Design of hydrophobic polyurethane–magnetite iron oxide-titanium dioxide nanocomposites for oil-water separation

Abstract

Efficacious oil-water separation has become a global challenge owing to regular oil spillage accidents and escalating industrial oily wastewater. In this study, we synthesized titanium dioxide and magnetite iron oxide nanoparticles to use as a precursor for the production of the nanocomposites. Hydrophobic nanocomposites were fabricated using polyurethane, hematite and magnetite iron oxide nanoparticles, and titanium dioxide nanoparticles through a sol-gel process. The formation of the obtained nanocomposites was confirmed by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM) analyses. In addition, the thermogravimetric and differential thermogravimetric (TGA/DTG) and BET surface area results exhibited enhanced thermal stability of the optimized nanocomposite which displayed mesoporous type materials feature with high porosity. Furthermore, the obtained outcomes demonstrated that the distribution of nanoparticles into a polymer matrix had a significant impact on enhancing superhydrophobicity and the separation efficiency against sunflower oil. Seeing the water contact angle of the nanocomposite-coated filter paper was about 157° compared to 0° for the uncoated filter paper and endowed separation efficiency of almost 90% for 5 consecutive cycles. Thereby, these nanocomposites could be an ideal candidate for self-cleaning surfaces and oil-polluted water purification.

1. Introduction

Sustainable clean water supply has emerged as one of the greatest issues facing the world since rapid industrial development and frequent crude oil leakages have led to the generation of large amounts of oily wastewater. These indecomposable waste oils not only threatened the ecosystem but also greatly aggravated water scarcity [[1], [2], [3]]. Conventional separation methods due to the high stability of oil–water mixtures, have limited effectiveness in treating oil-water mixtures. Therefore, it is urgent to develop new technology and advanced materials which efficiently separate oil/water mixtures [[4], [5], [6]].

Nanotechnology is cutting-edge science, having the potential to curb challenges confronted in the existing water treatment crisis. It has the aptitude to cater the solutions related to water pollution detection, remediation, and purification with the technological use of nanomaterials. Engineered nanocomposites with special wettability, particularly hydrophobic-oleophilic materials are an appealing choice for various applications [[7], [8], [9]].

Diverse strategies including chemical vapor deposition, phase separation, spray coating, ink-jet printing, and sol-gel method, have been utilized to make hydrophobic surfaces on variant substrates. The hydrophobic surfaces with high water contact angles and low sliding angles features have been broadly sought after due to the huge potential in various applications such as transparent and antireflective surfaces, fluidic drag reduction, anti-fog and anti-freezing to windows and antennas, self-cleaning and anti-biofouling surfaces, and oil-water separation [10,11].

Polyurethane (PU)-based nanocomposites are the most commonly used and beneficial coating materials that can be tailored for versatile applications, particularly in self-cleaning surfaces and oil-water separation fields. The flexibility in the variety of macro diols, di-isocyanates, and chain extenders in PU provide broad relevance of PU coatings with a widespread scope of physical, adhesion, and mechanical properties [[12], [13], [14], [15]].

Currently, there is a growing interest in nanoparticles of metal oxides. The excellent performance of titanium dioxide (TiO₂) nanoparticles in a wide range of disciplines such as heterogeneous photo-catalyst, water treatment, self-cleaning surfaces, food and pharmaceutics, UV blockers, and white pigment in paints, which are attributed to their good physical and chemical properties [[16], [17], [18]]. Tremendous sensitivity towards ammonia gas, high electrical conductivity, and good thermal stability of polypyrrole-silver-TiO₂ [19], polyaniline-cerium- TiO₂ [20], and polyaniline phenothiazine-TiO₂ [21] nanocomposites, the great self-cleaning activity of acrylic-TiO₂ nanocomposite [22], and appropriate electrical, thermal and mechanical properties of poly butyl methacrylate-neodymium-TiO₂ nanocomposites [23] were attributable to the existence of TiO₂ nanoparticles in these nanocomposites.

The attention of researchers is focused on magnetite iron oxide nanoparticles (Fe₃O₄), as a kind of spinel ferrite with stable magnetic permeability and superparamagnetism, that have versatile properties and possible applications in magnetic resonance imaging, data storage, biological labels for anticancer therapy, electromagnetic interference shielding, wastewater treatment and oil recovery, and catalysis [[24], [25], [26]]. The presence of Fe₃O₄ nanoparticles in fabricated polyvinylidene fluoride-Fe₃O₄ nanocomposite by electrospinning technique [27], prepared poly anthranilic acid-Fe₃O₄ nanocomposite via in situ chemical oxidative polymerization process [28], and produced polymer-Fe₃O₄@graphene nanocomposite using DLF procedure [29], enhanced electrical, magnetic, and mechanical properties of the nanocomposites.

Hematite iron oxide (Fe₂O₃) nanoparticle is of particular curiosity owing to its outstanding electrical and magnetic properties, low cost, and environmentally friendly features. By virtue of its fascinating structural, suitable surface, thermal, and chemical characteristics, has been widely explored in pigments, water treatment, biomedical (drug delivery, stimuli-responsive stents, and magnetic resonance imaging), lithium-ion battery, storage media, sensing, and photochemical applications [30,31]. The Fe₂O₃ nanoparticles in Fe₂O₃-poly-aminoanthraquinone@BiVO nanocomposite boosted the water-splitting efficiency by accelerating the proton-coupled electron transfer process [32].

Here, we synthesized TiO₂ and Fe₃O₄ nanoparticles first to use them as a precursor for the preparation of nanocomposites. Since there was no report on polyurethane-iron oxide-TiO₂ nanocomposite in literature, this study aimed to fabricate hydrophobic nanocomposites using synthesized TiO₂ and Fe₃O₄ nanoparticles, polyurethane pellets, and Fe₂O₃ nanoparticles. Fe₃O₄ and Fe₂O₃ nanoparticles were selected due to their good magnetic properties which improve hydrophobic property and TiO₂ nanoparticle was chosen due to its hydrophobic feature. The newly formed nanocomposites were characterized for their structural, surface morphology, and thermal stability. The hydrophobicity, the performance of the nanocomposites in oil-water separation, separation efficiency, and reusability were evaluated.

2. Experimental

2.1. Materials

Ferrous chloride tetrahydrate (FeCl₂ ⋅ 4H₂O, 98%), ferric chloride hexahydrate (FeCl₃ ⋅ 6H₂O, 99%) , hydrochloric acid (HCl, 37%), ethanol, and N, N-dimethylformamide (DMF) were acquired from Merck, Germany. Sodium hydroxide (NaOH) was purchased from Friedemann Schmidt (Parkwood, WA, USA). Titanium(IV) butoxide (Ti(O-Bu)4, 98%), ACROS Organics, Geel, Belgium. Polyurethane (PU) pellets (C₁₇H₁₆N₂O₄, molecular weight = 312 g/mol, density = 1.18 g/cm³) were gained from Lubrizol Advanced Material, Korea. Deionized water was used in the experiments. Sunflower oil was obtained from the local market.

2.2. Preparation of nanoparticles and nanocomposites

2.2.1. Synthesis of TiO₂ nanoparticles and Fe₃O₄ nanoparticles

TiO₂ nanoparticles were synthesized through an established sol-hydrothermal method [33] and the co-precipitation technique was used to prepare Fe₃O₄ nanoparticles as previously reported [34].

2.2.2. Synthesis of PU-Fe₃O₄- TiO₂ nanocomposite

An appropriate amount of PU pellets was dissolved in DMF under vigorous stirring at a temperature of 50^◦C. Then, the synthesized TiO₂ and Fe₃O₄ nanoparticles were added in two conical flasks which contain DMF and the samples were sonicated for 1 h. After the dispersion of the nanoparticles, the dispersed nanoparticles were slowly added to the PU solution and the mixture was stirred for 6 h at 40 °C. Finally, the mixture was dried in the oven at 60 °C for 48 h. The amount of Fe₃O₄ and TiO₂ nanoparticles was varied and the samples are referred to as PUMNPTNP1, PUMNPTNP2, and PUMNPTNP3 (Table 1 and Fig. 1).

Table 1.

PU, Fe₃O₄ nanoparticles, and TiO₂ nanoparticles compositions used for the preparation of the PUMNPTNP nanocomposites.

Nanocomposite	PU (g)	Fe3O4 nanoparticles (g)	TiO2 nanoparticles (g)
PUMNPTNP1	1	0.5	0.5
PUMNPTNP2	1	1	0.5
PUMNPTNP3	1	1	1

Fig. 1.

Schematic diagram of PU-Fe₃O₄–TiO₂ nanocomposite preparation.

2.2.3. Synthesis of PU-Fe₂O₃- TiO₂ nanocomposite

The PU-Fe₂O₃–TiO₂ nanocomposites nanocomposites were fabricated by a sol-gel process. First, Fe₂O₃ and synthesized TiO₂ nanoparticles were dispersed in DMF (15 mL) under ultrasonic for 1 h. Meanwhile, PU pellets were dissolved in DMF (25 mL) at a temperature of 50 °C with stirring for 2 h. Then the TiO₂ and Fe₂O₃ suspensions were gently poured into a PU solution and the mixture was stirred for 6 h at 40 °C. The resulting slurry was then oven-dried at 60 °C for 48 h. The samples containing different amounts of TiO₂ and Fe₂O₃ nanoparticles were labeled as PUHNPTNP1 and PUHNPTNP2 (Table 2).

Table 2.

PU, TiO₂ nanoparticles, and Fe₂O₃ nanoparticles compositions used for the preparation of the PUHNPTNP nanocomposites.

Nanocomposite	PU (g)	TiO2 nanoparticles (g)	Fe2O3 nanoparticles (g)
PUHNPTNP1	1	0.5	0.5
PUHNPTNP2	1	1	1

2.3. Characterization

Fourier transform infrared spectra of the samples were recorded over the range of 400–4000 cm⁻¹ on a Thermo Nicolet FTIR (AEM, Madison, WI, USA) with 4 cm⁻¹ resolution using the potassium bromide disc method. The morphologies of Fe₃O₄ and TiO₂ nanoparticles, and the nanocomposite were observed by scanning electron microscopy (SEM, JEOL, JSM-7610F, Japan). Surface characterization of the nanocomposite was carried out using the nitrogen gas adsorption–desorption technique at 77 K, using a Micromeritics ASAP 2000 (Norcross, GA, USA). The composition and crystalline structure of the materials were analyzed via an XRD-6000 diffractometer (Shimadzu, Tokyo, Japan) by studying the angles from 5 to 70° using CuKα radiation (λ = 1.5406° A) at 40 kV and 30 mA. Thermogravimetric and differential thermogravimetric analyses (TGA/DTG) were carried out using a Mettler-Toledo instrument (Greifensee, Switzerland) in the range of 20–1000 °C at a heating rate of 10 °C/min under a nitrogen atmosphere to determine the thermal behavior of the nanocomposite.

2.4. Preparation of oil-water emulsion

The oil-water emulsion was prepared using commercial sunflower oil and distilled water following vigorous mechanical stirring of the mixture for 25 min. The oil concentration of the emulsion was 150 g/L.

2.5. Coating of the nanocomposites on filter paper

The obtained sol-gel nanocomposite solution was kept in the oven for 24 h and after evaporation of some amount of solvent, we added the nanocomposite solution through ink injection on the filter paper with the pore size of 20–25 μm. It was then allowed to dry in the oven for 8 h at 60 °C.

3. Results and discussion

3.1. X-ray diffraction

The XRD patterns of PU, Fe3O4 nanoparticles, Fe2O3 nanoparticles, TiO2 nanoparticles, PUMNPTNP1 nanocomposite, PUMNPTNP2 nanocomposite, PUMNPTNP3 nanocomposite, PUHNPTNP1 nanocomposite, and PUHNPTNP2 nanocomposite were shown in Fig. 2, Fig. 3.

Fig. 2.

X-ray diffraction patterns for PU (A), Fe₃O₄ nanoparticles (B), TiO₂ nanoparticles (C), and PUMNPTNP1 nanocomposite (D), PUMNPTNP2 nanocomposite (E), and PUMNPTNP3 nanocomposite(F).

Fig. 3.

X-ray diffraction patterns for PU (A), Fe₂O₃ nanoparticles (B), TiO₂ nanoparticles (C), and PUHNPTNP1 nanocomposite (D), and PUHNPTNP2 nanocomposite (E).

The XRD pattern of PU (Fig. 2, Fig. 3A) revealed the broad peak at around 2θ = 20.3° which indicates a typical pattern for a low crystalline material [35].

In Fig. 2B, the characteristic diffraction peaks of synthesized Fe₃O₄ nanoparticles were marked by their indices (220), (311), (400), (422), (511), and (440) which proved the formation of cubic inverse spinal structure of Fe₃O₄ nanoparticles [36]. The crystallite size was calculated using the Debye -Scherrer's equation $\left(D=\frac{K\mathrm{ʎ}}{\beta \mathit{cos}\theta }\right)$, where D was the average particle size, K was the Scherrer constant (0.9), ʎ was the XRD wavelength (0.15418 nm), β was the peak width of half maximum intensity, and θ was the Bragg diffraction angle. Thus the mean grain size was obtained 29 nm [37].

The diffractogram of the fabricated TiO₂ (Fig. 2, Fig. 3C) exhibited diffraction peaks at (110), (101), (200), (111), (210), (211), (220), (002), (310), (301) and (112) crystallographic planes, which agree with those reported for the standard crystal structure of rutile (TiO₂). The crystal size (D) of the TiO₂ could be determined through Debye -Scherrer's equation and was around 31 nm [38,39].

As shown in Fig. 2D, the broad diffraction peak of pristine PU, typical diffraction peaks of Fe₃O₄, and characteristic bands of TiO₂ nanoparticles were indicated the presence of polymer, Fe₃O₄, and TiO₂ nanoparticles in the PUMNPTNP1 nanocomposite. It was observed that the intensity of Fe₃O₄ and TiO₂ nanoparticles peaks was increased by increasing the amount of Fe₃O₄ and TiO₂ nanoparticles loaded on PUMNPTNP2 nanocomposite (Fig. 2E), and PUMNPTNP3 nanocomposite (Fig. 2F), respectively.

In the case of Fe₂O₃ nanoparticles (Fig. 3B), the distinct peaks at 24.11°, 33.12°, 35.26°, 40.72°, 49.36°, 53.74°, 57.81°, 62.46°, and 63.86°, could be assigned to (012), (104), (110), (113), (024), (116), (018), (214), and (300) planes, which are consistent with the standard pattern of Fe₂O₃ (JCPDS, No. 86-0550) [40].

These characteristic peaks of PU, Fe₂O₃ nanoparticles, and TiO₂ nanoparticles were observed in PUHNPTNP1 nanocomposite (Fig. 3D), and the intensity of Fe₂O₃ and TiO₂ nanoparticles peaks was raised by the increasing amount of Fe₂O₃ nanoparticles and TiO₂ nanoparticles loaded on PUHNPTNP2 nanocomposite (Fig. 3E).

3.2. FTIR spectroscopy

Fig. 4, Fig. 5 displayed the FTIR spectra of PU, Fe₃O₄ nanoparticles, Fe₂O₃ nanoparticles, TiO₂ nanoparticles, PUMNPTNP1 nanocomposite, PUMNPTNP2 nanocomposite, PUMNPTNP3 nanocomposite, PUHNPTNP1 nanocomposite, and PUHNPTNP2 nanocomposite. The PU was used as a reference polymer to elucidate the successful preparation of the nanocomposites.

Fig. 4.

FTIR spectrums for PU (A), Fe₃O₄ nanoparticles (B), TiO₂ nanoparticles (C), and PUMNPTNP1 nanocomposite (D), PUMNPTNP2 nanocomposite (E), and PUMNPTNP3 nanocomposite (F).

Fig. 5.

FTIR spectrums for PU (A), Fe₂O₃ nanoparticles (B), TiO₂ nanoparticles (C), and PUHNPTNP1 nanocomposite (D), and PUHNPTNP2 nanocomposite (E).

In the FTIR spectrum of pure PU (Fig. 4, Fig. 5A), the stretching vibration of N–H appeared at 3295 cm⁻¹ and the peaks at 2926 cm⁻¹ and 2866 cm⁻¹ were related to the symmetric and asymmetric stretching vibration of CH₂ groups. The band at 1712 cm⁻¹ is attributed to the C=O group of the urethane bond, while the NH out-of-plane bending vibration and C–N stretching vibration, were recorded at 1512 cm⁻¹ and 1307 cm⁻¹. The peaks at 1594 cm⁻¹ and 1435 cm⁻¹ were due to the stretching vibration of C=C, and the characteristic band at 1220 cm⁻¹ was ascribed to the C–O stretching vibration [35,41].

In the FTIR spectrum of synthesized Fe₃O₄ nanoparticles (Fig. 4B), the band at 541 cm⁻¹ was defined as typical stretching vibration of Fe–O [37] and it was shifted to the higher wave numbers in the nanocomposites due to the interaction of Fe₃O₄ nanoparticles with PU.

In the FTIR spectrum of synthesized TiO₂ nanoparticles (Fig. 4, Fig. 5C), a distinct peak at 485-685 cm⁻¹ was regarded with the stretching and bending modes of Ti–O groups which are consistent with the previous reports in the literature for the rutile phase of TiO2 nanoparticles [42]. This band was shifted to the higher frequencies in the nanocomposites as a result of the interaction between PU and TiO₂ nanoparticles.

In the FTIR spectrum of Fe₂O₃ nanoparticles (Fig. 5B), the vibrational bands of Fe–O were recorded at 434 cm⁻¹ and 520 cm⁻¹ [43] and these peaks slightly shifted in the nanocomposites.

For the nanocomposites (Figs. 4D and E, and 5D–F), due to the interaction between Fe₃O₄, Fe₂O₃, and TiO₂ with PU, the typical peak of C=O functional group, NH out of plane bending vibration, C–O stretching vibration, and the stretching vibration of N–H were observed with low intensity and shifted to the lower frequencies in the presence of these nanoparticles. There was an interaction between the oxygen of the Fe₃O₄, Fe₂O₃, and TiO₂ nanoparticles with the NH of the urethane bond forming complexes. This was probably due to the formation of hydrogen bonding between PU with the nanoparticles and the hydrogen bonding enhances the adhesion between the nanoparticles and the polymer matrix.

3.3. Surface morphology and N2 adsorption-desorption analysis

The morphologies of synthesized Fe₃O₄ nanoparticles, PUMNPTNP3 nanocomposite, and prepared TiO₂ nanoparticles were observed by scanning electron microscope (SEM). Fig. 6(A–C) indicated that the synthesized TiO₂ nanoparticles had a spherical shape with a mean size of 31 nm. The SEM images of the prepared Fe₃O₄ nanoparticles were depicted in Fig. 6(D–F). The roughly monodispersed spherical particles with an average size of 29 nm were observed. In Fig. 6(G–I), it can be seen that by the distribution of Fe₃O₄ and TiO₂ nanoparticles to the PU matrix to form polymer nanocomposite, the PU density and surface roughness were increased. Besides, irregular and porous surfaces in PUMNPTNP3 nanocomposite were perceived which was ascribed to the dispersion of Fe₃O₄ and TiO₂ nanoparticles and strong interface adhesion in the PU matrix.

Fig. 6.

SEM images of TiO₂ nanoparticles (A–C), Fe₃O₄ nanoparticles (D–F), and PUMNPTNP3nanocomposite (G–I).

The surface area and porosity of the PUMNPTNP3 nanocomposite were analyzed using N₂ adsorption-desorption isotherms (Figs. 7A and 7B). According to Brunauer-Emmett-Teller, type IV isotherm was detected for the PUMNPTNP3 nanocomposite, indicating the formation of a mesoporous structure [44]. Additionally, it could be observed from Fig. 7A that the N₂ uptake gradually rose with the growth of the relative pressure and the BET surface of the nanocomposite was around 45.0 m²/g. Even more, the pore volume and average pore diameter of the PUMNPTNP3 nanocomposite which were obtained from Barret–Joyner–Halenda (BJH) desorption pore size distribution plot were 1.8 cm³/g and 326 nm, respectively (Fig. 7B).

Fig. 7.

N₂ adsorption–desorption isotherms (A) and Barret-Joyner-Halenda method pore size distribution (B) of PUMNPTNP3 nanocomposite.

3.4. Thermal analysis

In order to verify the formation of the PUMNPTNP3 nanocomposite and investigate its thermal stability, the thermal behavior of the nanocomposite was measured via thermogravimetric and differential thermogravimetric (TGA/DTG) analyses. Fig. 8 demonstrated that the decomposition of PUMNPTNP3 nanocomposite progressed through two major stages of weight loss, occurring at temperature maxima of 319.9 °C and 406.6 °C, respectively. The two steps of weight loss were attributed to the thermodynamical incompatibility of the soft and hard domains in PU. The first step of weight loss was due to the depolycondensation reaction of PU to produce precursors such as polyols and isocyanates, then isocyanates were dimerized to form carbodiimides, accompanied by the volatilization of small molecules such as carbon oxides, alcohols, and aldehydes (degradation of the soft domain in PU). Besides, the second step was associated with the decomposition of the substituted urea resulting from the reaction of carbodiimides with water or alcohol (degradation of the hard domain in PU). However, in the prior studies were reported that pure PU was decomposed in two steps at the maximum temperature of 327 °C and 370 °C, respectively. Therefore, it can be concluded from the above results that the addition of Fe₃O₄ and TiO₂ nanoparticles into the PU matrix enhanced the thermal stability of the PU in the nanocomposite [39,45].

Fig. 8.

TGA/DTG curves of PUMNPTNP3 nanocomposite.

3.5. Surface wettability and oil-water separation performance of the nanocomposite coated filter paper

The surface wetting properties of the modified and unmodified filter papers in the air and ambient temperature were examined (Fig. 9). The deionized water was randomly dropped on the surfaces of the modified and unmodified filter papers and the water contact angle was measured by selecting several points of the same sample arbitrarily. The water droplet quickly penetrated into the unmodified filter paper and the contact angle was 0°. However, for the PUMNPTNP3 nanocomposite-coated filter paper, the shape of droplets remained unchanged and the water contact angle reached around 157° and remained substantially constant.

Fig. 9.

Photographs of surface wettability of unmodified (A) and modified (B and C) filter papers and oil-water separation set-up of modified filter paper (D–F).

Next, the oil−water separation test was carried out on a device as shown in Fig. 9 to further verify the function of the nanocomposite hydrophobicity. The nanocomposite-coated filter paper was placed into the funnel. The artificial oily wastewater was poured through a funnel with a nanocomposite-coated filter paper inserted. In the separation process, oil permeated through the coated filter paper and dripped into the below-collecting beaker (Fig. 9), but the water was repelled and stayed above the coated filter paper. This phenomenon divulged that these nanocomposites have hydrophobic features and the composites coating can separate oil−water mixtures.

To evaluate the separation performance for nanofilter, the amount of feed and passed oil after the separation process was recorded and calculated using the following equation

$$\gamma =\frac{m_1}{m_0}\times 100$$

where γ is the separation efficiency, m₁ is the mass of permeate and m₀ presents the mass of feed oil during the separation process [46]. To investigate the cycling stability of the proposed nanofilter, the separation process was repeated over 5 cycles. To confirm the accuracy of the reusability test, the prepared nanofilter was dried under a vacuum oven at 60 °C for 3 h.

However, all of these results revealed that the PUMNPTNP3 nanocomposite had the greatest hydrophobic property among the nanocomposites with around 90% separation efficiency which could be related to the high amount of Fe₃O₄ nanoparticles with superior magnetic features and TiO₂ nanoparticles with good hydrophobic properties in PUMNPTNP3 nanocomposite. Hence, this makes PUMNPTNP3 nanocomposite, a unique and promising hydrophobic material for practical applications.

3.6. Discussions

PU-based materials with hydrophobic-oleophilic properties have been reported for oil−water separation. PU is a commercially available material suitable for oil-water separation applications after appropriate modifications [[47], [48], [49], [50], [51]]. As the wettability is controlled by the surface roughness and structure, thus the hydrophobic and magnetic materials (TiO₂ and Fe₃O₄ nanoparticles) introduced hydrophobic properties in the nanocomposite by modification of surface roughness and structure of the nanocomposite (Fig. 6G–I). TiO₂ and Fe₃O₄ nanoparticles showed hydrophobicity with a large surface area and were often-desired modifiers with low surface energy and stability that improved the surface hydrophobicity in a controllable manner. For instance, a surface roughness induced superhydrophobicity of graphene nanocomposite was reported by incorporating Fe₃O₄ nanoparticles and TiO₂ nanoparticles induced superhydrophobicity in cotton. TiO₂ and Fe₃O₄ nanoparticles were employed as a filler to PU matrix by the strong dipole–dipole attractions, changed the surface roughness, and significantly improved the surface durability towards drop impact.

Likewise [46,[52], [53], [54], [55]], the oil trapped between the nanocomposite pores augmented water repellency and furthered water rejection. The hydrophobic PUMNPTNP3 nanocomposite offered enormous space for absorption and a bigger specific surface area to allow oil absorptivity.

The PUMNPTNP3 nanocomposite separation capacity was compared with previous studies (Table 3). Various materials like polymers, metal oxide nanoparticles, carbon materials, and metal organic-frameworks have been applied to get desirable surface wettability for oily water treatment. Surely, the Table data expressed that the newly accomplished nanocomposite has shown a good performance in sunflower oil–water separation among peers.

Table 3.

Recent advancements in super-wetting materials in oil-water emulsion separation.

Nanocomposite/composite	Method	Special wettability	Contact angle	Oil-water separation efficiency	Emulsified oils	References
Polystyrene-Fe3O4	Electrospinning	Superhydrophobic	162.0°	95.00 92.00	Olive oil, Sesame oil	[46]
Polydopamine-polypyrrole-polyaniline-Fe-stearic acid-polyurethane	Dip-coating two-step	Superhydrophobic	164.0°	99.00	–	[51]
Polyvinyl alcohol cross-linked tannic acid-magnetic TiO2-MXene	Magnetic self-assembly	Superhydrophilic	–	99.85	–	[56]
Polybutylene succinate nanofiber	Solution blow spinning	Superhydrophobic	–	98.10–99.50	Soybean oil, diesel, gasoline mineral oil, silicon oil	[57]
CaCO3 coated-stainless steel meshe	Biomineralization	Superhydrophilic	161.0°	98.50	–	[58]
Mg-silicate@glass fiber fabric	Universal epitaxial growth	Superhydrophilic	156.3°	99.50	Industrial oily sewage	[59]
Bacterial cellulose-cellulose nanocrystals	Immersion	Superhydrophilic	–	97.60	–	[60]
Sugarcane bagasse-nanofiber	Electrospinning	Superhydrophilic	–	99.60	–	[61]
Co3O4-stainless steel mesh	Co-precipitation, spray coating	Superhydrophilic	160.0°	99.00	–	[62]
Melamine foam-polydimethylsiloxane	One-pot emulsion polymerization	Superhydrophobic	156.8°	99.50	–	[63]
Graphitic carbon nitride/metal-organic framework-cotton	Hydrothermal, socking	Superhydrophobic	153.0°	99.00	Castor oil	[64]
Polytetrafluoroethylene @ zeolite imidazole	Electrospinning	Superhydrophobic	–	99.99	Kerosene	[65]
Zeolite mesh	Seed deposition	Superhydrophilic	–	99.98	Diesel, kerosene, rapeseed oil	[66]
Cellulose II aerogels	Lithium bromide system	Superhydrophobic	150.0°	96.00	Various oils	[67]
Lignin-cellulose aerogel	Sol-gel, freeze-drying	Superhydrophilic	–	99.00	–	[68]
FPUF@MOF-LDH@HDTMS	In-situ growth	Superhydrophobic	153.0°	99.1	–	[69]
TiO2 cotton fabric	Immersion	Superhydrophobic	157.3°	98.4	–	[70]
Polyimide-cattail- derived active carbon aerogel	Freeze-drying and thermal imidization	Superhydrophobic	151.3°	98.3	Various oils	[71]
Melamine sponge-reduced graphene oxide- active carbon- polydimethylsiloxane	Two-step dipping	Superhydrophobic	164.0°	99.85	–	[72]
Sodium alginate-graphene oxide	–	Superhydrophilic	–	93.26	–	[73]
Polyurethane-Fe3O4–TiO2	Sol-gel	Superhydrophobic	157°	90	Sunflower oil	This work

4. Conclusion

The development of effectual and viable hydrophobic materials is a crucial issue and global demand for self-cleaning materials and oily water treatment. In the current research, we described the fabrication of hydrophobic nanocomposites by sol-gel technique and using PU, Fe₂O₃ nanoparticles, Fe₃O₄ nanoparticles, and TiO₂ nanoparticles. The preparation of precursors Fe₃O₄ and TiO₂ nanoparticles were performed via co-precipitation and sol-hydrothermal methods, respectively. The synthesized nanocomposites indicated hydrophobic properties. Hydrophobic PUMNPTNP3 nanocomposite coated on the surface of filter paper possessed good oil-water separation efficiency. The present exploration will open an avenue for the design and development of materials with special wettability and the relevant utilization in self-cleaning surfaces and oil-water separation.

Author contribution statement

Asma khnadan Barani: Performed the experiments; Contributed reagents, materials, analysis tools or data.

Ghodratollah Roudini: Conceived and designed the experiments; Contributed reagents, materials, analysis tools or data.

Farahnaz Barahuie: Conceived and designed the experiments; Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote the paper.

Siti Ujila Binti Masuri: Analyzed and interpreted the data.

Data availability statement

Data will be made available on request.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.