The rapid development of industries, particularly petrochemicals, machining, textiles, and food processing, generates massive oily wastewater. Global produced water production is estimated at 250 million barrels daily. Discharge standards for oil content in effluents are typically below 40 mg/L, creating a significant demand for advanced wastewater treatment. Membrane technology offers advantages due to its high separation efficiency and low energy cost, especially for treating stable oil-water emulsions with droplet sizes under 20 µm. However, membrane fouling is a major obstacle. Oil droplets tend to coalesce and spread on membrane surfaces, leading to reduced efficiency and increased operational costs. Graphene oxide (GO)-based membranes show promise due to their high flux; however, at high flux, concentration polarization leads to more oil droplets on the surface and stronger interactions with the membrane. Tailoring membrane surfaces to mitigate fouling at high flux is crucial. Minimizing interfacial interactions between the surface and pollutants is key to enhancing antifouling properties. A promising strategy involves constructing amphiphilic surfaces with hydrophobic (typically fluorinated) materials on hydrophilic surfaces to achieve both fouling resistance and release. Hydrophilic domains create hydration layers that resist pollutant spread, while hydrophobic domains facilitate pollutant release. However, achieving synergistic optimization of fouling resistance and release is challenging, as nonpolar hydrophobic domains often reduce surface hydration, promoting pollutant spread. Precise engineering of surface chemistry is needed to control interfacial interactions. Previous work focused on adjusting hydrophobicity, amount, and distribution of hydrophobic domains, with little focus on interfering with surface hydration. This study addresses this gap by constructing an amphiphilic GO membrane and employing a hydrophobic chain engineering strategy to regulate interfacial interactions with oil droplets, aiming for ultralow fouling at high flux.

The literature extensively explores antifouling strategies for membrane separation, particularly in oil-water separation. Several studies highlight the use of biomimetic super-lyophobic and super-lyophilic materials [1], hydgro-responsive membranes [2], and various membrane materials like PPS [5] and PVDF [6] modified to achieve superwettability for enhanced separation. The use of graphene oxide-based membranes for wastewater treatment [7-12] is also well-documented, with efforts focusing on enhancing their antifouling properties through surface modifications. These modifications include using Cupric phosphate nanosheets [8], reduced graphene oxide aerogels [9], and 2D heterostructure membranes with self-cleaning abilities [11]. Existing research emphasizes constructing amphiphilic surfaces to balance fouling resistance and release [17-23], but achieving synergistic optimization remains a challenge. The effect of varying hydrophobicity, amount, and distribution of hydrophobic domains has been explored [33-36], but the precise control of surface hydration through molecular-scale engineering has been less investigated. Furthermore, the molecular-level influence of hydrophobic domains on amphiphilic surface hydration remains elusive. Understanding the interplay between surface energy, hydration layers, and pollutant interactions is essential for designing effective antifouling membranes. The influence of surface roughness and contact angles, described by Wenzel's and Young's equations, on the underwater oil contact angle is also discussed in literature [41, 42, 44]. The study utilizes the existing knowledge to optimize surface properties for high-flux oil-water separation.

The study employed a hydrophobic chain engineering strategy to create an amphiphilic GO membrane for oil-water separation. The process began with the preparation of a hydrophilic GO membrane (hGO) by sequentially assembling a hydrophilic phytic acid (PA) layer onto GO nanosheets using a coordination-driven metal-bridging assembly method with Fe³⁺ ions. This approach leverages the metal-organic coordination between Fe³⁺ and PA to create a stable and uniform hydrophilic surface. Next, hydrophobic perfluorocarboxylic acids (with varying chain lengths: C4, C6, C8, C10) were sequentially assembled onto the hGO membrane, creating discrete hydrophobic domains within a continuous hydrophilic domain. The F/P ratio in XPS analysis was used to ensure the even distribution of the perfluorocarboxylic acids on the surface despite varying chain lengths. The wetting behavior of the resulting membranes (F-hGO) was characterized by measuring water contact angles, apparent surface energy (γsv), underwater oil contact angles, and the normalized contact area of oil droplets. Atomic Force Microscopy (AFM) was employed to measure the adhesive force between oil droplets and the membrane surfaces. Molecular dynamics (MD) simulations were conducted to elucidate the influence of perfluorocarboxylic acid chain length on the hydration structure of the amphiphilic surface. The MD simulations used a model system with a superhydrophilic substrate, water molecules, and perfluorocarboxylic acid chains of different lengths. The simulations tracked water density, perfluorocarboxylic acid density, and water orientation (α angle between water dipole and surface normal). Differential scanning calorimetry (DSC) was also used to analyze the crystallization temperature and enthalpy of water on the membrane surfaces, further assessing the amount of interfacial water. The antifouling efficacy of the membranes was evaluated using hexadecane-in-water emulsions. The membranes' oil repellency, permeance, flux decline ratio (DRr), and flux recovery ratio (FRR) were determined. The effect of perfluorocarboxylic acid density was also investigated by varying FeCl3 solution concentration. Various characterization techniques including FTIR, XPS, TEM, and AFM were employed to analyze the membrane structure and properties. For the MD simulations, the SPC/E model for water molecules and the SHAKE algorithm for maintaining water rigidity were employed. The force field for the perfluorocarboxylic acid chains was generated by LigParGen, and a Lennard-Jones potential was applied to model non-bonding interactions. The simulations were run using LAMMPS software.

The study revealed a nonlinear relationship between the length of perfluorocarboxylic acid chains and the membrane's antifouling properties. The water contact angle increased monotonically with chain length, while the apparent surface energy (γsv) decreased. Surprisingly, the underwater oil contact angle exhibited an inverted-U relationship with chain length, reaching a maximum at C6. The C6-perfluorocarboxylic acid-modified membrane (F6-hGO) showed the highest underwater oil contact angle (165.7°) and the smallest contact area, leading to the lowest adhesive force (0.54 µN), approximately half that of the C4-modified membrane. MD simulations confirmed that the hydration capacity varied nonlinearly with chain length, peaking at C6. This was attributed to a more uniform orientation of interfacial water molecules on the F6-hGO surface, creating a denser and more uniform hydration layer. The F6-hGO membrane exhibited superior antifouling performance in oil-water separation tests, with a flux decline ratio (DRr) below 10% and a flux recovery ratio (FRR) near 100%, even at a high permeance (~620 L m⁻² h⁻¹ bar⁻¹). The DRr for F6-hGO was significantly lower than those of GO (60.0%) and hGO (31.8%) membranes and other reported membranes. Furthermore, the F6-hGO membrane showed excellent cycling performance and maintained its antifouling capability even with different oil-in-water emulsions and gypsum scaling. The combination of a hydration barrier from the hydrophilic PA coating, the low surface energy of the perfluorocarboxylic acid chains, and the optimized interfacial interactions at C6 chain length synergistically contributed to the superior antifouling properties. This suggests that the medium-length perfluorocarboxylic acids (C6) are optimal for achieving the strongest hydration capacity and thus superior antifouling efficacy.

The findings demonstrate the importance of molecular-scale regulation of surface chemistry for optimizing antifouling properties in membrane separation. The nonlinear relationship between hydrophobic chain length and antifouling performance highlights the need for precise control over interfacial interactions. The synergistic effect of hydrophilic and hydrophobic domains, along with the optimized hydration layer at C6 chain length, offers a new strategy for designing high-performance antifouling membranes. The superior performance of the F6-hGO membrane, even at high permeance, suggests its potential for practical applications in oily wastewater treatment. The microscopic understanding of interfacial interactions gained in this study can be applied to other fields such as oil recovery, biomedicine, and nanofluidics. The success of this strategy suggests that similar approaches could be applied to other membrane materials and pollutant systems.

This study successfully demonstrated a hydrophobic chain engineering strategy to create highly antifouling graphene oxide membranes for oil-water separation. By precisely controlling the length of perfluorocarboxylic acid chains, the researchers achieved synergistic optimization of both fouling resistance and release, leading to a superior membrane with a flux decline ratio below 10% and flux recovery ratio near 100%, even at high permeance. The findings highlight the importance of understanding and controlling surface hydration for designing effective antifouling membranes. Future work could explore the application of this strategy to other membrane materials and pollutant types, optimizing chain length and density for a wider range of applications.

The study focused on a specific type of oil-in-water emulsion (hexadecane). The generalizability of the findings to other types of oils and emulsions needs further investigation. While the membrane showed excellent antifouling performance in the tested conditions, long-term stability and durability under real-world conditions remain to be evaluated. The molecular dynamics simulations were performed using a simplified model system, and the results may not fully capture the complexities of real membrane systems. Further investigation into the impact of other parameters like surface roughness and pore size distribution on antifouling performance would improve understanding.