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Article

Developing a Superhydrophilic/Underwater Superoleophobic Plasma-Modified PVDF Microfiltration Membrane with Copolymer Hydrogels for Oily Water Separation

by
Hasan Ali Hayder
1,
Peng Shi
1,* and
Sama M. Al-Jubouri
2,*
1
School of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu 610500, China
2
Department of Chemical Engineering, College of Engineering, University of Baghdad, Baghdad 10071, Iraq
*
Authors to whom correspondence should be addressed.
Appl. Sci. 2025, 15(12), 6654; https://doi.org/10.3390/app15126654
Submission received: 18 April 2025 / Revised: 20 May 2025 / Accepted: 26 May 2025 / Published: 13 June 2025
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Abstract

Polymer membranes often face challenges of oil fouling and rapid water flux decline during the separation of oil-in-water emulsions, making them a focal point of ongoing research and development efforts. Coating PVDF membranes with a hydrogel layer equips the developed membranes with robust potential to mitigate oil fouling. However, developing a controllable thickness of a stable hydrogel layer to prevent the blocking of membrane pores remains a critical issue. In this work, atmospheric pressure low-temperature plasma was used to prepare the surface of a PVDF membrane to improve its wettability and adhesion properties for coating with a thin hydrophilic film of an AM-NaA copolymer hydrogel. The AM-NaA/PVDF membrane exhibited superhydrophilic and underwater superoleophobic properties, along with exceptional anti-crude oil-fouling characteristics and a self-cleaning function. The AM-NaA/PVDF membrane achieved high separation efficiency, exceeding 99% for various oil-in-water emulsions, with residual oil content in the permeate of less than 10 mg/L after a single-step separation. Additionally, it showed a high-water flux of 5874 L/m2·h for crude oil-in-water emulsions. The AM-NaA/PVDF membrane showed good stability and easy cleaning by water washing over multiple crude oil-in-water emulsion separation and regeneration cycles. Adding CaCl2 destabilized emulsions by promoting oil droplet coalescence, further boosting flux. This strategy provides a practical pathway for the development of highly reusable and oil-fouling-resistant membranes for the efficient separation of emulsified oily water.

1. Introduction

The petroleum industry discharges large quantities of oily wastewater, most of which comes from oilfields [1,2]. This wastewater significantly threatens ecological systems and human health [3,4]. According to statistics, the volume of oilfield-produced water will increase to 243,000 million barrels per year by 2030, due to increased demand for crude oil and increased global production activity in the absence of economically competent renewable energy sources and their limitations, all of which will be reflected in an increasing volume of oily water associated with oil extraction [5,6]. The oilfield wastewater stream must be managed to reduce its harmful environmental impact and meet the agenda of the Sustainable Development Goals by 2030 [7].
The difficulty of separating oily water mainly arises from the size of the oil droplets in the water and the stability of the system. According to the size of the oil droplets, oil–water systems are usually divided into floating oil (>150 μm), dispersed oil (20–150 μm), emulsified oil (<20 μm), and dissolved oil [8]. Emulsified oil, particularly heavy crude oil (<20° API), contains more natural compounds as surfactants, such as asphaltenes and resins, which promote the stability of the emulsion, making the emulsified oil separation more difficult to handle [9,10]. Membrane technology presents a promising alternative for treating oilfield wastewater, offering higher-quality effluent, lower energy consumption, compact design, low chemical consumption, and cost efficiency compared to conventional methods [11,12]. However, a major technical obstacle faced by successfully implementing membranes for produced water treatment in oilfields is serious fouling by crude oil, since fouling causes flux decline due to its high adhesion to most membrane surfaces [13,14,15,16,17,18]. To date, there is a general lack of crude oil-repellent membranes in the oil and gas sector.
Polyvinylidene fluoride (PVDF) is an important polymer used in microfiltration and ultrafiltration membranes due to its excellent flexibility, chemical resistance, thermal stability, and mechanical strength [19,20,21]. These characteristics make PVDF membranes particularly suitable for oily water separation. However, PVDF membranes have strong hydrophobicity, so membrane fouling inevitably happens during separation processes [22,23,24]. In practical applications, the wettability of the membrane materials, whether hydrophilic or hydrophobic, can be tailored by constructing specially engineered structures on the surface of the substrate material or by introducing hydrophilic or hydrophobic components. Such development strategies improve the membrane separation performance to meet different research needs and industrial requirements [25,26]. Consequently, the modification of PVDF membranes, likewise other polymeric membranes, has emerged as a significant research area focused on altering their hydrophilic properties [27,28,29].
Various modification methods have gained significant attention in the development of super-wettable materials, such as surface grafting [30,31,32], surface coating [33,34,35], and grafting co-blending [27,36,37]. Among these strategies, surface coating with hydrophilic materials stands out for its simplicity and scalability. Hydrogels, in particular, have gained significant interest as promising materials for the hydrophilic modification of separation membranes, owing to their high hydrophilicity and exceptional design flexibility [38,39]. The high hydrophilicity of polymer hydrogels is attributed to the abundance of hydrophilic functional groups, including the hydroxyl (-OH), amino (-NH2), carboxyl (-COOH), and amide (-CONH2) groups. These functional groups facilitate the formation of a hydration layer on the membrane surface during the filtration process that repels crude oil droplets, thereby exhibiting a superoleophobic effect crucial for oil–water separation applications [40,41]. Coating filter membranes with hydrogels effectively combines the inherent hydrophilicity of the hydrogel with the high permeability of the membrane, enhancing both separation efficiency and antifouling properties [42,43].
Prior studies highlight the potential of hydrogel coatings. Gao et al., 2018 [44] coated a thin hydrogel film onto the surface of a PAA-g-PVDF ultrafiltration membrane through layer-by-layer self-assembly of sodium alginate and Cu2+. The resulting filtration film achieved a high water flux of up to 1230 L/m2·h.bar, with separation efficiency exceeding 99.8% and outstanding antifouling and cyclic performance. Xue et al., 2011 [45] coated micro-scale porous metal substrates with rough nanostructured hydrogel instead of directly applying a hydrogel coating to the filtration membrane surface. The hydrogel-coated mesh demonstrated excellent selective and effective separation of water from various oil/water mixtures, including crude oil, with separation efficiency exceeding 99. However, this method faces two key challenges: (1) controlling the coating thickness, because exceeding the optimal thickness of the layer can lead to pore blockage, reducing filtration efficiency; and (2) ensuring long-term stability, as weak adhesion can cause delamination or degradation over time [46,47]. Therefore, developing an effective and versatile surface modification strategy remains a critical challenge in advancing membrane technology.
Acrylamide (AM) and acrylic acid (in the form of sodium acrylate (NaA)) are two important raw monomers that form superior synthetic copolymer hydrogels that have separation ability for oil-in-water emulsions, metal ions, and organic dyes, along with notable antibacterial properties, thanks to the role of their functional groups, making them highly suitable for treating oilfield-produced water [48,49,50,51]. The ratio of these monomers significantly influences polymer chain packing, thereby controlling the thickness of the coated hydrogel layer. The presence of carbon–fluorine (C-F) bonds with strong bond energy (110 kcal.mol−1) in PVDF complicates chemical modification, resulting in weak interactions between the substrate and coating layer [52]. To address this issue, applying the low-temperature plasma treatment before the hydrogel coating process effectively enhances the wettability and chemical bonding between the PVDF membrane surface and the hydrogel. The mechanism involves the introduction of polar functional groups and an increase in the number of available binding sites, thereby facilitating improved interfacial interactions [53,54,55].
This work integrates atmospheric pressure low-temperature plasma treatment with AM/NaA copolymer hydrogels to develop a superhydrophilic and underwater superhydrophobic PVDF microfiltration membrane with coating stability and antifouling performance. The hydrogel is synthesized from AM and NaA copolymer in different AM/NaA monomer mass ratios by free radical polymerization in aqueous solutions. The hydrogel is coated onto a plasma-preconditioned PVDF surface and optimized by treatment time. The modified membranes are tested for crude oil-in-water emulsion separation, assessing reusability and antifouling with the optimal monomer ratio. This work also explores separation efficiency across oil types (kerosene and diesel) and the role of salts (calcium chloride, magnesium chloride hexahydrate, and sodium chloride) in promoting demulsification of crude oil-in-water emulsions within the membrane system. The superior performance and fabrication method make our AM-NaA/PVDF membrane an effective solution for industrialization.

2. Experimental

2.1. Materials and Chemicals

The substrate was a PVDF membrane with a mean pore size of 0.503 μm (Nengda Filter Inc., Haining, China). Acrylamide (AM, AR, 99.0%, Kelong Chemical, Chengdu, China) and sodium acrylate (NaA, 95.0%, Beijing OKA, Beijing, China) as monomers, N,N′-methylene di-acrylamide (BIS, AR, 98.05%, Kelong Chemical, Chengdu, China) as a crosslinker, and 2,2′-Azobis(2-methylpropionamide) dihydrochloride (V-50, AR, ≥97%, Aladdin) as an initiator were used as received. Inorganic salts, including sodium chloride (NaCl, AR, 99.8%, MW 58.44), calcium chloride anhydrous (CaCl2, AR, 99.9%, MW 110.98), and magnesium chloride hexahydrate (MgCl2·6H2O, AR, 98.0%, MW 203.30), were supplied by Kelong Chemical. A heavy crude oil sample with a density of 0.9691 g/cm3 at 15.6 °C was collected from an oilfield in the Bohai Sea of China. Diesel oil and kerosene were commercial products from the Chinese market. The purity of the nitrogen gas was more than 99.99%. Distilled water (DI) with a conductivity of 3.8 μS/cm was used in all experiments.

2.2. Characterizations

The surface morphology of the membranes was characterized via scanning electron microscopy (SEM, S-4800, Hitachi, Tokyo, Japan), and the elemental composition was analyzed via energy-dispersive X-ray spectroscopy (EDS, Hitachi Regulus 8100, Tokyo, Japan). The surface element composition was characterized using X-ray photoelectron spectroscopy (XPS, Escalab 250Xi, Thermo Fisher, Waltham, MA, USA). A laser confocal microscope (Nikon Eclipse Ti, Tokyo, Japan) was used to analyze the emulsion droplets and the droplet size in water with an accuracy of ±2%. The hydrogel was characterized via Fourier transform infrared spectroscopy (ATR-FTIR) on a Nicolet 6700 (Thermo Fisher, Waltham, MA, USA) instrument to detect its functional groups. The pore size of the membrane was determined using the bubble-point method with a membrane pore size analyzer (PSMA-20, GaoQ Functional Materials Co., Ltd., Nanjing, China). The contact angles (CA) of 10 μL droplets were measured using a drop shape analyzer (DSA 30s, Krüss, Hamburg, Germany). After examining the separation performance of all developed membranes, the membrane modified with a plasma exposure time of 4 min and the hydrogel in a monomer mass ratio of 2:3 AM/NaA was chosen for conducting all aforementioned characterizations. Each characterization was repeated 5 times for reliability.
This approach ensured that the characterization focused on the most promising candidate, aligning with the study’s objective to identify high-performing hydrogel membranes.
The degree of grafting was calculated as the percentage weight increase of the membrane after grafting, according to Equation (1) [56]:
G R = m 1 − m 0 m 0 × 100 %
where m1 is the weight of the grafted membrane, and m0 is the weight of the membrane.

2.3. Water Absorbency Measurement of the AM/NaA Hydrogel

A 1 g sample of dry AM/NaA hydrogel was immersed in water at room temperature until equilibrium was reached. The water absorbency was determined by weighing the swollen hydrogel after allowing it to drain on a sieve for 10 min. The water absorbency Q (g H2O/g sample) was calculated using Equation (2) [57]:
Q g H 2 O / g s a m p l e = m − m 0 m 0
where m and m0 denote the weight of the hydrogel swollen by water and the weight of the dry hydrogel, respectively. This test was performed on the hydrogel with an optimized 2:3 AM/NaA monomer mass ratio, incorporating 0.134 g of a crosslinker (BIS) and 0.1 g of an initiator (V-50), which was prepared at 60 °C with a reaction time of 1 h.

2.4. Preparation of the AM-NaA/PVDF Membrane

The polymer hydrogels AM, NaA, and AM-NaA in different AM/NaA monomer ratios were prepared by free radical polymerization in aqueous solutions. The AM and NaA monomers were mixed according to the mass ratios presented in Table 1. The solution was prepared by dissolving 0.134 g of the crosslinker BIS in 100 mL of distilled water. The water was first degassed with nitrogen gas for 15 min to remove any dissolved oxygen, as it inhibits radical polymerization by reacting with the active radicals and generates dead chain ends, which can interfere with the crosslinking process [58]. The resultant solutions were stirred by a heating magnetic stirrer at 40 °C for 30 min to ensure complete polymerization and the initiation of free radical reactions. Subsequently, 0.1 g of V-50 initiator was added dropwise to the mixture. The decomposition rate of the V-50 initiator is temperature-dependent and follows the Arrhenius equation (Equation (3)):
ln k A = − E a R . 1 T = f 1 T
where k is the rate constant (s−1), A is the pre-exponential factor (s−1), Ea is the activation energy (J/mol), R is the universal gas constant (8.314 J/(mol.K)), and T is the absolute temperature (K). While the free radicals can be generated at a temperature of 40 °C, the experimental attempts indicated that a temperature of 60 °C promoted solid gel formation. Therefore, decomposition into free radicals commences even at relatively moderate temperatures and is accelerated by temperature increase. The PVDF membrane was exposed to surface pretreatment using atmospheric plasma to activate its surface before hydrogel grafting, as chemical bonding with dissimilar materials is highly complicated in the presence of few available bonding sites. The PVDF membrane was sonicated for 10 min in water and dried at room temperature for 12 h before treatment with atmospheric glow discharge plasma (CTP-2000K, Coronalab, Nanjing, China). The electric field between the electrodes was 1000 V/mm, and the input power was 30 W. The rod electrode was wiped back and forth on the PVDF membrane for 4 min. Subsequently, the plasma-treated PVDF membrane (LEP-H1) was immersed in a hydrogel precursor solution for 30 min. After that, the membranes were withdrawn and subjected to further crosslinking via oven drying at 60 °C for 30 min, resulting in the modified membranes referred to as AM-NaA/PVDF. The preparation process is shown in Figure 1.

2.5. Oily Water Preparation

The oil-in-water emulsion (1000) mg/L was prepared by homogenizing the crude oil and deionized water. The crude oil and water were heated separately to a stable 60 °C. The oil was added dropwise into the water under rigorous shearing by a homogenizer (Waring, LB20EG, Stamford, CT, USA) at 6000 rad/min for 15 min to obtain a uniform model oily water with an oil content of approximately 1000 mg/L. The surface tension of the resultant oily water model was measured using pendant drop shape analysis, and it was found to be 56 mN/m at a temperature of 25 °C. The size distribution of the oil droplets in the emulsion was examined after 2 h of preparation using a laser confocal microscope, and ImageJ 1.53e software was then used to test 100 selected spherical oil drops to check for good stability of the emulsion. The mean diameter of oil droplets (D) was 1.64 μm (as shown in Figure S1). These results confirm that the oil solution can be considered an emulsified oil solution.

2.6. Oily Water Separation

The separation performance of the oil-in-water emulsion by the modified PVDF membrane (separation area: 15.2 cm2) was tested by a vacuum-driven dead-end filtration system at 0.05 MPa. The permeate flux, J (L/m2·h), and the rejection, R (%), were determined using Equations (4) and (5), respectively. The membrane was pre-wetted with water before the filtration test to create a hydration layer that served as a barrier to oil fouling.
J = Δ v A Δ t
R ( % ) = C 1 − C 2 C 1 × 100
where v is the permeate volume (L), A is the effective membrane area (m2), and t is the operation time (h). C1 is the oil concentration in the feed solution (mg/L), and C2 is the oil concentration in the permeate (mg/L).

2.7. Analysis of the Residual Oil Concentration in the Permeate

The residual oil content in the permeate was quantitatively determined using UV spectrophotometry (UV-1800, Shimadzu, Shanghai, China). The detailed procedure for the calibration curve preparation is outlined as follows: n-hexane is used as a solvent to prepare a series of standard oil solutions with five different concentrations. Subsequently, the absorbance values of these standard solutions are measured within a range of wavelengths (200–500 nm) to establish the linear relationship between the absorbance and the corresponding oil concentration, which gives the calibration curve. Finally, the collected filtrate is extracted with an equal volume of n-hexane, and the resulting extract is analyzed using the abovementioned UV spectrophotometer to measure its absorbance. The residual oil concentration in the permeate can then be determined based on the calibration equation.

3. Results and Discussion

3.1. Effect of Reaction Time on Grafting and Wettability

The AM-NaA copolymer hydrogel was grafted onto the plasma-treated PVDF membrane surface via free radical polymerization, initiated by the thermal decomposition of an azo compound. The grafting yield for a typical plasma-induced free radical polymerization depends on reaction time. Figure 2a shows that the grafting yield increases steadily from 17.234% at 60 min to 19.068% at 210 min. It was found that increased reaction time alters the wettability of the membrane surface (Figure 2b). The contact angle (CA) was measured immediately after droplet contact with the AM-NaA/PVDF membrane. At a reaction time of 60 min, the water contact angle (WCA) was 46.603°, indicating good water absorption; however, this absorption decreases as the grafting yield increases, with the WCA rising to 56.767° at 210 min. This shift may result from changes in surface chemistry or morphology induced by prolonged reaction times.

3.2. Surface Morphology

SEM was used to analyze the surface morphology of the prepared membranes, as shown in Figure 3. The untreated PVDF membrane (H-1) (Figure 3a) exhibited a relatively smoother surface with disordered porous structures. In contrast, the LEP-H1 membrane (Figure 3b) displayed a rougher surface with more pronounced porosity due to the plasma etching process. [59,60]. Figure 4b shows that the mean pore size of the LEP-H1 increased slightly to 0.574 µm compared with 0.503 µm for the H-1 (Figure 4a), suggesting that plasma treatment alters membrane pore morphology [61,62]. The values of the surface roughness (Ra) increased considerably from 558 nm (Figure 3g) for the H-1 to 1046 nm (Figure 3h) for the LEP-H1, further evidencing the impact of plasma treatment.
The cross-section of the AM-NaA/PVDF membrane with sample H-6 (Figure 3f) revealed that the pores remained unblocked by the hydrogel, with a total membrane thickness of 95.23 ± 5.14 μm. The surface of sample H-6 (Figure 3e) exhibited a more porous and uniformly structured morphology compared to H-3 and H-4 (Figure 3c,d), respectively, consisting of a sponge-like skin layer, with an average pore size of 0.549 μm (Figure 4c) and Ra of 770 nm (Figure 3i). The morphology uniformity can be attributed to the hydrophilic surface of LEP-H1, which facilitated the adhesion of the AM-NaA solution.
For the copolymer P(AM-NaA), the reactivity ratios of AM and NaA are reported as 1.326–1.003 and 0.228–0.295, respectively [63]. It can be predicted that the distribution of the two monomer units on the copolymer chain is relatively uniform, and their content is related to the monomer feed ratio, which is more obvious in Figure 3c–e. In sample H-3, with a 1:1 AM/NaA mass ratio, the relatively balanced incorporation of both monomers likely contributed to a certain uniform pore size distribution due to a consistent chain composition. In contrast, for sample H-4, since the content of the AM monomer in the copolymer is larger than that of NaA, the hydrogen bonding between the copolymer chain segments is stronger, but due to the coexistence of -COO−, the electrostatic repulsion between the chain segments cannot be ignored. However, the presence of -COO− groups from NaA introduced electrostatic repulsion, counteracting the hydrogen bonding and resulting in a slightly larger and less uniform pore size. Thus, the microstructure of the copolymer hydrogel is governed by the balance between hydrogen bonding and electrostatic repulsion, influencing polymer chain packing, as schematically depicted in Figure S2.
The EDS spectra of the LEP-H1 (Figure 5(a3,a6)) demonstrated a uniform oxygen distribution on the membrane surface, suggesting that the plasma process effectively introduced oxygen-containing functional groups on the membrane surface. Furthermore, the EDS spectrum of the H-6 indicated that the elements F, O, C, and Na are uniformly distributed across the membrane surface (Figure 5(b1–b6)). The appearance of Na elements further verified that acrylate was successfully loaded on the PVDF membrane surface.

3.3. Structural Characterization of the Membranes by FT-IR Analysis

The FT-IR spectra of the prepared membranes are presented in Figure 6, where the key peaks are analyzed and their corresponding functional groups identified. The LEP-H1 confirms the presence of hydroxyl (-OH), ester (O-C), and carboxyl (O=C) functional groups, indicating the successful introduction of these groups onto the membrane surface using plasma treatment for hydrogel grafting. As shown in Figure 6, the peaks at 1232 and 1712 cm−1 are representative of C-O and C=O, respectively.
In comparison with H-1 and LEP-H1, the spectrum of H-6 exhibits a broad peak at 3396 cm−1, attributed to the overlapping stretching vibrations of hydroxyl (O-H) and amide (N-H) groups. Additionally, an absorption peak at 1550 cm−1 was observed, which corresponds to the stretching vibration of the sodium acrylate (-COONa) group [48,64].
Moreover, the preserved PVDF peaks at 1403 cm−1 (-CH2-), 1173 cm−1 (-CF2-), and 876 cm−1 (C-C) indicate that the modification process retained the PVDF base structural integrity while introducing functional groups. These observations confirm the copolymer hydrogel attachment on the PVDF membrane surface.

3.4. Structural Characterization of the Membranes by XPS Analysis

X-ray photoelectron spectroscopy (XPS) was used to detect the changes in surface chemistry resulting from plasma treatment and hydrogel coating by identifying new functional groups. Figure 7 presents the XPS survey spectra of H-1, LEP-H1, and H-6. The analysis of major peaks corresponding to fluorine (F1s), carbon (C1s), and oxygen (O1s) serves as a key indicator of the chemical composition and structural properties of the polymer. As expected, H-1 exhibited dominant F1s and C1s peaks, along with a low atomic ratio of O1s, reflecting the intrinsic composition of the polymer.
Figure 8 focuses on the XPS core-level spectra of the C1s and O1s regions, which provide further details about the chemical modifications across the different membrane states. In panel (a), the C1s spectrum for H-1 shows peaks at 290.72 eV, 286.22 eV, and 284.61 eV, corresponding to the CF2, CH2, and CH groups, respectively, which are consistent with its inherent chemical structure [65]. Panel (b) presents the C1s spectrum for LEP-H1, where the CF2 peak at 290.03 eV is less intense (27.06%) compared to H-1, suggesting a reduction in fluorine content. The CH2 peak at 286.57 eV (34.38%) shifts slightly, and its intensity decreases, indicating structural modifications. A new peak at 286.70 eV (22.76%) is attributed to carbonyl (C=O) groups, signifying that the LEP treatment introduces oxidation or functionalization at the PVDF surface. Additionally, a C=O peak at 289.30 eV (6.46%) further supports this observation. In panel (c), the C1s spectrum for H-6 shows a reduced CF2 peak (290.97 eV, 16.62%) compared to H-1 and LEP-H1, suggesting a reduction in fluorine content in the modified PVDF. The CH2 peak at 284.86 eV (49.57%) becomes significantly more intense, reflecting a higher concentration of CH2 groups on the surface after the AM-NaA treatment. The appearance of C=O peaks at 288.39 eV (16.91%) indicates surface oxidation and functionalization during the AM-NaA treatment.
The presence of small oxygen peaks in the untreated PVDF membrane (panel (d)) (such as C=O at 532.33 eV and C-O at 533.66 eV) could be attributed to surface contamination or adsorbed oxygen from the environment. Panel (e) shows the O1s spectrum for LEP-H1, where the peak at 532.23 eV (90.99%) is more pronounced, signifying a higher degree of surface oxidation and the presence of more carbonyl groups, consistent with the changes observed in the C1s spectrum. Panel (f) presents the O1s spectrum for H-6, where the peak at 531.27 eV (70.08%) corresponds to oxygen in carbonyl (C=O) groups, further confirming surface oxidation. The binding energy at 536.02 eV confirms the presence of -OH groups in the prepared AM-NaA/PVDF membrane.
XPS analysis of the H-6 membrane also reveals a nitrogen content of 2.72%, attributed to amide groups in the hydrogel, as detected in the N 1s XPS spectrum (Figure S3). Additionally, a significant sodium content (10.87%) was detected in the Na 1s spectrum (Figure S3), corresponding to sodium acrylate units, where sodium exists as a counterion to the carboxylate groups in the copolymer structure.
The atomic compositions and O/C ratios, as summarized in Table 2, demonstrate the progressive chemical modifications of the PVDF membranes. H-1 shows a low O/C ratio of 0.077, reflecting its inherently hydrophobic nature. Plasma treatment increased the O/C ratio to 0.122, indicating the successful introduction of oxygen-containing functional groups. The O/C ratio further rose significantly to 0.427, highlighting substantial surface functionalization and enhanced hydrophilicity due to the hydrogel coating.

3.5. Wettability of the Membranes

The interfacial energy and wettability difference of the hydrocarbon and water on the solid surfaces provide a significant route for fabricating superhydrophilic and underwater superoleophobic membranes [37,66]. Controlling these properties minimizes oil adhesion underwater, which is quantitatively described by the work of adhesion. The work of adhesion, defined as the energy required to separate oil when it adheres to a surface in water, is calculated using a simplified form of the Young–Dupré equation (Equation (6)):
W = γ 23 1 + cos θ 0
where γ23 is the interfacial tension between crude oil and water (N/m), and θ0 is the oil contact angle (OCA) on the surface, as illustrated in Figure 9g. Thus, W becomes zero when the θ0 reaches 180°, signifying that a larger OCA correlates with lower adhesion work [67,68]. Based on this principle, superhydrophilic and underwater superoleophobic membranes can be fabricated by incorporating hydrophilic functional materials on a PVDF substrate.
The untreated PVDF membrane displayed hydrophobicity with a WCA of 120° and underwater oil contact angle (UOCA) of 98° (see Figure 9a,d). To measure the UOCA, a droplet of crude oil was injected from below the submerged membrane using a microsyringe and raised to contact the membrane surface, as depicted in Figure S4. Finally, the three-phase contact angle of the membrane/crude oil/water at room temperature was measured. Plasma surface treatment decreased the WCA to 47°, and the UOCA increased to 148° (see Figure 9b,e), which is related to increased surface wettability. This change is attributed to the introduction of polar groups and increased surface roughness following plasma treatment, which amplifies hydrophilicity on the LEP-H1, consistent with the Wenzel model [69]. These results confirm that the surface treatment was successful, and the surface is ready for the subsequent coating step. H-6 exhibited a WCA of 0° and a UOCA exceeding 151° (see Figure 9c,f), confirming its superhydrophilic and underwater superoleophobic characteristics.
The enhanced wettability of the membrane surface caused by plasma treatment significantly improved the water flux and the separation efficiency of crude oil-in-water emulsions by PVDF membranes coated with a crosslinked AM/NaA copolymer (i.e., when V-50 = 0.0 g). Figure S5 demonstrates that the permeate of crude oil-in-water emulsion increased to 1435 L/m2·h at 0.01 MPa after 4 min of plasma treatment, and the oil removal ratio reached 99%.

3.6. Oil-in-Water Emulsion Separation and Reusability Performance

A series of oil-in-water emulsions, including crude oil, diesel oil, and kerosene-in-water emulsions, were prepared and filtered through the AM-NaA/PVDF membranes. For the crude oil-in-water emulsion, as shown in Figure 10a, the permeate flux of the membranes exhibited an increasing trend, reaching a maximum of 6579 L/m2·h when the monomer mass ratio of AM to NaA was adjusted from H-2 to H-5. However, further increasing the proportion of NaA monomer units (AM/NaA) to H-6 resulted in a gradual decline in permeate flux to 5874 L/m2·h while maintaining nearly zero oil fouling, as demonstrated in Figure S6. This behavior can be attributed to the ionic strength of acrylate monomers and the surface morphology of membranes. This result demonstrates the successful separation of the oil-in-water emulsion by achieving a separation efficiency of 99.5%. Consequently, the AM-NaA/PVDF membrane fabricated at H-6 was employed for the subsequent experiments and analyses.
The cycling performance of the AM-NaA/PVDF membrane was evaluated by monitoring permeate flux and oil content during multiple separation cycles (with a cycle period of 30 s) of a crude oil-in-water emulsion. After each cycle, the membrane was rinsed with water for 1–2 min to remove any residual oil. As shown in Figure 10b, the membrane maintained a removal efficiency of more than 99% within five-run separations, and the permeate flux decreased by about 12%, i.e., from 5874 L/m2·h to 5180 L/m2·h. These results can be attributed to the formation of an oil layer on the membrane surface that increases with reusability and consequently raises the resistance of the membrane against the flow. This phenomenon is particularly pronounced in dead-end filtration processes, as the absence of hydrodynamic forces prevents the removal of the deposited oil layer [70,71].
The oil-in-water microscopic and optical images of the crude oil-in-water emulsion, diesel oil-in-water emulsion, and kerosene-in-water emulsion, before and after separation, are presented in Figure 11a–c. The milky white diesel oil- and kerosene-in-water emulsions, as well as the turbid brown crude oil-in-water emulsion, became transparent and clear after one cycle of separation using the AM-NaA/PVDF membrane. The oil content in the permeate for all three oil-in-water emulsions was below 10 mg/L, corresponding to a separation efficiency of just above 99% (Figure 11d). These results indicate that the AM-NaA/PVDF membrane can meet the safe environmental discharge limits of oil and grease content in wastewater across various regions, including China, the USA, and others [72], by removing almost all oil droplets from the oil-in-water emulsion, demonstrating its high separation efficiency. The high oil rejection of the AM-NaA/PVDF membrane is attributed to its superior underwater oil-repellent properties and proper mean pore size. The AM-NaA/PVDF membrane also exhibited high permeate flux during the separation of oil-in-water emulsions: 5874 L/m2·h for crude oil-in-water emulsion, 5276 L/m2·h for diesel oil-in-water emulsion, and 5798 L/m2·h for kerosene-in-water emulsion. This high flux is further supported by the superabsorption properties of the AM-NaA hydrogel, as evidenced by its water uptake capacity of 98 g H2O per gram of gel (Figure S7).

3.7. Chemical Stability and Self-Cleaning Function

The self-cleaning function and underwater oil adhesion behavior of the AM-NaA/PVDF membrane were evaluated by dropping crude oil droplets onto the membrane surface, placed in a beaker containing water. The oil droplets rapidly detached from the H-6 membrane surface, indicating a low adhesion effect on the AM-NaA/PVDF membrane (Supplementary Movie S1). To further assess the self-cleaning function, crude oil was dropped onto a pre-wetted, coated membrane placed in the air, followed by rinsing with water. The oil droplets were completely and instantly removed from the membrane surface, leaving no residue, as demonstrated in Figure 12a,b (Supplementary Movie S2). In contrast, when using the PVDF membrane modified with acrylamide (H-2), oil detachment was slower, requiring several seconds, as shown in Figure 12c. Long-term stability of the AM-NaA/PVDF membrane was assessed by immersing a sample in water for one month. The membrane showed the same performance because the hydrogel layer covering the PVDF surface remained unchanged after one month, remaining intact and maintaining its performance, with oil droplets detaching instantly (Supplementary Movie S3).
The chemical stability of membrane materials is a key indicator for evaluating their long-term performance in practical applications, especially in the process of treating complex emulsions and acidic environments, as it influences separation efficiency and durability. To evaluate this, the AM-NaA/PVDF membrane was immersed in aqueous solutions (pH of 3–12) for 24 h to assess changes in wetting properties. As shown in Figure S8, the WCA at pH = 7 was 5° after 0.5 s, indicating excellent hydrophilicity. When the pH decreases, the WCA gradually increases, and the hydrophilicity of the material gradually weakens, but the WCA remains below 20°. The UOCA showed little variation, consistently measuring 150°, confirming excellent underwater oleophobicity. These results show that the prepared membranes can maintain good chemical stability under acidic conditions.

4. Demulsification of Crude Oil-in-Water Emulsions for Enhanced Membrane Separation

Demulsifiers reduce the cohesion and electrostatic repulsion between oil droplets in oil-in-water emulsions, thereby enhancing membrane separation performance. This study investigated the effects of adding the divalent salt calcium chloride (CaCl2) as a demulsifier to crude oil-in-water emulsions, evaluating zeta potential, particle size distribution, and separation performance. CaCl2 was added at concentrations of 0.1, 0.5, and 1.0 g per 100 mL of emulsion at 60 °C. The zeta potential was measured using Brookhaven Instruments with an accuracy of ±2%.
Figure 13a shows that the zeta potential of the emulsion increased with the addition of CaCl2, approaching 0 mV from −16.14 mV in the absence of CaCl2. Increasing the zeta potential reduces electrostatic repulsion between oil droplets, promoting emulsion instability [73]. This led to droplet clustering and coalescence, as evidenced by increased oil droplet sizes (1.64 μm, 13.77 μm, 33.74 μm, and 48.94 μm at CaCl2 concentrations of 0.0, 0.1, 0.5, and 1 mg/100 mL, respectively). Microscopic images display this behavior in Figure 14, where oil droplets aggregate to form flocs, which subsequently coalesce into larger droplets. Higher CaCl2 concentrations enhance the flocculation rate, correlating with increased coalescence. The separation performance of the emulsion using 1.0 g/100 mL CaCl2 was tested after 2 h of gravity separation and oil skimming, then fed to the filtration system.
The influence of various electrolytes (CaCl2, MgCl2·6H2O, and NaCl) on the separation efficiency of crude oil-in-water emulsions was systematically evaluated under consistent experimental conditions. Figure 13b compares their impact on the permeate flux of a saline crude oil-in-water emulsion at neutral pH using an AM-NaA/PVDF membrane. The addition of CaCl2 to the oil-in-water emulsion significantly raised the permeate flux to 14,818 L/m2·h. This represents a significant increase compared to the permeate fluxes observed with MgCl2·6H2O (11,748 L/m2·h) and NaCl (11,823 L/m2·h) while maintaining >99% separation efficiency. According to the Debye–Hückel theory [74], divalent Ca2+ ions more effectively screen surface charges, compressing the electrical double layer and promoting droplet coalescence, thus enhancing flux.
The double-layer thickness (k−1) decreases inversely with the square root of the ionic strength. Table 3 presents the calculated k−1 in water at 25 °C as a function of electrolyte concentration. At 1.0 g/100 mL, CaCl2 exhibits the highest ionic strength (270.3 mol/m3), resulting in the smallest double-layer thickness ( k − 1 = 5.85 × 10 − 10   m ) , which explains its superior flux enhancement due to greater compression of the double layer compared to MgCl2·6H2O and NaCl. The ionic strength (Ic) is given as Equation (7):
I c = 1 / 2 ∑ Z i 2 C i
The double-layer thickness (in meters) is given by Equation (8):
κ = N A e 2 ∑ Z i 2 C i ε 0 ε r k B T 1 / 2 = 2 e 2 N A I S ε 0 ε r k B T 1 / 2
where NA is the Avogadro constant, e is the electronic charge, kB is the Boltzmann’s constant, ε0 is the permittivity of vacuum, εr is the relative permittivity of the solution, Ci is the concentration (mol·m−3) of the i th type of ion, and Zi is the valence of the i th type of ion.

5. Comparison Study

Only a few studies have been concerned with treating oily water containing heavy crude oil via hydrogels as a coating for the membrane surface, as investigated in this work. Most research focuses on lighter oils, such as organic solvents and gasoline, due to the challenges associated with causing crude oil fouling, especially in long-term tests. This issue arises from the lack of a stable and robust hydration layer, which depends on the hydrogel coating method. This method must ensure strong interfacial adhesion between the hydrogel coating and the substrate, as well as robust cohesive strength within the hydrogel coating itself. Moreover, when the oil becomes heavier, the performance of the material reported by other studies cannot fit the emission/reuse requirement (the oil cannot be removed completely). A pioneering study on microfiltration membranes conducted 30 years ago highlighted the challenges associated with this approach [75]. The AM-NaA/PVDF membrane in the present study could achieve high oil removal efficiency and outstanding anti-oil-fouling properties within a very thin hydrogel layer, with a short reaction time and requiring fewer chemicals, offering a better cost-effectiveness balance. Table 4 compares the current work with other previous studies that utilized modified membranes for the separation of crude oil-in-water emulsions.

6. Conclusions

In conclusion, this work presents an effective strategy for fabricating crude oil-repellent membranes by coating AM-NaA copolymer hydrogel on a plasma-activated PVDF substrate. The surface of the obtained AM-NaA/PVDF membrane showed superhydrophilicity, underwater superoleophobicity, and anti-crude oil fouling performance. The wettability and permeate flux of the AM-NaA/PVDF membrane was optimized by adjusting the monomer mass ratio of AM to NaA, with the optimal ratio determined to be 2:3. The AM-NaA/PVDF membrane showed an outstanding capacity to separate different stabilized oil-in-water emulsions with high water flux, even heavy crude oil-in-water emulsion, with a high separation efficiency of 99.5% and a high permeate flux of 5874 L/m2·h, as well as good cycling stability after five cycles. The AM-NaA/PVDF membrane also exhibited long-term stability after thirty days due to the strong adhesion between the AM-NaA-coated layer and the PVDF membrane. Furthermore, the CaCal2 anhydrate used for destabilizing emulsified oil droplets was employed to enhance the flux rate and minimize fouling in microfiltration by maximizing mass transfer at the surface of the PVDF membrane. This approach can enhance the long-term operational life of the membrane and prevent oil fouling, making it a promising candidate for the treatment of oilfield-produced water.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/app15126654/s1.

Author Contributions

Conceptualization, P.S. and H.A.H.; methodology, H.A.H.; software, H.A.H.; validation, formal analysis, investigation, resources, and data curation, P.S. and H.A.H.; writing—original draft preparation, H.A.H.; writing—review and editing, S.M.A.-J.; visualization, H.A.H.; supervision, P.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding authors.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Schematic of the AM-NaA/PVDF membrane fabrication and filtration process.
Figure 1. Schematic of the AM-NaA/PVDF membrane fabrication and filtration process.
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Figure 2. (a) Grafting yield (GR%) and (b) water contact angle (WCA) of H-6 as a function of reaction time.
Figure 2. (a) Grafting yield (GR%) and (b) water contact angle (WCA) of H-6 as a function of reaction time.
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Figure 3. Morphology analysis of (a) SEM of H-1 (10,000×), (b) SEM of LEP-H1 (10,000×), (c) SEM of H-3 (5000×), (d) SEM of H-4 (5000×), (e) SEM of H-6 (5000×), (f) cross-sectional SEM of H-6 (2000×), and (g–i) AFM images of H-1, LEP-H1, and H-6, respectively.
Figure 3. Morphology analysis of (a) SEM of H-1 (10,000×), (b) SEM of LEP-H1 (10,000×), (c) SEM of H-3 (5000×), (d) SEM of H-4 (5000×), (e) SEM of H-6 (5000×), (f) cross-sectional SEM of H-6 (2000×), and (g–i) AFM images of H-1, LEP-H1, and H-6, respectively.
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Figure 4. Pore size and pore size distribution (PSD) of (a) H-1, (b) LEP-H1, and (c) H-6.
Figure 4. Pore size and pore size distribution (PSD) of (a) H-1, (b) LEP-H1, and (c) H-6.
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Figure 5. Elemental mapping of (a1–a5) LEP-H1 and (b1–b5) H-6, and the elemental distribution of (a6) LEP-H1 and (b6) H-6.
Figure 5. Elemental mapping of (a1–a5) LEP-H1 and (b1–b5) H-6, and the elemental distribution of (a6) LEP-H1 and (b6) H-6.
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Figure 6. FT-IR characterization of the H-1, LEP-H1, and the H-6.
Figure 6. FT-IR characterization of the H-1, LEP-H1, and the H-6.
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Figure 7. XPS survey spectra of H-1, LEP-H1, and H-6.
Figure 7. XPS survey spectra of H-1, LEP-H1, and H-6.
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Figure 8. XPS core-level spectra of H-1, LEP-H1, and H-6. The panels on the left display the C1s spectra, while those on the right depict the O1s spectra.
Figure 8. XPS core-level spectra of H-1, LEP-H1, and H-6. The panels on the left display the C1s spectra, while those on the right depict the O1s spectra.
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Figure 9. Contact angles: (a–c) WCA of H-1, LEP-H1, and H-6, respectively; (d–f) UOCA of H-1, LEP-H1, and H-6, respectively; and (g) the schematic shape of a crude oil droplet on a membrane surface underwater.
Figure 9. Contact angles: (a–c) WCA of H-1, LEP-H1, and H-6, respectively; (d–f) UOCA of H-1, LEP-H1, and H-6, respectively; and (g) the schematic shape of a crude oil droplet on a membrane surface underwater.
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Figure 10. (a) Permeate flux and oil rejection performance of AM-NaA/PVDF membranes with different monomer mass ratios for the separation of crude oil-in-water emulsions. (b) The reusability of the H-6 membrane over multiple cycles for crude oil-in-water emulsion separation.
Figure 10. (a) Permeate flux and oil rejection performance of AM-NaA/PVDF membranes with different monomer mass ratios for the separation of crude oil-in-water emulsions. (b) The reusability of the H-6 membrane over multiple cycles for crude oil-in-water emulsion separation.
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Figure 11. Microscopic and photographic images of the emulsion feeds and their permeate water for H-6 membranes. (a) crude oil-in-water emulsion, (b) diesel oil-in-water emulsion, and (c) kero-sene-in-water emulsion. (d) Permeate flux and oil rejection performance of H-6 membranes with different oil-in-water emulsions.
Figure 11. Microscopic and photographic images of the emulsion feeds and their permeate water for H-6 membranes. (a) crude oil-in-water emulsion, (b) diesel oil-in-water emulsion, and (c) kero-sene-in-water emulsion. (d) Permeate flux and oil rejection performance of H-6 membranes with different oil-in-water emulsions.
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Figure 12. Digital photographs illustrating the resistance to crude oil adhesion and self-cleaning function of the AM-NaA/PVDF membrane. (a) Underwater self-cleaning test: crude oil stain on the H-6 membrane detaches rapidly in water, showing low oil adhesion. (b) Self-cleaning in air: (1) a clean H-6 membrane is (2) stained with crude oil and (3) fully cleaned after water rinsing, with residual oil droplets visible in the water. (c) Oil detachment comparison: (1–2) the H-2 membrane shows gradual oil droplet detachment after 3–5 s, while (3) the H-6 membrane exhibits instant detachment.
Figure 12. Digital photographs illustrating the resistance to crude oil adhesion and self-cleaning function of the AM-NaA/PVDF membrane. (a) Underwater self-cleaning test: crude oil stain on the H-6 membrane detaches rapidly in water, showing low oil adhesion. (b) Self-cleaning in air: (1) a clean H-6 membrane is (2) stained with crude oil and (3) fully cleaned after water rinsing, with residual oil droplets visible in the water. (c) Oil detachment comparison: (1–2) the H-2 membrane shows gradual oil droplet detachment after 3–5 s, while (3) the H-6 membrane exhibits instant detachment.
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Figure 13. (a) Effect of CaCl2 concentration on the zeta potential and particle size distribution of crude oil-in-water emulsions. (b) The permeate flux of a saline crude oil-in-water emulsion containing different salts (NaCl, MgCl2·6H2O, and CaCl2).
Figure 13. (a) Effect of CaCl2 concentration on the zeta potential and particle size distribution of crude oil-in-water emulsions. (b) The permeate flux of a saline crude oil-in-water emulsion containing different salts (NaCl, MgCl2·6H2O, and CaCl2).
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Figure 14. Microscopic and photographic images: (a1–a4) crude oil-in-water emulsions after 2 h of preparation with CaCl2 dosages of 0.1, 0.5, and 1.0 g/100 mL, respectively; and (b1–b4) crude oil-in-water emulsions after 24 h of preparation with CaCl2 dosages of 0.1, 0.5, and 1.0 g/100 mL, respectively.
Figure 14. Microscopic and photographic images: (a1–a4) crude oil-in-water emulsions after 2 h of preparation with CaCl2 dosages of 0.1, 0.5, and 1.0 g/100 mL, respectively; and (b1–b4) crude oil-in-water emulsions after 24 h of preparation with CaCl2 dosages of 0.1, 0.5, and 1.0 g/100 mL, respectively.
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Table 1. Composition of AM/NaA hydrogel coating on PVDF membranes.
Table 1. Composition of AM/NaA hydrogel coating on PVDF membranes.
Membrane CodeH-1H-2H-3H-4H-5H-6H-7
AM/NaA (monomer mass ratio)0:05:01:14:13:22:30:5
Table 2. The atomic composition of H-1, LEP-H1, and H-6, as determined by XPS.
Table 2. The atomic composition of H-1, LEP-H1, and H-6, as determined by XPS.
MembraneC1sO1sO/C
Binding Energy (eV)Atomic (%)Binding Energy (eV)Atomic (%)
PVDF286.2352.03531.744.030.077
LEP/PVDF285.5486.8532.1510.570.122
AM-NaA/PVDF284.8960.54531.2925.870.427
Table 3. Ionic strength and double-layer thickness for each salt at specified concentrations.
Table 3. Ionic strength and double-layer thickness for each salt at specified concentrations.
SaltConcentration (g/100 mL)Ionic Strength (Ic) (mol/m3)Double-Layer Thickness (k−1) (m)
CaCl20.127.03 1.85 × 10 − 9
0.5135.15 8.28 × 10 − 10
1.0270.3 5.85 × 10 − 10
MgCl2·6H2O0.114.76 2.51 × 10 − 9
0.573.8 1.12 × 10 − 9
1.0147.6 7.92 × 10 − 10
NaCl0.117.1 2.33 × 10 − 9
0.585.6 1.04 × 10 − 9
1.0171 7.36 × 10 − 10
Table 4. Performance comparison of the AM-NaA/PVDF membrane (fabricated at H-6) with recently developed membranes for crude oil-water emulsion separation.
Table 4. Performance comparison of the AM-NaA/PVDF membrane (fabricated at H-6) with recently developed membranes for crude oil-water emulsion separation.
Porous MaterialsSubstrate TypeFabrication MethodModification Conditions (h/°C) Inlet Oil Content (mg/L)Oil Removal (%)Flux
L/m2·h 		Ref.
DMPA@SMAPVDF Grafting and co-blending9/601034.699.2420[36]
PAAS@PVDFPVDF Grafting and blending 48/851:99 oil/water99.97350[37]
SA-TiO hydrogelPVDF-g-ILSurface coating8/601:99 oil/water991658[40]
F127@SMAPVDF Grafting and co-blending9/70100099.1272.4[76]
PVDF/PHEMA, TA/AEPPSnon-woven fabricVapor-induced phase co-deposited–/851:99 oil/water991056[77]
Oxides (ZnO, Al2O3, TiO2 and SnO2)PVDF Atomic layer deposition (ALD)0.0125/100284597.11800[78]
Alginate hydrogelFilter paperSurface coating8/–1:20 oil/water99.9~4000[43]
AM-NaA copolymer hydrogel PVDF Surface coating1/40100099.55874This study
Note: The symbol (–) indicates that the modification condition was not specified in the study.
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MDPI and ACS Style

Hayder, H.A.; Shi, P.; Al-Jubouri, S.M. Developing a Superhydrophilic/Underwater Superoleophobic Plasma-Modified PVDF Microfiltration Membrane with Copolymer Hydrogels for Oily Water Separation. Appl. Sci. 2025, 15, 6654. https://doi.org/10.3390/app15126654

AMA Style

Hayder HA, Shi P, Al-Jubouri SM. Developing a Superhydrophilic/Underwater Superoleophobic Plasma-Modified PVDF Microfiltration Membrane with Copolymer Hydrogels for Oily Water Separation. Applied Sciences. 2025; 15(12):6654. https://doi.org/10.3390/app15126654

Chicago/Turabian Style

Hayder, Hasan Ali, Peng Shi, and Sama M. Al-Jubouri. 2025. "Developing a Superhydrophilic/Underwater Superoleophobic Plasma-Modified PVDF Microfiltration Membrane with Copolymer Hydrogels for Oily Water Separation" Applied Sciences 15, no. 12: 6654. https://doi.org/10.3390/app15126654

APA Style

Hayder, H. A., Shi, P., & Al-Jubouri, S. M. (2025). Developing a Superhydrophilic/Underwater Superoleophobic Plasma-Modified PVDF Microfiltration Membrane with Copolymer Hydrogels for Oily Water Separation. Applied Sciences, 15(12), 6654. https://doi.org/10.3390/app15126654

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