Effect of surface grafting on the oil-water mixture passing through a nanoslit: A Molecular Dynamics Simulation Study

Effect of surface grafting on the oil-water mixture passing through a nanoslit: A Molecular Dynamics Simulation Study | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Effect of surface grafting on the oil-water mixture passing through a nanoslit: A Molecular Dynamics Simulation Study Wende Tian, Yanwei Wang, Zhexenbek Toktarbay This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5098627/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 08 Nov, 2024 Read the published version in Advanced Composites and Hybrid Materials → Version 1 posted 10 You are reading this latest preprint version Abstract Graphene oxide-based membranes hold great promise in composite materials for applications such as wastewater treatment and oil-water separation. In this study, we employed molecular dynamics simulations to investigate the separation of water from an oil-water mixture using a two-layer graphene oxide membrane. We explored the effects of random and stripe-like grafting patterns on penetration efficiency, focusing on varying grafting densities. Our results show that increasing grafting density reduces flux and permeability of both oil and water molecules, highlighting the critical role of surface functionalization in membrane design. Notably, the stripe grafting pattern significantly enhances penetration efficiency by optimizing steric interactions around the nanoslit. These findings contribute to the development of nanocomposite materials and surface modification techniques, offering insights into the design of high-performance membranes for oil-water separation. Understanding the relationship between grafting density, surface patterning, and membrane performance is crucial for advancing hybrid materials that address industrial challenges such as wastewater treatment and oil spill remediation. The insights gained from this study can be further refined by exploring different functional groups and surface modifications, broadening the applications of these membranes in industrial separation processes. Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 1 Introduction Massive amounts of oily wastewater are continuously generated due to the rapid development of industry and daily life. For example, global wastewater production is estimated to be around 250 million barrels per day [ 1 ]. Therefore, there is a significant demand for advanced treatment methods for oily wastewater [ 1 ]. Several methods [ 2 , 3 ] exist for water-oil separation, including distillation, electro-dialysis, solvent extraction, reverse osmosis, and carbon nanotubes [ 4 ]. Membrane technology offers unique advantages in treating oily wastewater due to its high separation efficiency and low energy cost [ 5 ]. Two-dimensional graphene oxide (GO)-based membranes show great promise in wastewater treatment due to their high flux [ 6 ]. In experimental studies, a group of researchers used a metal mesh membrane coated with graphene oxide nanoparticles to separate water from oil [ 7 , 8 ]. They found that stainless steel mesh [ 9 ] coated with nano-graphene oxide particles was hydrophilic when exposed to air but super-oleophobic when immersed in a water environment [ 7 ]. The pore diameter of graphene is a significant factor in the water flux in the membrane; membranes with hydrophilic pores show greater water flux than those with hydrophobic pores [ 10 ]. Hydrophilic pores also increase the probability of hydrogen bonding [ 11 ] during passage, causing water molecules to pass through. Recently, two parallel graphene layers with pores were used for water purification [ 12 ]. The results showed that the interlayer spacing affects water flux, with intervals of about 5 Å resulting in almost zero water flux, while larger distances increased the flux of water [ 13 ]. The major obstacle for the membrane separation technique is the fouling issue [ 14 ]. Spreadable oil droplets are prone to coalesce and spread on membrane surfaces, leading to severe membrane fouling and lower separation efficiency [ 15 ]. Tailoring membrane surfaces to mitigate severe fouling at high flux is, therefore, a critical challenge in developing membranes for oil-water separation [ 2 ]. Recently, an amphiphilic GO membrane was constructed to regulate interfacial interactions with oil droplets and achieve ultralow fouling at high fluxes [ 16 ]. However, the impact of surface modification on water flux is not yet fully understood, and molecular-scale information is lacking. The energetic barrier of hydration layers induced by the hydrophilic domains resists the spread of pollutants, while the low surface energy of the hydrophobic domains facilitates the release of pollutants [ 17 ]. Nonpolar hydrophobic domains used to enhance fouling-release ability usually reduce surface hydration [ 16 ]. Despite these advances, there are still few simulations studying the effect of grafting density and patterns. In this study, we perform molecular dynamics (MD) simulations to investigate the effect of surface modification of GO membranes on water and oil flux. The surface of the GO membrane was grafted with perfluoroalkyl chains at various fractional areas. Perfluoroalkyl chains are hydrophobic and favor the release of oils. We focus on the effect of grafting area fractions and the arranging patterns. Especially, two types of grafting patterns are studied: random and stripe. We found that the flux and permeability of oil and water molecules decrease with increasing grafting density. However, the penetration efficiency also depends on the grafting pattern. The random pattern is lower in efficiency than that of the stripe pattern due to the steric hindrance of inclined molecules around the nanoslit. Our findings are helpful to design more effective oil-water separation membranes through surface modification. 2 Model and Methods The schematic of the simulation box is shown in Fig. 1 . One layer of graphene oxide (GO, shown in red) and one layer of graphene (shown in yellow) are used as a membrane located in the simulation box. The gap between them is about 1 nm. Each layer has a nano-slit with a width of 0.8 nm. The shift distance between the two nanoslits is 1 nm. The surface of the GO is randomly modified by perfluorocarboxylic acid (CF₃CF₂CF₂CF₂COOH, shown in green) with the -OH group close to the surface. On the right side of the membrane, there is a mixture of water and oil containing hexane molecules (C 4 H₁₄). Water molecules are transparent, and hexane molecules appear as blue points. The numbers of water and oil molecules are 5000 and 64, respectively. The oil-water mixture was kept constant in the initial configuration in all systems. Two graphene sheets, shown in azure, are used as pistons, one of which is behind the water and oil emulsion. The simulation box is periodic only in the x and y directions, while the z direction is non-periodic. Two arrows indicate the direction of the applied atmospheric pressure. Different numbers of perfluoroalkyl chains (20, 30, 42) were grafted on the neutral or negatively charged surface with a charge of -10e. For the negatively charged surface, 10 Na + ions were added to the box. We mainly focused on the random modification, but systems with stripe arrangements of perfluoroalkyl chains on the surface were also studied for comparison. All systems we performed are listed in Table 1 . Table 1 Summary of the simulation systems showing the number of perfluoroalkyl chains grafted on neutral and negatively charged graphene oxide surfaces. Systems are categorized by grafting pattern (random or stripe) and by the density of grafted chains (zero, moderate, and high density). The surface charge (neutral or negatively charged) and arrangements of grafted chains are explored to study their effects on the behavior of the water-oil mixture in the simulation box. Number of grafted chains Zero Moderate density High density 0 a 20 30 42 Random Neutral b √ √ √ √ Negative √ √ √ √ Stripe Neutral - √ √ √ Negative - √ √ √ a Values present the number of grafted molecules, which were also used to indicate the corresponding systems. b The neutral and negative indicate the surface electricity. The negatively charged surface is with charge of -10e. Our MD simulations were performed using the LAMMPS package [ 18 ]. The box size is 3.9 nm x 3.9 nm in the X and Y directions. The entire simulation was carried out at a fixed volume and constant temperature using the NVT statistical ensemble. To stabilize the temperature at 300 K, the Nose-Hoover thermostat was used with a decay ratio of 0.1 ps⁻¹ [ 19 ]. The Verlet algorithm was used to solve Newton's equations of motion for the particles at each time step of 1 fs. The cutoff radius for both the Coulombic and Lennard-Jones energy was set to 12.0 Å. Each simulation was performed for 2 ns with 1 bar applied on both sides for equilibrium, followed by 8 ns for data production with 200 bar on the right side. In this simulation, the SPC/E model was used for water molecules, and the OPLS force field was used to describe bond, angular, dihedral, van der Waals, and electrostatic interactions between hexane hydrocarbon molecules [ 20 ]. To describe the interactions between atoms in graphene oxide and the graphene piston, the force field developed by Cheng and Steele [ 21 ] was used. The PPPM method was used to correct the potential of electrostatic interactions of Coulombic and long-range Lennard-Jones interactions. Using the SHAKE algorithm [ 22 ], the O-H bond length and H-O-H angle of water molecules were kept constant at 1.0 Å and 104.59°, respectively. 3 Results and discussions We first focus on the randomly grafting systems. The distribution of oil, brush, and water in the equilibrium state without a pressure difference in the Z direction is given in Fig. 2 . The height of the brush is about 1.1 nm for all cases. With the increase in grafted density, the distribution profile of the brush slightly broadens in the Z direction due to steric interactions. It can also be found that neutral surface favor the brush due to the hydrophobic effect. The oil distribution is broad. There is more oil in the brush layer for cases of low grafting densities because of the greater free space near the surface, as seen in the spatial density map of the brush (bottom, Fig. 2 ). For higher grafted density, the oils are mainly distributed ~ 1.0nm far from the surface, close to the brush terminal. It can be observed that there are more perfluorocarboxylic acid molecules around the nanoslit for higher grafted densities. There is a water layer near the surface, indicated by the first peaks of water density. Additionally, there appears to be more water on the negatively charged surface compared to the neutral one due to electrostatic-dipole interactions. Further, the distributions of water close to the brush terminals shows no significant difference. It should be pointed out that we do not observe significant aggregation of oil molecules at the terminal. This is in good agreement with the finding by Yang et al. [ 16 ], who studied the antifouling effect of grafted membrane. Now we turn to the oil and water penetration behavior. Special systems without grafted perfluorocarboxylic acids were also included for comparison. The number of oil and water molecules passing through the membrane as a function of time is calculated and shown in Fig. 3 to evaluate the performance at different grafting densities. It can be seen that, without surface modification, oil and water penetrate through the nanoslit quickly. When the surface is grafted with perfluorocarboxylic acids, the penetration efficiency of oil decreases. High grafting density is more effective at blocking oil penetration (Fig. 3 a). The behavior is similar for the penetration of water (Fig. 3 b). Qualitatively, there is no significant difference between neutral and negatively charged surfaces. However, water molecules are not stuck at high grafting densities due to their small size. It appears that negatively charged modification is better for water penetration due to its high hydration effect. The efficiency of the flow of oil and water molecules through the channels of various systems is shown in Fig. 4 . Efficiency is defined as the ratio of the number of molecules (water or oil) passing through the second nanoslit within 6 ns. It can be observed that without modification, almost all oil and water molecules penetrate through the membrane regardless of surface charge. Moderate grafting density nearly blocks half of the molecules. For high grafting density, the efficiency is close to zero for the neutral surface. For the negatively charged surface, the efficiency for water is about 10%. This result is consistent with the number of penetrated molecules. To study the effect of the grafting pattern, we also designed a stripe-like structure on the surface. The Z-direction density profile in the equilibrium state is similar to that of the random system (see Fig. 5 a and Fig. 2 c). The distribution of grafted molecules is relatively regular, with almost no grafted molecules tilting in the nanoslit region (Fig. 5 b). Qualitatively, the penetration efficiency of oil and water decreases as the grafting density increases (Fig. 5 c). About 80% of the molecules penetrate through the nanoslit for all cases we studied (Fig. 5 d). Surprisingly, it is obvious that the penetration efficiency at high density is larger than that of the random system (Fig. 4 and Fig. 5 c). For moderate grafting density, the efficiency is also slightly higher than that of the random system. This implies that grafting pattern plays a crucial role in penetration efficiency. To explore the mechanism behind the different efficiencies at high density, we carefully examined the water penetration process by analyzing the simulation trajectories. The tilt of grafted molecules plays a crucial role in blocking oil and water. To quantitatively compare the degree of tilting, we defined a region of 1 nm × 4 nm × 3.9 nm above the nanoslit (Fig. 6 a) and counted the average number of carbon atoms of perfluorocarboxylic acids in this region for the negatively charged systems (Fig. 6 b) using data from the last 2 ns of the trajectories. It can be observed that the number of carbon atoms varies with different patterns. At high density, the carbon atom count for the random pattern is higher than that for the stripe pattern. The time evolution of the carbon atoms in the region is shown in Fig. 6 c. It can be seen that the grafted molecules close to the nanoslit tilt more quickly for the random pattern, whereas for the stripe pattern, the carbon atom number increases gradually. This difference may be due to the varying distances between grafted molecules and the nanoslit. The tilt of grafted molecules depends not only on their stiffness, but on the hydrodynamic flows in the nanoslit due to the pressure difference. To further investigate the differences between the two systems, we also plotted the spatial density maps of the randomly and stripe-like brush (Fig. 7 ). Evidently, there are more grafted molecules distributed in the nanoslit region for the random pattern. When designing membranes for oil-water separation, key properties such as pore size, slit size, and interlayer spacing are critical for determining the separation efficiency [ 1 ]. Given that filtration is primarily a physical process, these parameters must be carefully controlled to prevent contaminants from passing through the membrane. Control over these dimensions allows for enhanced separation performance, especially when functional groups are incorporated into the membrane structure [ 23 ]. The addition of functional groups can dramatically alter membrane behavior by modifying its interactions with water and oil molecules, enhancing specific properties such as water permeability, ion rejection, and pore polarity. This is particularly important for composite materials, where functionality and performance can be tailored for specific applications. For instance, membranes with superhydrophobic-superoleophilic or superhydrophilic-superoleophobic properties have proven highly effective for oil-water separation [ 24 ]. These materials either filter molecules by blocking their passage or selectively absorb them, depending on the membrane’s surface chemistry. Hydrophilic functional groups typically enhance water permeability by facilitating water passage through the membrane, whereas hydrophobic groups tend to reduce permeability by repelling water molecules and decreasing membrane wettability. This dual behavior enables a more tailored approach to membrane design, making functionalization an essential component of next-generation nanocomposite materials. Our findings demonstrate that grafting density plays a crucial role in determining the performance of GO-based membranes. As grafting density increases, the surface becomes more densely covered with functional groups, which can directly affect the membrane's hydrophilicity and interactions with water and oil molecules. At higher densities, steric hindrance is introduced, leading to a more densely packed surface layer that blocks the passage of molecules. This results in reduced permeability, which is consistent with prior studies showing that increased surface coverage can impede transport through the membrane [ 16 ]. Moreover, our study reveals that the nanoscale pattern of functional groups significantly impacts membrane performance. A random grafting pattern creates a heterogeneous surface with unevenly distributed functional groups, leading to varying interactions with water and oil molecules. In contrast, stripe-like patterns produce a more uniform distribution, which optimizes the interaction between the membrane and the permeating molecules, thereby enhancing separation efficiency. This suggests that the design of the grafting pattern is as important as the grafting density in optimizing membrane performance. The differences in penetration efficiency due to grafting patterns are primarily driven by steric effects, where the spatial arrangement of functional groups around the nanoslit influences the ability of molecules to pass through. The physical arrangement of these groups can either block or facilitate the passage of water and oil molecules through the membrane. Additionally, hydrodynamic interactions during non-equilibrium transport processes can cause the grafted molecules to tilt or reorient, further affecting separation efficiency. This tilting behavior, which varies between random and stripe patterns, emphasizes the role of both molecular interactions and fluid dynamics in membrane functionality. 4 Conclusion Graphene oxide-based membranes have shown great potential in composite materials for applications ranging from wastewater treatment to oil-water separation. In the present work, we have studied the separation of water from an oil-water mixture using a two-layer graphene oxide membrane through molecular dynamics simulations. We focus on the effects of random grafting density on penetration efficiency and compare this with a stripe-like grafting pattern. Our simulation results demonstrate that increasing grafting density leads to decreased flux and permeability of oil and water molecules, emphasizing the critical role of surface functionalization in composite membrane design. Furthermore, the grafting pattern, particularly the stripe configuration, significantly enhances penetration efficiency by optimizing steric interactions around the nanoslit. These findings contribute to the advancement of nanocomposite materials and surface modification techniques, offering insights into the design of high-performance membranes for oil-water separation. Understanding the interplay between grafting density, surface patterning, and membrane performance is crucial for developing advanced hybrid materials that can address industrial challenges such as wastewater treatment and oil spill remediation. Future research could explore the integration of alternative functional groups and patterns, as well as the real-world application of these membranes in large-scale industrial processes. Such advancements would not only improve the efficiency of separation technologies but also broaden the scope of composite materials in environmental and industrial applications. Declarations Author contribution Wende Tian wrote the main manuscript text and did all the simulation works; Yanwei Wang wrote some part of the manuscript text and revised the draft; Zhexenbek Toktarbay did experiments conceptualization, general management, and manuscript revision. Funding This work was supported by the Ministry of Science and Higher Education of the Republic of Kazakhstan under the project AP19679745 “Design of mechanically strong, biodegradable membrane for separation process”. Competing interests: The authors declare no competing interests. Ethical Approval: not applicable. References Kommu A and Singh JK (2020) A Review on Graphene-Based Materials for Removal of Toxic Pollutants from Wastewater, Soft Materials 18:297-322. https://doi.org/10.1080/1539445x.2020.1739710 Lejars M, Margaillan A, and Bressy C (2012) Fouling Release Coatings: A Nontoxic Alternative to Biocidal Antifouling Coatings, Chem. Rev. 112:4347-4390. https://doi.org/10.1021/cr200350v Rana D and Matsuura T (2010) Surface Modifications for Antifouling Membranes, Chem. Rev. 110:2448–2471. https://doi.org/10.1021/cr800208y Fornasiero F, Park HG, Holt JK, Stadermann M, Grigoropoulos CP, Noy A, and Bakajin O (2008) Ion Exclusion by Sub-2-Nm Carbon Nanotube Pores, Proc. Natl. Acad. 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Cite Share Download PDF Status: Published Journal Publication published 08 Nov, 2024 Read the published version in Advanced Composites and Hybrid Materials → Version 1 posted Editorial decision: Revision requested 08 Oct, 2024 Reviews received at journal 08 Oct, 2024 Reviews received at journal 06 Oct, 2024 Reviewers agreed at journal 06 Oct, 2024 Reviewers agreed at journal 05 Oct, 2024 Reviewers agreed at journal 03 Oct, 2024 Reviewers invited by journal 03 Oct, 2024 Editor assigned by journal 01 Oct, 2024 Submission checks completed at journal 27 Sep, 2024 First submitted to journal 16 Sep, 2024 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-5098627","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":363681207,"identity":"a05bce95-a51f-49c2-bc45-595741cd49e3","order_by":0,"name":"Wende Tian","email":"","orcid":"","institution":"Soochow University","correspondingAuthor":false,"prefix":"","firstName":"Wende","middleName":"","lastName":"Tian","suffix":""},{"id":363681208,"identity":"bc04a01d-a8e4-470d-b02c-39426c749ee0","order_by":1,"name":"Yanwei Wang","email":"","orcid":"","institution":"Nazarbayev University","correspondingAuthor":false,"prefix":"","firstName":"Yanwei","middleName":"","lastName":"Wang","suffix":""},{"id":363681209,"identity":"96fc897b-90b2-4089-b08e-074108e0cfc2","order_by":2,"name":"Zhexenbek Toktarbay","email":"data:image/png;base64,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","orcid":"","institution":"Satbayev University","correspondingAuthor":true,"prefix":"","firstName":"Zhexenbek","middleName":"","lastName":"Toktarbay","suffix":""}],"badges":[],"createdAt":"2024-09-16 16:12:11","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5098627/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5098627/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s42114-024-01055-6","type":"published","date":"2024-11-08T15:57:57+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":68164257,"identity":"55a27346-3c8f-45c9-a044-379a01131d20","added_by":"auto","created_at":"2024-11-04 09:28:08","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":589993,"visible":true,"origin":"","legend":"\u003cp\u003eA schematic illustration of the simulation box. Blue beads represent oil molecules, green brush, red graphene oxide (GO), yellow graphene. There are two walls for applying pressure of 1.0 and 200 bar, respectively.\u003c/p\u003e","description":"","filename":"Fig1.png","url":"https://assets-eu.researchsquare.com/files/rs-5098627/v1/4b84d2f8902a73e6059b0604.png"},{"id":68164474,"identity":"d5bdd2b8-ab03-4b60-b108-604eab9200aa","added_by":"auto","created_at":"2024-11-04 09:36:08","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":203697,"visible":true,"origin":"","legend":"\u003cp\u003eDensity profiles of random systems with moderate (20, a) and high (42, b) grafting densities. The top shows density distribution of oil, brush, and water along z-axis. The bottom shows spatial density map of brush on the surface. The red rectangles denote the region of nanoslit. The color bar is monomer number per nm\u003csup\u003e2\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"Fig2.png","url":"https://assets-eu.researchsquare.com/files/rs-5098627/v1/e77a94567b49380138753e7f.png"},{"id":68164258,"identity":"c3e54ad1-e8ec-4b17-8a43-f85bc35123e3","added_by":"auto","created_at":"2024-11-04 09:28:08","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":154321,"visible":true,"origin":"","legend":"\u003cp\u003eThe penetrated numbers of oil (a) and water (b) passing through the first nanoslit as a function of time at a pressure of 200 bar. The colorful regions are error bars.\u003c/p\u003e","description":"","filename":"Fig3.png","url":"https://assets-eu.researchsquare.com/files/rs-5098627/v1/f602b8836e2da8c6902e1ebd.png"},{"id":68164265,"identity":"33387ea5-e75c-4a63-a497-2dacaa786a1e","added_by":"auto","created_at":"2024-11-04 09:28:09","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":34359,"visible":true,"origin":"","legend":"\u003cp\u003eEfficiency of water and oil molecules passed through the channel for neutrally (black number)- and negatively (red number)- charged surface.\u003c/p\u003e","description":"","filename":"Fig4.png","url":"https://assets-eu.researchsquare.com/files/rs-5098627/v1/c809af9b5314dce6605653e0.png"},{"id":68164475,"identity":"fb993b0e-f369-4ac1-849b-18b2b2335af7","added_by":"auto","created_at":"2024-11-04 09:36:09","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":195323,"visible":true,"origin":"","legend":"\u003cp\u003eDensity profiles of stripe systems of high grafting density (system 42). Density distribution of oil, brush, and water along z-axis (a) and spatial density map (b) of brush on the surface. The red rectangles denote the region of nanoslit. The color bar is monomer number per nm\u003csup\u003e2 \u003c/sup\u003e.\u003csup\u003e \u003c/sup\u003eThe penetrated numbers of water (c) passing through the first nanoslit as a function of time at a pressure of 200 bar. Efficiency (d) of water and oil molecules passed through the channel for neutrally- and negatively- charged surface.\u003c/p\u003e","description":"","filename":"Fig5.png","url":"https://assets-eu.researchsquare.com/files/rs-5098627/v1/0b68d8ed4b859dbf27dc9098.png"},{"id":68164263,"identity":"f82af5ac-4784-4802-8a96-23a7369e822f","added_by":"auto","created_at":"2024-11-04 09:28:09","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":297431,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Typical snapshot of oil and water passing through the nanoslit for the negatively-charged system at high density. The black box is eye-guiding region for counting C-atom number of perfluorocarboxylic acids. (b) The C-atom number for the negatively-charged system at various density at 6ns. (c) The time evolution of C-atom number for the negatively-charged system at high density.\u003c/p\u003e","description":"","filename":"Fig6.png","url":"https://assets-eu.researchsquare.com/files/rs-5098627/v1/81c7b36ae6da75ff3b0288fe.png"},{"id":68164261,"identity":"b990397c-5027-4c80-8d33-d2bcbe73e1a4","added_by":"auto","created_at":"2024-11-04 09:28:09","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":288631,"visible":true,"origin":"","legend":"\u003cp\u003eComparison of spatial density map of (a) randomly- and (b) stripe-like brush on the surface. The red rectangles denote the region of nanoslit. The color bar is monomer number per nm\u003csup\u003e2 \u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"Fig7.png","url":"https://assets-eu.researchsquare.com/files/rs-5098627/v1/5445cde2558692e1fde94cf5.png"},{"id":68750142,"identity":"1624ed95-b1b7-411f-a07e-c361b7a1be01","added_by":"auto","created_at":"2024-11-11 16:11:05","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1896081,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5098627/v1/2e7c33fb-b40d-4ea4-b20f-789538c069bc.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Effect of surface grafting on the oil-water mixture passing through a nanoslit: A Molecular Dynamics Simulation Study","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003eMassive amounts of oily wastewater are continuously generated due to the rapid development of industry and daily life. For example, global wastewater production is estimated to be around 250\u0026nbsp;million barrels per day [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. Therefore, there is a significant demand for advanced treatment methods for oily wastewater [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. Several methods [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e] exist for water-oil separation, including distillation, electro-dialysis, solvent extraction, reverse osmosis, and carbon nanotubes [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. Membrane technology offers unique advantages in treating oily wastewater due to its high separation efficiency and low energy cost [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. Two-dimensional graphene oxide (GO)-based membranes show great promise in wastewater treatment due to their high flux [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. In experimental studies, a group of researchers used a metal mesh membrane coated with graphene oxide nanoparticles to separate water from oil [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. They found that stainless steel mesh [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e] coated with nano-graphene oxide particles was hydrophilic when exposed to air but super-oleophobic when immersed in a water environment [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. The pore diameter of graphene is a significant factor in the water flux in the membrane; membranes with hydrophilic pores show greater water flux than those with hydrophobic pores [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. Hydrophilic pores also increase the probability of hydrogen bonding [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e] during passage, causing water molecules to pass through. Recently, two parallel graphene layers with pores were used for water purification [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. The results showed that the interlayer spacing affects water flux, with intervals of about 5 \u0026Aring; resulting in almost zero water flux, while larger distances increased the flux of water [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe major obstacle for the membrane separation technique is the fouling issue [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. Spreadable oil droplets are prone to coalesce and spread on membrane surfaces, leading to severe membrane fouling and lower separation efficiency [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. Tailoring membrane surfaces to mitigate severe fouling at high flux is, therefore, a critical challenge in developing membranes for oil-water separation [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. Recently, an amphiphilic GO membrane was constructed to regulate interfacial interactions with oil droplets and achieve ultralow fouling at high fluxes [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. However, the impact of surface modification on water flux is not yet fully understood, and molecular-scale information is lacking. The energetic barrier of hydration layers induced by the hydrophilic domains resists the spread of pollutants, while the low surface energy of the hydrophobic domains facilitates the release of pollutants [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. Nonpolar hydrophobic domains used to enhance fouling-release ability usually reduce surface hydration [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. Despite these advances, there are still few simulations studying the effect of grafting density and patterns.\u003c/p\u003e \u003cp\u003eIn this study, we perform molecular dynamics (MD) simulations to investigate the effect of surface modification of GO membranes on water and oil flux. The surface of the GO membrane was grafted with perfluoroalkyl chains at various fractional areas. Perfluoroalkyl chains are hydrophobic and favor the release of oils. We focus on the effect of grafting area fractions and the arranging patterns. Especially, two types of grafting patterns are studied: random and stripe. We found that the flux and permeability of oil and water molecules decrease with increasing grafting density. However, the penetration efficiency also depends on the grafting pattern. The random pattern is lower in efficiency than that of the stripe pattern due to the steric hindrance of inclined molecules around the nanoslit. Our findings are helpful to design more effective oil-water separation membranes through surface modification.\u003c/p\u003e"},{"header":"2 Model and Methods","content":"\u003cp\u003eThe schematic of the simulation box is shown in Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e. One layer of graphene oxide (GO, shown in red) and one layer of graphene (shown in yellow) are used as a membrane located in the simulation box. The gap between them is about 1 nm. Each layer has a nano-slit with a width of 0.8 nm. The shift distance between the two nanoslits is 1 nm. The surface of the GO is randomly modified by perfluorocarboxylic acid (CF₃CF₂CF₂CF₂COOH, shown in green) with the -OH group close to the surface. On the right side of the membrane, there is a mixture of water and oil containing hexane molecules (C\u003csub\u003e4\u003c/sub\u003eH₁₄). Water molecules are transparent, and hexane molecules appear as blue points. The numbers of water and oil molecules are 5000 and 64, respectively. The oil-water mixture was kept constant in the initial configuration in all systems. Two graphene sheets, shown in azure, are used as pistons, one of which is behind the water and oil emulsion. The simulation box is periodic only in the x and y directions, while the z direction is non-periodic. Two arrows indicate the direction of the applied atmospheric pressure. Different numbers of perfluoroalkyl chains (20, 30, 42) were grafted on the neutral or negatively charged surface with a charge of -10e. For the negatively charged surface, 10 Na\u0026thinsp;+\u0026thinsp;ions were added to the box. We mainly focused on the random modification, but systems with stripe arrangements of perfluoroalkyl chains on the surface were also studied for comparison. All systems we performed are listed in Table \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e\n\u003cdiv class=\"gridtable\"\u003e\n \u003ctable id=\"Tab1\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eSummary of the simulation systems showing the number of perfluoroalkyl chains grafted on neutral and negatively charged graphene oxide surfaces. Systems are categorized by grafting pattern (random or stripe) and by the density of grafted chains (zero, moderate, and high density). The surface charge (neutral or negatively charged) and arrangements of grafted chains are explored to study their effects on the behavior of the water-oil mixture in the simulation box.\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colspan=\"2\" rowspan=\"2\"\u003e\n \u003cp\u003eNumber of grafted chains\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eZero\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colspan=\"2\"\u003e\n \u003cp\u003eModerate density\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eHigh density\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0 \u003csup\u003ea\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e20\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e30\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e42\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" rowspan=\"2\"\u003e\n \u003cp\u003eRandom\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eNeutral \u003csup\u003eb\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u0026radic;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u0026radic;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u0026radic;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u0026radic;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eNegative\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u0026radic;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u0026radic;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u0026radic;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u0026radic;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" rowspan=\"2\"\u003e\n \u003cp\u003eStripe\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eNeutral\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u0026radic;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u0026radic;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u0026radic;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eNegative\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u0026radic;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u0026radic;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u0026radic;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n\u003c/div\u003e\n\u003cp\u003e\u003csup\u003ea\u003c/sup\u003e Values present the number of grafted molecules, which were also used to indicate the corresponding systems.\u003c/p\u003e\n\u003cp\u003e\u003csup\u003eb\u003c/sup\u003e The neutral and negative indicate the surface electricity. The negatively charged surface is with charge of -10e.\u003c/p\u003e\n\u003cp\u003eOur MD simulations were performed using the LAMMPS package [\u003cspan class=\"CitationRef\"\u003e18\u003c/span\u003e]. The box size is 3.9 nm x 3.9 nm in the X and Y directions. The entire simulation was carried out at a fixed volume and constant temperature using the NVT statistical ensemble. To stabilize the temperature at 300 K, the Nose-Hoover thermostat was used with a decay ratio of 0.1 ps⁻\u0026sup1; [\u003cspan class=\"CitationRef\"\u003e19\u003c/span\u003e]. The Verlet algorithm was used to solve Newton\u0026apos;s equations of motion for the particles at each time step of 1 fs. The cutoff radius for both the Coulombic and Lennard-Jones energy was set to 12.0 \u0026Aring;. Each simulation was performed for 2 ns with 1 bar applied on both sides for equilibrium, followed by 8 ns for data production with 200 bar on the right side. In this simulation, the SPC/E model was used for water molecules, and the OPLS force field was used to describe bond, angular, dihedral, van der Waals, and electrostatic interactions between hexane hydrocarbon molecules [\u003cspan class=\"CitationRef\"\u003e20\u003c/span\u003e]. To describe the interactions between atoms in graphene oxide and the graphene piston, the force field developed by Cheng and Steele [\u003cspan class=\"CitationRef\"\u003e21\u003c/span\u003e] was used. The PPPM method was used to correct the potential of electrostatic interactions of Coulombic and long-range Lennard-Jones interactions. Using the SHAKE algorithm [\u003cspan class=\"CitationRef\"\u003e22\u003c/span\u003e], the O-H bond length and H-O-H angle of water molecules were kept constant at 1.0 \u0026Aring; and 104.59\u0026deg;, respectively.\u003c/p\u003e"},{"header":"3 Results and discussions","content":"\u003cp\u003eWe first focus on the randomly grafting systems. The distribution of oil, brush, and water in the equilibrium state without a pressure difference in the Z direction is given in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. The height of the brush is about 1.1 nm for all cases. With the increase in grafted density, the distribution profile of the brush slightly broadens in the Z direction due to steric interactions. It can also be found that neutral surface favor the brush due to the hydrophobic effect. The oil distribution is broad. There is more oil in the brush layer for cases of low grafting densities because of the greater free space near the surface, as seen in the spatial density map of the brush (bottom, Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). For higher grafted density, the oils are mainly distributed ~ 1.0nm far from the surface, close to the brush terminal. It can be observed that there are more perfluorocarboxylic acid molecules around the nanoslit for higher grafted densities. There is a water layer near the surface, indicated by the first peaks of water density. Additionally, there appears to be more water on the negatively charged surface compared to the neutral one due to electrostatic-dipole interactions. Further, the distributions of water close to the brush terminals shows no significant difference. It should be pointed out that we do not observe significant aggregation of oil molecules at the terminal. This is in good agreement with the finding by Yang \u003cem\u003eet al.\u003c/em\u003e [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e], who studied the antifouling effect of grafted membrane.\u003c/p\u003e\u003cp\u003eNow we turn to the oil and water penetration behavior. Special systems without grafted perfluorocarboxylic acids were also included for comparison. The number of oil and water molecules passing through the membrane as a function of time is calculated and shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e to evaluate the performance at different grafting densities. It can be seen that, without surface modification, oil and water penetrate through the nanoslit quickly. When the surface is grafted with perfluorocarboxylic acids, the penetration efficiency of oil decreases. High grafting density is more effective at blocking oil penetration (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea). The behavior is similar for the penetration of water (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb). Qualitatively, there is no significant difference between neutral and negatively charged surfaces. However, water molecules are not stuck at high grafting densities due to their small size. It appears that negatively charged modification is better for water penetration due to its high hydration effect.\u003c/p\u003e\u003cp\u003eThe efficiency of the flow of oil and water molecules through the channels of various systems is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e. Efficiency is defined as the ratio of the number of molecules (water or oil) passing through the second nanoslit within 6 ns. It can be observed that without modification, almost all oil and water molecules penetrate through the membrane regardless of surface charge. Moderate grafting density nearly blocks half of the molecules. For high grafting density, the efficiency is close to zero for the neutral surface. For the negatively charged surface, the efficiency for water is about 10%. This result is consistent with the number of penetrated molecules.\u003c/p\u003e\u003cp\u003eTo study the effect of the grafting pattern, we also designed a stripe-like structure on the surface. The Z-direction density profile in the equilibrium state is similar to that of the random system (see Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea and Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec). The distribution of grafted molecules is relatively regular, with almost no grafted molecules tilting in the nanoslit region (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb). Qualitatively, the penetration efficiency of oil and water decreases as the grafting density increases (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec). About 80% of the molecules penetrate through the nanoslit for all cases we studied (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed). Surprisingly, it is obvious that the penetration efficiency at high density is larger than that of the random system (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e and Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec). For moderate grafting density, the efficiency is also slightly higher than that of the random system. This implies that grafting pattern plays a crucial role in penetration efficiency.\u003c/p\u003e\u003cp\u003eTo explore the mechanism behind the different efficiencies at high density, we carefully examined the water penetration process by analyzing the simulation trajectories. The tilt of grafted molecules plays a crucial role in blocking oil and water. To quantitatively compare the degree of tilting, we defined a region of 1 nm × 4 nm × 3.9 nm above the nanoslit (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea) and counted the average number of carbon atoms of perfluorocarboxylic acids in this region for the negatively charged systems (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eb) using data from the last 2 ns of the trajectories. It can be observed that the number of carbon atoms varies with different patterns. At high density, the carbon atom count for the random pattern is higher than that for the stripe pattern. The time evolution of the carbon atoms in the region is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ec. It can be seen that the grafted molecules close to the nanoslit tilt more quickly for the random pattern, whereas for the stripe pattern, the carbon atom number increases gradually. This difference may be due to the varying distances between grafted molecules and the nanoslit. The tilt of grafted molecules depends not only on their stiffness, but on the hydrodynamic flows in the nanoslit due to the pressure difference. To further investigate the differences between the two systems, we also plotted the spatial density maps of the randomly and stripe-like brush (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e). Evidently, there are more grafted molecules distributed in the nanoslit region for the random pattern.\u003c/p\u003e\u003cp\u003eWhen designing membranes for oil-water separation, key properties such as pore size, slit size, and interlayer spacing are critical for determining the separation efficiency [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. Given that filtration is primarily a physical process, these parameters must be carefully controlled to prevent contaminants from passing through the membrane. Control over these dimensions allows for enhanced separation performance, especially when functional groups are incorporated into the membrane structure [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. The addition of functional groups can dramatically alter membrane behavior by modifying its interactions with water and oil molecules, enhancing specific properties such as water permeability, ion rejection, and pore polarity. This is particularly important for composite materials, where functionality and performance can be tailored for specific applications.\u003c/p\u003e\u003cp\u003eFor instance, membranes with superhydrophobic-superoleophilic or superhydrophilic-superoleophobic properties have proven highly effective for oil-water separation [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. These materials either filter molecules by blocking their passage or selectively absorb them, depending on the membrane’s surface chemistry. Hydrophilic functional groups typically enhance water permeability by facilitating water passage through the membrane, whereas hydrophobic groups tend to reduce permeability by repelling water molecules and decreasing membrane wettability. This dual behavior enables a more tailored approach to membrane design, making functionalization an essential component of next-generation nanocomposite materials.\u003c/p\u003e\u003cp\u003eOur findings demonstrate that grafting density plays a crucial role in determining the performance of GO-based membranes. As grafting density increases, the surface becomes more densely covered with functional groups, which can directly affect the membrane's hydrophilicity and interactions with water and oil molecules. At higher densities, steric hindrance is introduced, leading to a more densely packed surface layer that blocks the passage of molecules. This results in reduced permeability, which is consistent with prior studies showing that increased surface coverage can impede transport through the membrane [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eMoreover, our study reveals that the nanoscale pattern of functional groups significantly impacts membrane performance. A random grafting pattern creates a heterogeneous surface with unevenly distributed functional groups, leading to varying interactions with water and oil molecules. In contrast, stripe-like patterns produce a more uniform distribution, which optimizes the interaction between the membrane and the permeating molecules, thereby enhancing separation efficiency. This suggests that the design of the grafting pattern is as important as the grafting density in optimizing membrane performance.\u003c/p\u003e\u003cp\u003eThe differences in penetration efficiency due to grafting patterns are primarily driven by steric effects, where the spatial arrangement of functional groups around the nanoslit influences the ability of molecules to pass through. The physical arrangement of these groups can either block or facilitate the passage of water and oil molecules through the membrane. Additionally, hydrodynamic interactions during non-equilibrium transport processes can cause the grafted molecules to tilt or reorient, further affecting separation efficiency. This tilting behavior, which varies between random and stripe patterns, emphasizes the role of both molecular interactions and fluid dynamics in membrane functionality.\u003c/p\u003e"},{"header":"4 Conclusion","content":"\u003cp\u003eGraphene oxide-based membranes have shown great potential in composite materials for applications ranging from wastewater treatment to oil-water separation. In the present work, we have studied the separation of water from an oil-water mixture using a two-layer graphene oxide membrane through molecular dynamics simulations. We focus on the effects of random grafting density on penetration efficiency and compare this with a stripe-like grafting pattern. Our simulation results demonstrate that increasing grafting density leads to decreased flux and permeability of oil and water molecules, emphasizing the critical role of surface functionalization in composite membrane design. Furthermore, the grafting pattern, particularly the stripe configuration, significantly enhances penetration efficiency by optimizing steric interactions around the nanoslit.\u003c/p\u003e \u003cp\u003eThese findings contribute to the advancement of nanocomposite materials and surface modification techniques, offering insights into the design of high-performance membranes for oil-water separation. Understanding the interplay between grafting density, surface patterning, and membrane performance is crucial for developing advanced hybrid materials that can address industrial challenges such as wastewater treatment and oil spill remediation. Future research could explore the integration of alternative functional groups and patterns, as well as the real-world application of these membranes in large-scale industrial processes. Such advancements would not only improve the efficiency of separation technologies but also broaden the scope of composite materials in environmental and industrial applications.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAuthor contribution\u003c/strong\u003e Wende Tian wrote the main manuscript text and did all the simulation works; Yanwei Wang wrote some part of the manuscript text and revised the draft; Zhexenbek Toktarbay did experiments conceptualization, general management, and manuscript revision.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u0026nbsp;\u003c/strong\u003eThis work was supported by the Ministry of Science and Higher Education of the Republic of Kazakhstan under the project AP19679745 \u0026ldquo;Design of mechanically strong, biodegradable membrane for separation process\u0026rdquo;.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests:\u003c/strong\u003e The authors declare no competing interests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthical Approval:\u003c/strong\u003e not applicable.\u0026nbsp;\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eKommu A and Singh JK (2020) A Review on Graphene-Based Materials for Removal of Toxic Pollutants from Wastewater, Soft Materials 18:297-322. https://doi.org/10.1080/1539445x.2020.1739710 \u003c/li\u003e\n\u003cli\u003eLejars M, Margaillan A, and Bressy C (2012) Fouling Release Coatings: A Nontoxic Alternative to Biocidal Antifouling Coatings, Chem. 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Mater. 28:1706545. https://doi.org/10.1002/adfm.201706545 \u003c/li\u003e\n\u003cli\u003eYou X, Wu H, Zhang R, Su Y, Cao L, Yu Q, Yuan J, Xiao K, He M, and Jiang Z (2019) Metal-Coordinated Sub-10 Nm Membranes for Water Purification. Nat. Commun. 10:4160. https://doi.org/10.1038/s41467-019-12100-0 \u003c/li\u003e\n\u003cli\u003eSong X, Chen Y, Rong M, Xie Z, Zhao T, Wang Y, Chen X, and Wolfbeis OS (2016) A Phytic Acid Induced Super‐Amphiphilic Multifunctional 3D Graphene‐Based Foam, Angew. Chem. Int. Ed. 55:3936-3941. https://doi.org/10.1002/anie.201511064 \u003c/li\u003e\n\u003cli\u003eDong Y, Li J, Shi L, Wang X, Guo Z, and Liu W (2014) Underwater Superoleophobic Graphene Oxide Coated Meshes for the Separation of Oil and Water, Chem. 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Chem. 13:952-962. https://doi.org/10.1002/jcc.540130805 \u003c/li\u003e\n\u003cli\u003eFu L, Liao K, Tang B, Jiang L, and Huang W (2020) Applications of Graphene and Its Derivatives in the Upstream Oil and Gas Industry: A Systematic Review, Nanomaterials 10:1013. https://doi.org/10.3390/nano10061013 \u003c/li\u003e\n\u003cli\u003eZahedi H and Foroutan M (2019) Separation of Water\u0026ndash;Oil Mixture on Poly Methyl Methacrylate Surface Using TiO2 Nanoparticles via Molecular Dynamics Simulation, Adsorption 25:1019-1031. https://doi.org/10.1007/s10450-019-00119-0 \u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"advanced-composites-and-hybrid-materials","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"achm","sideBox":"Learn more about [Advanced Composites and Hybrid Materials](https://link.springer.com/journal/42114)","snPcode":"42114","submissionUrl":"https://submission.nature.com/new-submission/42114/3","title":"Advanced Composites and Hybrid Materials","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-5098627/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5098627/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eGraphene oxide-based membranes hold great promise in composite materials for applications such as wastewater treatment and oil-water separation. In this study, we employed molecular dynamics simulations to investigate the separation of water from an oil-water mixture using a two-layer graphene oxide membrane. We explored the effects of random and stripe-like grafting patterns on penetration efficiency, focusing on varying grafting densities. Our results show that increasing grafting density reduces flux and permeability of both oil and water molecules, highlighting the critical role of surface functionalization in membrane design. Notably, the stripe grafting pattern significantly enhances penetration efficiency by optimizing steric interactions around the nanoslit. These findings contribute to the development of nanocomposite materials and surface modification techniques, offering insights into the design of high-performance membranes for oil-water separation. Understanding the relationship between grafting density, surface patterning, and membrane performance is crucial for advancing hybrid materials that address industrial challenges such as wastewater treatment and oil spill remediation. The insights gained from this study can be further refined by exploring different functional groups and surface modifications, broadening the applications of these membranes in industrial separation processes.\u003c/p\u003e","manuscriptTitle":"Effect of surface grafting on the oil-water mixture passing through a nanoslit: A Molecular Dynamics Simulation Study","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-11-04 09:28:04","doi":"10.21203/rs.3.rs-5098627/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2024-10-08T13:34:45+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-10-08T13:26:20+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-10-06T12:25:31+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"195775753562740034426540890132234787087","date":"2024-10-06T12:11:00+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"296531561129249586489063253070359340316","date":"2024-10-05T16:02:16+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"301755003290600652591653714780942562090","date":"2024-10-04T02:50:28+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-10-03T15:12:28+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-10-01T12:44:08+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2024-09-27T09:31:04+00:00","index":"","fulltext":""},{"type":"submitted","content":"Advanced Composites and Hybrid Materials","date":"2024-09-16T16:07:27+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"advanced-composites-and-hybrid-materials","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"achm","sideBox":"Learn more about [Advanced Composites and Hybrid Materials](https://link.springer.com/journal/42114)","snPcode":"42114","submissionUrl":"https://submission.nature.com/new-submission/42114/3","title":"Advanced Composites and Hybrid Materials","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"42f9fa55-22be-40bc-abf5-95c716d4e704","owner":[],"postedDate":"November 4th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2024-11-11T16:05:48+00:00","versionOfRecord":{"articleIdentity":"rs-5098627","link":"https://doi.org/10.1007/s42114-024-01055-6","journal":{"identity":"advanced-composites-and-hybrid-materials","isVorOnly":false,"title":"Advanced Composites and Hybrid Materials"},"publishedOn":"2024-11-08 15:57:57","publishedOnDateReadable":"November 8th, 2024"},"versionCreatedAt":"2024-11-04 09:28:04","video":"","vorDoi":"10.1007/s42114-024-01055-6","vorDoiUrl":"https://doi.org/10.1007/s42114-024-01055-6","workflowStages":[]},"version":"v1","identity":"rs-5098627","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5098627","identity":"rs-5098627","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}