Tunable Hydrophilicity in PES-Based Nanofiber Membranes via Oxygen Plasma Treatment

TL;DR

This paper shows how oxygen plasma treatment can change the surface properties of nanofiber membranes, making them super-hydrophilic and tunable for various uses.

Contribution

The study introduces oxygen plasma treatment as a method to modify nanofiber membranes for tunable hydrophilicity and surface reactivity.

Findings

Oxygen plasma treatment added functional groups like SO3H, C=O, and OH to membrane surfaces.

The treatment converted membranes from hydrophobic to super-hydrophilic.

Plasma engineering proved effective for creating customizable nanofiber membranes.

Abstract

To tailor surface chemistry and wettability for advanced membrane applications, this study investigates PES-, PES–PVP-, and PES–GO-based nanofiber membranes modified through oxygen plasma treatment. The plasma process introduced reactive functional groups, including SO3H, C=O, and OH, onto the fiber surfaces, converting the membranes from hydrophobic to super-hydrophilic and enhancing their surface reactivity. This modification enabled tunable wettability, allowing controlled adjustment of the membrane’s hydrophilic behavior. Overall, the results demonstrate the effectiveness of plasma engineering in developing versatile nanofiber membranes with customizable surface properties for a wide range of applications.

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Full text

1. Introduction

Freshwater scarcity is an increasingly critical challenge worldwide, intensified by human activities and climate change. According to long-term hydroclimatic projections, this problem is expected to worsen in the coming years. Statistical data from Oman’s oil fields indicate that substantial volumes of produced water are generated during hydrocarbon extraction [1]. The Oman Observer (2024) reports that Petroleum Development Oman (PDO) produces approximately one million cubic meters of produced water—water co-extracted with oil from underground reservoirs—every day [2]. This water is contaminated with oil and hydrocarbons, making it unsuitable for agricultural use or human consumption. To enable its treatment and reuse, there is a need to develop advanced separation technologies, such as super-hydrophilic and hydrophobic membranes, capable of efficiently separating water from hydrocarbon contaminants.

Distillation is one of the most established traditional desalination methods used to convert wastewater into freshwater. The process relies on heat to vaporize water, leaving salts and impurities behind, followed by condensation to produce purified water. Although effective and reliable, distillation is highly energy-intensive and often costly to operate and maintain, making it less practical for large-scale or resource-limited applications [3]. Membrane technologies provide a more economical and effective way to purify water than conventional techniques. As membrane technologies continue to advance, more research is necessary to fully realize their potential [3,4]. Among the different kinds of membranes, poly-ether-sulfone (PES) has remarkable qualities for membrane materials, including high mechanical resilience, superior chemical resistance, and thermal stability. PES membranes are frequently employed in microfiltration and ultrafiltration processes, which makes them appropriate for a variety of uses, such as wastewater treatment and desalination [5].

Pristine PES membranes are inherently hydrophobic, limiting their wettability and increasing their susceptibility to fouling during filtration processes. To overcome these drawbacks, PES can be modified either chemically—through the incorporation of functional groups—or physically by altering its surface properties. Plasma-based surface modification has recently emerged as an effective and environmentally friendly approach for enhancing membrane hydrophilicity and overall performance. Studies using H_2_O plasma, corona air plasma, radiofrequency, and microwave plasmas consistently demonstrate improved wettability and reduced fouling of PES materials. For example, corona air plasma followed by grafting of HB-PEG polymers has been shown to produce highly anti-fouling ultrafiltration membranes effective in oil-in-water separation, while RF plasma polymerization with organosilicon and oxygen has yielded superhydrophilic surfaces with significantly enhanced water flux and dye rejection. Collectively, the literature highlights plasma modification as a green and versatile technique for improving the efficiency, longevity, and application potential of PES-based membranes and textiles [6,7,8,9,10,11]. Building on these advancements, the present study introduces oxygen plasma as a novel and efficient treatment for PES-based nanofiber membranes. Oxygen plasma generates reactive species capable of breaking bonds such as C–C, C–H, and C–S on the membrane surface, creating sites that subsequently convert into stable oxygen-containing functional groups (e.g., carbonyl, carboxyl, and hydroxyl). These newly formed groups improve hydrophilicity, enhance permeability, and reduce fouling. In addition, further improvements to PES membranes can be achieved by blending them with hydrophilic polymers or nanomaterials, offering additional control over surface chemistry and filtration performance [12].

This work fabricates and evaluates three types of PES-based nanofiber membranes using a nano-spinner: pristine PES, PES blended with polyvinylpyrrolidone (PVP), and PES incorporated with graphene oxide (GO). Pristine PES serves as the baseline material, representing the inherent hydrophobicity and limited surface functionality of unmodified PES membranes. Incorporating PVP introduces a hydrophilic polymer that enhances wettability and promotes uniform fiber formation, while the addition of GO contributes oxygen-rich functional groups—such as hydroxyl, epoxy, and carboxyl—that further modify surface chemistry and improve water affinity [12,13,14,15]. These compositional variations are designed to systematically alter membrane morphology, chemical characteristics, and filtration behavior. A central focus of this study is comparing how pristine PES, PES–PVP, and PES–GO membranes perform both before and after oxygen plasma treatment. Oxygen plasma is applied as a surface activation method to enhance hydrophilicity, introduce additional functional groups, and strengthen surface reactivity. By evaluating all three membrane types in their untreated and plasma-treated states, this study provides comprehensive insight into how polymer blending, nanomaterial incorporation, and plasma activation collectively influence the performance and functionality of PES-based nanofiber membranes.

The key findings of this study demonstrate the substantial influence of surface engineering on the performance of PES-based nanofiber membranes. Pristine PES, being inherently hydrophobic, exhibits limited wettability; however, its properties can be markedly improved through the incorporation of PVP or graphene oxide and further enhanced through oxygen plasma treatment. These combined modifications introduce oxygen-containing functional groups—including –OH, –C=O, and –SO_3_H—that significantly increase membrane hydrophilicity and enable finely tuned control over surface wettability, a critical parameter for advanced water treatment processes. Unlike many previous studies that focused solely on polymer blending or chemical modification, this work highlights oxygen plasma treatment as a simple, rapid, and highly effective strategy for tailoring membrane surface chemistry. The integration of material blending with plasma activation provides a versatile and scalable platform for designing membranes with customizable interfacial properties. Overall, this approach represents a meaningful advancement in membrane engineering and offers valuable insights for the development of next-generation filtration materials for diverse industrial and environmental applications.

2. Materials and Methods

2.1. Materials

Polyethersulfone (PES, nominal granule size 3 mm) was obtained from Goodfellow Limited, Cambridge, UK (PE29 6WR), and polyvinylpyrrolidone (PVP, molecular weight 360 kDa) was purchased from Sigma-Aldrich (Taufkirchen, Germany). Graphene oxide (GO) nanopowder was sourced from US Research Nanomaterials, Inc. (Houston, TX, USA). Poly(sodium 4-styrenesulfonate) (PSS, average Mw ~200,000, 30 wt% in H_2_O, CAS No. 25704-18-1) was obtained from ALFA. N,N-dimethylformamide (DMF, ≥99.8% purity) and N-methyl-2-pyrrolidone (NMP, ≥99.5% purity) were obtained from Merck (Darmstadt, Germany) and used as solvents without further purification. All chemicals and reagents were used as received.

2.2. Preparation of PES-Based Nanofiber Precursor Solutions

The nanofiber membranes were fabricated using precursor solutions prepared from polyethersulfone (PES) granules dissolved in a mixed-solvent system of N-methyl-2-pyrrolidone (NMP) and N,N-dimethylformamide (DMF). A solvent ratio of 7:3 (v/v) DMF:NMP was used for all formulations to ensure optimal solubility and electrospinning performance. To modify the physicochemical properties of the membranes, graphene oxide (GO) and polyvinylpyrrolidone (PVP) were incorporated into the PES matrix at controlled concentrations. Three distinct precursor solutions were prepared: (i) 25 wt% PES; (ii) 25 wt% PES containing 1 wt% PVP; (iii) 25 wt% PES containing 0.5 wt% GO. The lower concentration of GO relative to PVP was selected to maintain an appropriate solution viscosity suitable for processing with the nanospinner, while still providing effective surface functionalization. All mixtures were magnetically stirred until complete polymer dissolution and uniform dispersion of the additives were achieved. The detailed compositions of the precursor solutions used for membrane fabrication are presented in Table 1.

2.3. Electrospun Nanofiber-Based Membrane

The Nanospinner24 (Inovenso, Istanbul, Turkey) was employed for electrospinning under the operating parameters listed in Table 2 [16,17]. The parameters were optimized for each membrane type to achieve a stable jet and uniform nanofiber morphology. Although the basic setup was the same, the different solution properties of PES, PES-PVP, and PES-GO required adjustments in flow rate and voltage to ensure continuous fiber formation. As shown in Table 2, PES membranes were electro-spun at a flow rate of 1.0 mL/h and 23.5 kV, providing smooth and defect-free fibers. PES-PVP membranes, with higher viscosity due to PVP, required a flow rate of 2.0 mL/h and 24 kV to maintain a stable jet. PES-GO solutions, being more conductive and less viscous, needed a lower flow rate of 0.7 mL/h at 24 kV to avoid bead formation and produce uniform nanofibers. Relative humidity (60%) and collector rotation speed (220 rpm) were kept constant for all membrane types to ensure consistent solvent evaporation and fiber alignment. These tailored parameters enabled the fabrication of well-formed nanofibers suitable for subsequent structural characterization and performance evaluation.

The nanocomposite solutions were loaded into 10 mL plastic syringes and mounted on a syringe pump connected to the nano-spinner. The resulting nanofibers were collected on aluminum foil placed on the grounded collector. Each membrane required approximately nine hours for electrospinning, followed by 24 h of drying before being carefully detached from the aluminum substrate.

2.4. Oxygen Plasma Treatment

The fabricated membranes were treated with oxygen plasma using a PlasmaNT system for three minutes. The process was carried out at a pressure of 0.2 mbar with an oxygen (O_2_) flow rate of 100 sccm. This plasma treatment generates reactive oxygen species that interact with the membrane surface, introducing oxygen-containing functional groups and significantly enhancing surface hydrophilicity.

2.5. Membrane Charectrisation

The membranes were characterized using a range of techniques to evaluate their structural, morphological, and chemical properties. Surface morphology was examined using a JEOL JSM-7600F scanning electron microscope (JEOL Ltd., Tokyo, Japan) operated at an accelerating voltage of 15 kV. Prior to imaging, samples were sputter-coated with a thin layer of platinum to minimize charging effects. Nanofiber diameters were quantified from the SEM micrographs using ImageJ 1.53 software, with measurements performed on at least 50 fibers per sample to ensure statistical reliability.

Water contact angle (WCA) of the membranes was measured using an Optical Tensiometer (Krüss DSA100, Hamburg, Germany) to evaluate the surface hydrophilicity of the membranes. A sessile drop of 5 μL deionized water was placed on membrane samples of 2 cm × 3 cm, which were mounted on a glass slide. Three measurements were taken for each membrane sample, and the average value was reported.

The thermal stability of the fabricated membranes was assessed using thermogravimetric analysis (TGA), which monitors weight loss as a function of temperature to provide quantitative insight into degradation behavior and thermal decomposition. Measurements were performed using a SDT Q600 TGA/DSC instrument (TA Instruments, New Castle, DE, USA; Serial No. 0600-0868) equipped with the DSC-TGA Standard module under a nitrogen atmosphere with a flow rate of 50 mL/min. Samples of approximately 5–10 mg were heated from 25 °C to 800 °C at a constant rate of 10 °C/min. The resulting thermograms were used to identify characteristic weight-loss steps and decomposition temperatures, enabling a comparative evaluation of the thermal stability of pristine and modified PES-based membranes.

Additionally, the functional groups present in the membranes were analyzed using Fourier-transform infrared spectroscopy (FTIR), which identifies characteristic chemical bonds and molecular structures by measuring infrared light absorption at specific wavelengths. Measurements were performed using a PerkinElmer SpectraOne FTIR spectrometer (Waltham, MA, USA). Spectra were recorded over the range of 400–4000 cm^−1^ with a signal resolution of 4 cm^−1^, averaged over 40 scans. This analysis allowed identification of functional groups introduced through PVP, graphene oxide, and plasma treatment, providing detailed insight into the chemical modifications of the PES-based membranes.

Finally, the chemical composition and surface elemental analysis of the membranes were carried out using X-ray Photoelectron Spectroscopy (XPS), which provides detailed information on surface elemental composition, chemical bonding, and oxidation states. Measurements were performed using a multiprobe photoelectron spectroscopy system (Scienta Omicron, Taunusstein, Germany) with a monochromatic Al Kα X-ray source (1486.6 eV). Survey scans were acquired with an analyzer pass energy of 50 eV, while high-resolution scans for individual elements were recorded at 20 eV. The C 1s peak at 284.6 eV was used as a reference for binding energy calibration. Spectral deconvolution and data analysis were performed using CasaXPS 2.3.24.PR1 software (Casa Software Ltd., Teignmouth, UK), enabling identification of surface functional groups and chemical states introduced through PVP, graphene oxide, and plasma treatment.

2.6. Antibacterial Activity

The antibacterial activity of the fabricated membranes was investigated by testing their effects on the Gram-negative bacterium Escherichia coli (E. coli) and the Gram-positive bacterium Staphylococcus aureus (S. aureus). These bacteria are commonly studied in the context of membranes designed for wastewater filtration applications. The membranes were characterized based on their ability to inhibit the growth of these bacteria. Evaluating membrane performance against these bacteria is crucial to ensure effective filtration and prevent microbial growth on the membrane surfaces.

The bacteria were cultured in Luria–Bertani (LB) broth (Difco, Bergen County, NJ, USA; pH 7.5) at 37 °C for 24 h. After incubation, bacterial cells were harvested by centrifugation at 5000× g and 25 °C for 10 min, and the resulting pellets were resuspended in sterile deionized water. Photocatalysis experiments were conducted using 24-well plates (Costar, Tewksbury, MA, USA), with membrane samples (0.5 cm × 0.5 cm) placed at the bottom of each well. Each well was then filled with 1.5 mL of the bacterial culture containing one membrane sample. Bacterial growth was monitored by measuring the optical density (OD) of the culture at 600 nm (OD600) using a spectrophotometer. The OD600 value increases as bacteria grow, scattering more light. The OD600 measurement reflects the turbidity of the culture, with higher values corresponding to greater bacterial density. Typically, an OD600 between 0.1–1.0 indicates moderate bacterial growth, while a value between 1.0–2.0 suggests a dense culture [18].

2.7. Evaluation of Membrane Hydrophilicity

The hydrophilic properties of the membranes were further assessed by measuring the permeation flux under gravity-driven dead-end filtration. Experiments were conducted using three types of feed solutions: deionized water, a PSS solution, and an oil–water mixture obtained from Oman’s petroleum production fields (hereafter referred to as the “mother solution”), as well as a mixture containing both PSS and oil–water. The flux, representing the rate of water permeation through the membrane, was calculated using Equation (1):

$$[eqn]$$

where J is the average flux, V is the volume of water moving through the membrane, A is the membrane area, and Δt is the time interval. The unit used for flux is L/h·m^2^ [19,20,21].

To assess the effect of membrane surface properties on selective permeation, the filtration behavior of superhydrophilic and hydrophobic membranes was evaluated. Superhydrophilic membranes, such as PES–PVP nanofiber membranes, preferentially allow water to permeate while repelling oil, making them suitable for water–oil separation studies. In contrast, hydrophobic membranes selectively permit oil passage while restricting water flow, enabling evaluation of oil recovery from water–oil mixtures. This was tested using a model mixture prepared by combining cooking oil with water. This approach allows systematic investigation of how membrane surface chemistry influences flux and selective permeability.

3. Result and Discussion

3.1. Scanning Electron Microscopy

Scanning Electron Microscopy (SEM) image (Figure 1a) shows the structure of PES nanofibers, highlighting their interconnected arrangement and smooth surfaces. The fibers appear uniform with very few visible defects, such as irregular shapes. The histogram (Figure 1b) shows the diameter distribution of PES nanofibers measured from the SEM images using ImageJ software [22]. The average fiber diameter is about (0.45 ± 0.02) µm. There are a few larger fibers that might be due to variations in the solution viscosity or jet instability [23]. These factors can lead to uneven fiber formation, causing some fibers to be thicker than the average diameter.

The SEM image (Figure 1c) shows the structure of PES-PVP nanofibers, which form a dense and interconnected network, unlike the smoother PES nanofibers. The histogram (Figure 1d) presents the diameter distribution of the PES-PVP nanofibers, as measured from the SEM images. The average diameter of the fibers is (1.07 ± 0.05) µm, which is twice as large as that of PES nanofibers. This is likely due to the interaction between PES and PVP influences the stretching and formation of the polymer jet, resulting in thicker fibers.

The SEM image of PES-GO nanofibers shown in (Figure 1e) appears similar to PES nanofibers, with a smooth and uniform surface. Additionally, the diameter distribution in (Figure 1f) shows an average width of (0.3 ± 0.1) µm, falling within the range of PES nanofibers.

The similarity in the SEM images and diameter distribution between PES and PES-GO arises because both materials interact well, with GO reinforcing the PES matrix without significantly altering the electrospinning process. The consistent surface tension of PES-GO blends leads to more uniform nanofiber formation. In contrast, the addition of PVP to PES resulting irregular fiber morphologies. PVP’s more flexible, hydrophilic nature leads to uneven fiber diameters and bead formation. Thus, PES-PVP blends show distinct differences in fiber morphology compared to PES and PES/GO blends.

3.2. Water Contact Angle

Water contact angle measurements, shown in Figure 2, indicate that pristine PES exhibits hydrophobic behavior, with a high contact angle of 135 ± 3° (Figure 2a), reflecting its strong tendency to repel water. This hydrophobicity arises from the non-polar aromatic backbone of PES and the absence of polar functional groups on its surface, which results in low surface energy and limited wettability [24]. In contrast, incorporating PVP into PES produces a highly hydrophilic surface, as PVP is a strongly polar, water-attracting polymer. This modification increases the surface energy and significantly lowers the water contact angle, promoting rapid water absorption (Figure 2b). In fact, the water droplet is completely absorbed upon contact, indicating the formation of a superhydrophilic membrane surface. On the other hand, the incorporation of GO into PES maintains the hydrophobic characteristic similar to pristine PES, as GO’s surface, despite its polar functional groups, does not significantly alter the overall surface energy of the composite, keeping the water-repelling properties intact (Figure 2c).

After three minutes of oxygen plasma treatment, all membranes—PES, PES-PVP, and PES-GO—exhibit super-hydrophilic behavior. The water droplet is completely absorbed upon contact, indicating the formation of a highly hydrophilic surface. The plasma treatment introduces oxygen-containing functional groups on the membrane surface, significantly increasing surface energy and enhancing water wettability. This transformation improves the ability of the membranes to interact with water, making them more efficient in applications such as filtration and separation. For oily wastewater treatment, the superhydrophilic surface significantly aids in the rejection of oil and other hydrophobic contaminants, while allowing water to pass through more easily. This feature provides a significant advantage in enhancing membrane performance during oil-water separation, reducing fouling and achieving higher filtration efficiency.

In most cases, hydrophobic membranes are used to separate oil from water due to their water repellent and oil transmitting properties, while hydrophilic membranes are effective in separating water from oil, as they allow water to pass through and prevent oil [25,26,27].

3.3. Thermogravimetric Analysis

Thermogravimetric analysis (TGA) was performed to evaluate the thermal stability of the PES, PES–PVP, and PES–GO, based nanofiber membranes before and after 3 min of oxygen plasma treatment (Figure 3a–c). As shown in Figure 3a, pristine PES exhibits an initial weight loss of approximately 2% up to 200 °C, attributed to adsorbed moisture and residual solvent evaporation. In contrast, plasma-treated PES shows negligible mass loss in this region, indicating improved low-temperature stability due to the removal of loosely bound surface species during plasma exposure. Both pristine and treated PES membranes display a gradual weight loss of ~6% between 200–350 °C, corresponding to the onset of polymer chain scission [19]; however, the reduced early-stage degradation in the treated sample confirms that plasma treatment enhances PES stability at lower temperatures. For the PES–PVP nanofiber, (Figure 3b), the pristine and plasma-treated membranes exhibit nearly identical behavior in the initial stages, demonstrating similar early thermal responses. As temperature increases, however, the curves begin to diverge: in the 400–600 °C region, the pristine PES–PVP membrane exhibits higher thermal stability, whereas the plasma-treated membrane shows a faster degradation rate, as highlighted in the inset of the original graph. This decrease in high-temperature stability is likely due to plasma-induced oxidation or partial modification of PVP domains, which reduces their resistance to thermal breakdown. Conversely, in the PES–GO membranes (Figure 3c), plasma-treated samples demonstrate consistently enhanced thermal stability compared with untreated PES–GO across the entire temperature range. This improvement is attributed to plasma-driven interactions between GO nanosheets and the PES matrix, which strengthen interfacial bonding and delay thermal decomposition [28,29]. Across all membranes, the first major weight-loss event occurs between 250–350 °C, associated with the degradation of side groups and early backbone scission [23], while a sharp mass loss above ~600 °C corresponds to complete structural decomposition. Overall, oxygen plasma treatment plays a significant role in modifying surface chemistry, improving interfacial adhesion, and enhancing the thermal robustness of selected nanofiber systems, particularly PES and PES–GO.

3.4. Fourier Transform Infrared (FTIR) Spectroscopy (Just to Add Main Functions)

Fourier Transform Infrared (FTIR) spectroscopy was used to study the chemical structure and functional groups of PES, PES-PVP, and PES-GO nano-fiber before and after oxygen plasma treatment, as shown in Figure 4a,b, respectively.

The FTIR spectra of PES, PES-PVP, and PES-GO before oxygen plasma treatment show unique chemical features for each material. In the case of pure PES, peaks are observed at around 1150 cm^−1^ and 1240 cm^−1^, which correspond to the stretching vibrations of the sulfone (SO_2_) groups in its structure. A smaller peak at 1577 cm^−1^ represents the stretching of C=C bonds in the aromatic rings of PES [29,30,31]. When PVP is added to PES (PES-PVP), a new peak appears around 1660 cm^−1^, indicating the presence of a carbonyl (C=O) group from the amide structure of PVP [32]. A similar peak is also observed in PES-GO, originating from the carboxyl (-COOH) groups, as shown in the enlarged region of Figure 4a on the right. Additionally, a broad peak around 1402 cm^−1^ corresponds to O-H bonds, which are present in the hydroxyl groups on the surface and edges of GO. Furthermore, the C=C bond peak at 1577 cm^−1^ becomes much stronger, showing the presence of aromatic carbon structures in GO, which interact with the PES matrix [33,34,35,36]. Although the oxygen plasma treatment enhances the hydrophilicity of the membranes, the FTIR spectra for PES, PES-PVP, and PES-GO show no significant changes in their surface properties, as shown in Figure 4b.

3.5. X-Ray Photoelectron Spectroscopy

The XPS analysis of PES, PES-PVP, and PES-GO membranes provides detailed insights into their surface chemical composition, offering information that cannot be obtained from FTIR. The S 2p, C 1s, and O 1s spectra were studied for the fabricated membranes before and after oxygen plasma treatment. Figure 5 shows the XPS spectra of PES membrane. Prior to plasma treatment, the sulfur (S 2p) spectra show distinct peaks corresponding to SO_3_ groups, confirming the presence of oxidized sulfur species intrinsic to the PES backbone (Figure 5a). Carbon (C 1s) spectra highlight C–C bond in the PES backbone, in addition to C–O bonds in the ether functional groups PES structure (Figure 5b). The oxygen (O 1s) spectra reveal peaks for C-O-C (ether linkages) and S=O (sulfone groups), as seen in Figure 5c [37,38].

The calculated oxygen-to-carbon (O/C) ratio is 0.22. The C/O ratio was calculated using the fitting protocols of de-convolution of the C 1s spectra, containing carbon-oxygen bonds. The total area (A_tot_) of all C 1s peaks is directly proportional to the number of carbon atoms present. The area of each single peak (A_i_) is proportional to the number of carbon atoms in the relative functional groups. In this way, it is possible to estimate the overall O/C ratio given by the contribution of the different species present, such as hydroxyl (1-1), epoxy (1-2), carbonyl (1-1) and carboxyl (2-1), according to the Formula (2) [39].

$$[eqn]$$

By using this approach, there is no need to monitor O 1s peak, thus avoiding any contribution from oxygen that is not chemically bound to carbon.

The XPS analysis of the PES membrane after three minutes of O_2_ plasma treatment reveals significant changes in the surface chemistry compared to the untreated membrane, as shown in Figure 5. In the sulfur (S 2p) spectra, the emergence of a new peak corresponding to SO_2_^−^ [40]. The peak associated with SO_3_^−^ is still present, as shown in Figure 5d. In the carbon (C 1s) spectra, new contributions from C=O (carbonyl) groups are evident, along with the existing peaks for C-C. This indicates that the plasma treatment not only oxidized the surface but also modified the chemical environment of the carbon atoms, as shown in Figure 5e. Additionally, the oxygen-to-carbon (O/C) ratio increases from 0.22 (untreated) to 0.52 (treated), confirming enhanced oxidation of the surface. In the oxygen (O 1s) spectra, shows the introduction of carbonyl functional groups due to plasma-induced surface oxidation. Additionally, the C-O-C peak remains prominent, showing the retention of ether linkages, as seen in Figure 5f.

Overall, the 3 min O_2_ plasma treatment effectively introduces new oxygen-containing groups, such as C=O and SO_2_^−^ and increases the oxidation state of the surface. These modifications can improve the surface properties, such as hydrophilicity and reactivity, making the membrane more suitable for applications requiring enhanced surface functionality. This analysis provides a clear understanding of the chemical changes induced by the plasma treatment.

XPS analysis of PES-PVP in Figure 6 reveals key functional groups that provide insight into its chemical structure. In the sulfur (S 2p) spectra, the presence of a distinct peak corresponding to SO_3_ groups confirms the incorporation of sulfone functionality within the membrane, as shown in Figure 6a. The carbon (C 1s) spectra show peaks for C-C bonds, indicative of aromatic carbon in the PES backbone, as well as C-H bonds, which are associated with the aliphatic CH_2_ groups in the PVP polymer, as shown in Figure 6b [41]. In the oxygen (O 1s) spectra, hydroxyl groups (–OH) and C-O-C (ether linkages) peaks are observed, highlighting the complex chemical environment of the membrane. These findings confirm the presence of both PES and PVP components, with characteristic functional groups that contribute to the membrane’s overall structure and properties.

After PES-PVP membrane was treated with oxygen plasma for three minutes, significant changes were observed in the XPS spectra. In the sulfur (S 2p) spectra, the SO_3_ group peak remained, indicating that the sulfone functionality in PES was preserved despite the plasma treatment. The peak in S 2p indicates sulfur in more highly oxidized states, sulfonic acid (–SO_3_H). This peak arises from changes in the sulfur species on the surface of PES due to the oxidation caused by the plasma treatment, as shown in Figure 6d. The carbon (C 1s) spectra revealed new functional groups, suggesting the formation of additional oxygen-containing groups such as carbonyl (C=O) groups due to the plasma-induced oxidation, as shown in Figure 6e. After plasma treatment the O/C ratio is 0.82 confirming oxidation of membrane. In the oxygen (O 1s) spectra, the peak associated with hydroxyl (OH) groups appeared, originating from the PVP structure, confirming that the plasma treatment not only altered the PES surface but also enhanced the presence of hydrophilic functional groups. These modifications contribute to the improved surface reactivity and hydrophilicity of the PES-PVP membrane, making it more suitable for applications requiring enhanced surface properties.

XPS analysis of PES-GO in Figure 7 provides valuable insights into the surface chemical composition, revealing key interactions between PES and graphene oxide (GO). In the sulfur (S 2p) spectra, the presence of the SO_3_ group confirms the sulfone functionality within the PES backbone, as shown in Figure 7a. The carbon (C 1s) spectra show peaks for C-O, C=O, and C-C bonds, indicating the presence of ether linkages, carbonyl groups, and aromatic carbon structures from both the PES matrix and the GO, as shown in Figure 7b [42,43,44,45,46]. Additionally, the oxygen (O 1s) spectra display peaks for S=O, associated with sulfone groups in PES, and C-O-C, corresponding to the ether linkages in the polymer. These findings demonstrate the successful incorporation of GO into the PES matrix, with the presence of both the PES and GO-related functional groups, which can influence the membrane’s properties and surface reactivity.

After the PES-GO membrane was treated with oxygen plasma for three minutes, significant changes in the surface chemistry were observed through XPS analysis, as shown in Figure 7. In the sulfur (S 2p) spectra, a new peak corresponding to the sulfonic acid (–SO_3_H) group appeared alongside the existing SO_3_ group, indicating further oxidation of the sulfone functionality on the PES surface, similar to the changes observed in PES-PVP after plasma treatment, as shown in Figure 7d [47,48]. The carbon (C 1s) spectra revealed the presence of oxygenated carbon species such as carbonyl groups in GO or oxidized PES, in addition to hybridized carbon bond, as shown in Figure 7e. Furthermore, the oxygen-to-carbon (O/C) ratio increases from 0.27 (untreated) to 0.35 (treated), indicating a significant enhancement in surface oxidation. In the oxygen (O 1s) spectra, peaks for hydroxyl (OH) and C-O-C groups appeared, indicating the formation of new hydrophilic groups on the surface. These modifications suggest that the plasma treatment significantly enhanced the surface reactivity of the PES-GO membrane.

By using oxygen plasma treatment, new functional groups were introduced to PES, PES-PVP, and PES-GO membrane surfaces, including SO_3_H, C=O, and OH. During this treatment, plasma creates reactive species that interact with membrane surfaces, promoting oxidation and the formation of these hydrophilic and reactive functional groups. Therefore, plasma treatments enhance membrane surface reactivity and functionality, resulting in better performance in separation and filtration [49,50,51].

3.6. Antibacterial Activity

Figure 8 shows the OD600 values for both E. coli and S. aureus. It can be observed that all membrane types for both bacteria exhibit OD600 values around 0.1, indicating low bacterial density. Interestingly, the plasma-treated PES membrane shows inhibition of 14% for E. coli and 20% for S. aureus compared to non-treated PES. Plasma treatment of PES membranes makes them superhydrophilic by introducing polar groups, which enhance water affinity. While increased wettability may initially promote bacterial adhesion, it also creates an environment that inhibits bacterial growth. Plasma treatment generates reactive oxygen species on the membrane surface, which possess antimicrobial properties [52,53,54]. Similarly, in PES-GO, the distribution of graphene oxide (GO) within the membrane could be uneven, limiting its ability to effectively interact with bacteria and reduce bacterial growth. The PES membranes were treated for only three minutes, does this mean longer oxygen plasma treatment could further enhance bacterial inhibition? To investigate this further, PES and PES-PVP membranes were treated for 5 min inhibition of 14% for E. coli was measured again, as it can be seen in Figure S2 in Supplementary. It can be seen clearly that longer plasma treatment inhibits bacterial growth. Therefore, despite the superhydrophilic nature of the plasma-treated PES membrane, plasma treatment can reduce bacterial viability and prevent their growth.

3.7. Evaluation of Membrane Hydrophilicity

The flux of water, PSS solution, an oil–water mixture from an Omani petroleum field (labeled as mother solution), and a mixture containing both PSS and oil–water was measured through PES nanofiber-based membranes using a dead-end filtration system under gravity, are shown in in Figure 9. Initially, the pristine PES membrane exhibited no permeation of any of these solutions, which can be attributed to its intrinsic hydrophobic nature. However, following three minutes of oxygen plasma treatment, PES membrane became hydrophilic and allowed permeation of all the tested solutions. Flux measurements were conducted for pure water, PSS solution, oil–water mixture, and the PSS/oil–water mixture, yielding values of 800, 700, 200, and 150 L/h m^2^, respectively. The PES-PVP membrane has flux values of 900, 600, 50, and 125 L/h m^2^, similar to the plasma-treated PES-PVP membranes. The results show the importance of plasma treatment and improving membrane permeability, especially in oil related applications. A striking result showed that the plasma treated PES membrane produced a higher flux of oil-field solution compared to the PES-PVP membrane. This indicates that treating the PES membrane with oxygen plasma improves its water absorption capacity, which contributes to enhancing its permeability to complex mixture.

Super-hydrophilic and hydrophobic membranes offer distinct advantages for liquid–liquid separation. Super-hydrophilic membranes, such as PES–PVP nanofiber membranes, preferentially allow water to permeate while repelling oil, making them highly effective for separating water from oil. Conversely, hydrophobic membranes selectively permit oil passage while blocking water, enabling efficient oil recovery from water–oil mixtures. As demonstrated in QR code video in Section S3 in Supplementary, the PES–PVP membrane successfully separated water from an oil–water mixture, highlighting its potential for practical applications in wastewater treatment and oil–water separation processes.

3.8. Water Recovery and Fouling

In the study of membrane fouling, membrane performance was evaluated by quantifying and comparing the permeate water flux before and after filtration of a polystyrene sulfonate (PSS) solution, as shown in Figure 10. In the case of the pristine PES membrane, known for its hydrophobic nature, no water permeated was observed. The oxygen plasma-treated PES membrane-maintained water flux and recovery, with its hydrophilic surface contributing to a notable improvement in fouling resistance. The slight increase in water flux after filtration is attributed to improved wettability of the membrane surface, which facilitates water penetration into the pores and reduces flow resistance, and this change is considered within the experimental error range. Water flux through the PES–PVP membrane decreased from 819 to 707 L/h m^2^, representing a 13.7% reduction, while the plasma-treated PES–PVP membrane showed a smaller decrease from 848 to 754 L/h m^2^, corresponding to an 11.1% reduction. This indicates that both membranes experience some flux decline, likely due to initial fouling or solution resistance, but the plasma-treated membrane retains water flux more effectively. The results highlight the beneficial effect of plasma treatment in enhancing membrane stability and maintaining higher performance during filtration.

3.9. SEM Investigate of Membrane Surface Morphology and Fouling

SEM analysis provided detailed insights into the surface morphology of the four membranes after performing dead-end filtration with a heavy Omani oil–water mixture. SEM images of pristine and plasma-treated PES revealed clean surfaces with no visible fouling, indicating that plasma treatment effectively enhanced surface hydrophilicity and promoted water permeation without contaminating the surface, are shown in Figure 11a.

In contrast, SEM analysis of PES-PVP membranes, both untreated and plasma-treated membrane showed distinct evidence of adsorbed and adhered particles on the membrane surface. These fouling features suggest that the incorporation of PVP, while typically used to improve hydrophilicity, may have also increased surface affinity for oil or emulsified contaminants, making the membrane more susceptible to fouling.

3.10. Literature Comparison of Plasma-Treated Membranes

Across the literature, plasma treatment consistently enhances membrane hydrophilicity and water flux, though the degree of improvement depends on membrane type, plasma gas, and any grafting steps. Studies using H_2_O, Ar, and mixed-gas plasmas [6,8,9,11] generally report large drops in water contact angle, often to 0°, and noticeable increases in flux. More complex treatments, such as Ar plasma with acrylic acid grafting [8] or corona air plasma with PEG grafting [10], also improve surface wettability and permeability, though sometimes less dramatically (Table 3).

In comparison, this study shows one of the strongest responses to plasma modification. Oxygen plasma reduced the PES membrane contact angle from 135° to 0°, increasing flux from 0 to 702.9 L/h m^2^, while the PES-PVP membrane also improved slightly in flux after treatment. These results highlight the high effectiveness of short O_2_ plasma exposure in introducing hydrophilic functional groups and enhancing performance.

Overall, the comparison suggests that O_2_ plasma is a powerful tool for engineering PES-based membranes. Future work should explore different plasma gases and gas mixtures to further tune membrane hydrophilicity, antifouling properties, and long-term stability for advanced water treatment applications.

4. Conclusions

This study demonstrates the successful engineering of PES-based nanofiber membranes (PES, PES–PVP, and PES–GO) using oxygen plasma treatment to precisely tailor their surface chemistry and wettability. Plasma modification introduced new functional groups and transformed the initially hydrophobic membranes into super-hydrophilic ones without altering their fiber morphology, thereby enhancing their surface reactivity and antimicrobial potential. Overall, the findings show that plasma-engineered nanofiber membranes offer a versatile and promising platform for advanced separation, water purification, and multifunctional membrane applications.