi, A.; Alrehili, M.; Hakim, A.; Oztekin, A. Twisted Hollow Fiber Membrane Modules for Reverse Osmosis-Driven Desalination. Desalination 2018, 441, 21-34. 31. Dick, V. V.; Klein, P. Molecular Simulation of the Hydrodynamics of Water in Contact with Hydrophilized Poly(Vinylidene Fluoride) Surfaces. J. Colloid Interface Sci. 2014, 432, 70-76. 32. Huo, J.; Chen, Z.; Zhou, J. Zwitterionic Membrane via Nonsolvent Induced Phase Separation: A Computer Simulation Study. Langmuir , 2019, 35(5),1973-1983. 33. Ling, C. J.; Liang, X. Y.; Fan, F. C.; Yang, Z. Diffusion Behavior of the Model Diesel Components in Different Polymer Membranes by Molecular Dynamic Simulation. Chem. Eng. Sci. 2012, 84, 292-302. 34. Zhou, Z.; Rajabzadeh, S.; Shaikh, A. R.; Kakihana, Y.; Ma, W. Z.; Matsuyama, H. Effect of 28 ACS Paragon Plus Environment

Page 29 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Surface Properties on Antifouling Performance of Poly(Vinyl Chloride-Co-Poly(Ethylene Glycol)Methyl Ether Methacrylate)/PVC Blend Membrane. J. Membr. Sci. 2016, 514, 537-546. 35. Ohkubo, T.; Kidena, K.; Takimoto, N.; Ohira, A. Molecular Dynamics Simulations of Nafion and Sulfonated Poly Ether Sulfone Membranes II. Dynamic Properties of Water and Hydronium. J. Mol. Model. 2012, 18 (2), 533-540. 36. Shaikh, A. R.; Rajabzadeh, S.; Matsuo, R.; Takaba, H.; Matsuyama, H. Hydration Effects and Antifouling Properties of Poly(Vinyl Chloride-Co-PEGMA) Membranes Studied Using Molecular Dynamics Simulations. Appl. Surf. Sci. 2016, 369, 241-250. 37. Xiang, Y.; Xu, R. G.; Leng, Y. S. Molecular Simulations of the Hydration Behavior of a Zwitterion Brush Array and Its Antifouling Property in an Aqueous Environment. Langmuir 2018, 34 (6), 2245-2257. 38. Myat, D. T.; Stewart, M. B.; Mergen, M.; Zhao, O.; Orbell, J. D.; Gray, S. Experimental and Computational Investigations of the Interactions Between Model Organic Compounds and Subsequent Membrane Fouling. Water Res. 2014, 48, 108-118. 39. Shaikh, A. R.; Karkhanechi, H.; Yoshioka, T.; Matsuyama, H.; Takaba, H.; Wang, D. M. Adsorption of Bovine Serum Albumin on Poly(vinylidene fluoride) Surfaces in the Presence of Ions: A Molecular Dynamics Simulation. J. Phys. Chem. B 2018, 122 (6), 1919-1928. 40. Liu, J. X.; Wang, Z. H.; Tang, C. Y.; Leckie, J. O. Modeling Dynamics of Colloidal Fouling of RO/NF Membranes with A Novel Collision-Attachment Approach. Environmental Science & Technology 2018, 52 (3), 1471-1478. 41. Liu, C.; Chen, L.; Zhu, L. Fouling Behavior of Lysozyme on Different Membrane Surfaces during the MD Operation: An Especial Interest in the Interaction Energy Evaluation. Water Res. 2017, 119, 33-46. 42. Zhang, H.; Hu, L. M.; Lin, C. G.; Wang, L.; Yuan, S. L. Molecular Dynamics Simulation of Interaction between Lysozyme and Non-fouling Polymer Membranes. Acta Polym. Sin. 2014,

(1), 99-106. 29 ACS Paragon Plus Environment


Page 30 of 31

43. Xiang, Y.; Xu, R. G.; Leng, Y. S. Molecular Dynamics Simulations of a Poly(ethylene glycol)-Grafted Polyamide Membrane and Its Interaction with a Calcium Alginate Gel. Langmuir 2016, 32 (18), 4424-4433. 44. Stewart, M. B.; Myat, D. T.; Kuiper, M.; Manning, R. J.; Gray, S. R.; Orbell, J. D. A Structural Basis for the Amphiphilic Character of Alginates-Implications for Membrane Fouling. Carbohydr. Polym. 2017, 164, 162-169. 45. Hua, H.; Xiong, Y.; Fu, C.; Li, N. PH-Sensitive Membranes Prepared with Poly(Methyl Methacrylate) Grafted Poly(Vinylidene Fluoride) via Ultraviolet Irradiation-Induced Atom Transfer Radical Polymerization. RSC Adv. 2014, 4 (74), 39273-39279. 46. Luo, N.; Xu, R.; Yang, M.; Yuan, X.; Zhong, H.; Fan, Y. Preparation and Characterization of PVDF-Glass Fiber Composite Membrane Reinforced by Interfacial UV-Grafting Copolymerization. J. Environ. Sci-China 2015, 38, 24-35. 47. Van der Spoel, D.; Lindahl, E.; Hess, B.; Groenhof, G.; Mark, A. E.; Berendsen, H. J. C. GROMACS: Fast, Flexible, and Free. J. Comput. Chem. 2005, 26 (16), 1701-1718. 48. Ameduri, B. From Vinylidene Fluoride (VDF) to the Applications of VDF-Containing Polymers and Copolymers: Recent Developments and Future Trends. Chem. Rev. 2009, 109 (12), 6632-6686. 49. Cheng, G.; Liao, M. R.; Zhao, D. H.; Zhou, J. Molecular Understanding on the Underwater Oleophobicity of Self Assembled Monolayers: Zwitterionic versus Nonionic. Langmuir 2017, 33 (7), 1732-1741. 50. Liao, M. R.; Cheng, G.; Zhou, J. Underwater Superoleophobicity of Pseudozwitterionic SAMs: Effects of Chain Length and Ionic Strength. J. Phys. Chem. C 2017, 121 (32), 17390-17401. 51. Ge, J. L.; Zong, D. D.; Jin, Q.; Yu, J. Y.; Ding, B. Biomimetic and Superwettable Nanofibrous Skins for Highly Efficient Separation of Oil-in-Water Emulsions. Adv. Funct. Mater. 2018, 28 (10), 10.


Page 31 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table of Contents


Understanding the Antifouling Mechanism of Zwitterionic Monomer

Recommend Documents