INTRODUCTION
Nature serves as a profound source of inspiration for scientists and engineers, leading to innovative inventions that enhance human life By observing and mimicking the materials, structures, and systems found in the natural world, researchers engage in a creative process known as biomimicry A prominent example of biomimicry is the development of superhydrophobic surfaces, which are inspired by the waterproof, self-cleaning, and anti-adhesion properties of natural elements like lotus leaves and butterfly wings These remarkable properties are being studied and utilized across various fields, including the creation of anti-fogging materials.
2018), anti-freezing materials (Chevallier et al., 2011), self-cleaning materials (Satapathy et al., 2018), or environmental treatment (Sriram et al., 2020)
Figure 1.1 Superhydrophobic surfaces in nature
Superhydrophobic surfaces have garnered significant attention from researchers due to their wide-ranging applications Various fabrication methods, including sol-gel, self-assembly, chemical etching, plasma etching, and vapor deposition, have been developed to create these surfaces Studies have demonstrated the successful creation of superhydrophobic coatings on diverse substrates such as fabric, glass, silicon, and metal The choice of fabrication method is often determined by the specific substrate, ensuring the effective development of superhydrophobic surfaces tailored for different applications.
Superhydrophobic surfaces play a crucial role in oil-water separation, particularly in addressing the environmental challenges posed by oil spills, which have become more frequent due to rapid industrialization and oil transportation Oil pollution significantly threatens marine life, human health, and ecosystems, as exemplified by the Arctic Oil Spill in 2020, where approximately 21,000 tons of oil contaminated the environment near Norilsk, resulting in severe ecological damage Traditional methods for oil spill remediation, such as combustion, chemical treatment, and bioremediation, often lead to secondary pollution, with combustion releasing harmful emissions like CO2 and SO2 In contrast, the use of superhydrophobic materials offers an innovative and effective solution for oil spill management without contributing to further environmental harm.
This study focuses on filter paper, a widely utilized material in everyday life and research laboratories, selected as the substrate for experimentation Composed primarily of cellulose, a polysaccharide rich in hydroxyl groups, filter paper exhibits strong hydrophilic properties, enabling it to absorb water effortlessly.
Transforming filter paper from hydrophilic to hydrophobic enhances its applications, particularly in separating oil from water in mixtures Unlike other substrates like fabric, silicon, or metal, superhydrophobic filter paper offers significant advantages, including flexibility, biodegradability, and cost-effectiveness Despite its potential for oil pollution treatment, research on the environmental applications of superhydrophobic filter paper remains limited.
This study aims to develop an anti-wetting surface on filter paper substrates, with the goal of applying this innovation to environmental treatment This research opens up new avenues for future exploration in the field.
LITERATURE REVIEW
Wettability and water contact angle
The static water contact angle (WCA) is a key parameter for measuring surface wettability, defined as the angle formed at the intersection of the liquid-solid and liquid-vapor interfaces This angle is geometrically determined by drawing a tangent line from the contact point along the liquid-vapor interface in the droplet profile The concept was first introduced by Young in 1805, who proposed a fundamental equation to quantify the hydrophobicity and hydrophilicity of surfaces based on the static contact angle.
The static water contact angle, denoted as θ, is influenced by the interfacial tensions of solid-vapor (γSV), solid-liquid (γSL), and liquid-vapor (γLV) interfaces This relationship was established by Young in 1805, making him a pioneer in the scientific research on wettability and the water contact angle.
A surface is classified as hydrophilic if the water contact angle (WCA) is less than 90° (indicating that the solid-vapor surface energy is greater than the solid-liquid surface energy), while it is deemed hydrophobic when the WCA is 90° or greater (indicating that the solid-vapor surface energy is less than or equal to the solid-liquid surface energy) (Sethi et al., 2019) Visual representations of hydrophilic and hydrophobic surfaces can be found in Figure 2.1.
Figure 2.1 Hydrophilic and hydrophobic surface
Young's equation is applicable solely to perfectly smooth and chemically homogeneous solid surfaces In contrast, the wettability of rough or chemically heterogeneous surfaces, which are more commonly encountered in practical applications, presents a significantly more complex scenario Two prominent models have been developed to address this complexity.
5 describing the influence of surface roughness to water contact angle are Wenzel model and Cassie & Baxter model (Subhash Latthe et al., 2012)
In 1936, Wenzel introduced a model demonstrating how water droplets can infiltrate the grooves of rough surfaces His experiments revealed that surface roughness enhances the hydrophilicity of hydrophilic surfaces while increasing the hydrophobicity of hydrophobic surfaces Wenzel proposed an equation that relates the water contact angle on both smooth and rough surfaces, highlighting the significant impact of surface texture on wettability.
The Wenzel equation describes how surface roughness affects water contact angles, where the contact angle on a rough surface is influenced by the contact angle on a smooth surface and the surface roughness factor This factor, defined as the ratio of the actual area of the solid surface to its projected area, is equal to 1 for smooth surfaces and greater than 1 for rough surfaces According to the equation, when the contact angle is less than 90°, surface roughness enhances hydrophilicity, while it increases hydrophobicity when the contact angle exceeds 90° (Wenzel, 1936).
In 1944, following the introduction of Wenzel’s model, Cassie and Baxter identified its limitations in accurately predicting the water contact angle of droplets on rough surfaces containing air pockets To address this issue, they proposed an alternative model that considers the grooves beneath the droplet to be filled with vapor rather than liquid This led to a modification of Wenzel’s equation, resulting in a new equation capable of predicting contact angles in such scenarios.
(3) in which, and represents the contact angle of the smooth solid surface, represents the contact angle for air
Figure 2.2 Schematic of (a) Young’s equation, (b) Wenzel’s model and (c) Cassie-
Baxter’s model (Edalatpour et al., 2018)
A superhydrophobic surface is characterized by a high water contact angle of 150° or more, a low sliding angle of 10° or less, and properties such as anti-sticking, anti-contamination, and self-cleaning (Kumar & Nanda, 2019) Numerous natural surfaces, including lotus leaves, rice leaves, butterfly wings, cicada wings, and rose petals, exhibit remarkable hydrophobic or superhydrophobic qualities These unique properties of superhydrophobic surfaces hold significant potential for various real-life applications, including the development of anti-fogging materials, anti-freezing solutions, and self-cleaning products.
Fabrication methods of superhydrophobic surface
The concept of superhydrophobic surface fabrication was first introduced in 1996 by Satoshi Shibuishi's research group in Japan, sparking significant interest among scientists due to its wide applicability Wettability of a surface is influenced by two main factors: surface chemistry and surface roughness Building on this understanding, numerous methods have been researched and developed to create superhydrophobic surfaces on various materials.
Chemical etching is an efficient wet method that utilizes acidic or basic solutions to create surface roughness through molecular reactions This process is advantageous due to its low-cost materials and straightforward equipment, resulting in a high etching rate and selectivity However, it does have drawbacks, including the need for large quantities of etching chemicals, potential substrate contamination, and challenges in controlling the etching rate.
Numerous studies have explored methods for creating superhydrophobic surfaces A notable example is the research by Qian and Shen (2005), where aluminum, copper, and zinc surfaces were chemically etched using Beck’s dislocation etchant, Livingston’s dislocation etchant, and HCl solution, respectively Following the etching process, the surfaces were treated with tridecafluoroctyltriethoxysilane solution to modify their composition This treatment resulted in impressive static water contact angles of 156° for aluminum, 153° for copper, and 155° for zinc.
Xie and Li (2011) developed a superhydrophobic aluminum plate by utilizing a boiling aqueous NaOH solution for etching, which created surface roughness Following this process, they applied lauric acid as a chemical modifier The resulting aluminum surface exhibited excellent hydrophobic properties, achieving a water contact angle exceeding 150°.
In a study by Varshney et al (2017), a superhydrophobic steel mesh was developed for effective oil-water separation The preparation involved two key steps: etching with hydrochloric and nitric acids, followed by altering the surface chemistry using lauric acid The resulting mesh demonstrated impressive stability—mechanically, chemically, and thermally—with a static water contact angle of 171 ± 4.5° and a low sliding angle.
Lee et al (2011) developed a technique to create superhydrophobic silicon surfaces with a water contact angle approaching 180° This process involved increasing the roughness of silicon wafers by immersing them in a copper plating solution, followed by treatment in a mixture of hydrofluoric acid (HF) and hydrogen peroxide (H2O2) to generate micro and nanostructures Finally, the silicon surface was coated with Teflon to achieve the desired superhydrophobic properties.
8 an excellent superhydrophobic surface that can be applied in self-cleaning and microfluidic transportation
The sol-gel technique is a widely used method for fabricating superhydrophobic materials applicable to various solid substrate surfaces This process involves dispersing colloidal particles, ranging from 1 to 100 nm in size, within gels that feature an interconnected rigid network of micro/nano pores and polymeric chains longer than 1 μm In this technique, monomers are converted into a colloidal solution (sol), which acts as an initiator for the formation of a gel network comprised of polymers or particles.
Numerous studies have explored the application of superhydrophobic materials, with Fan et al (2012) demonstrating a successful method using the sol-gel technique to create a superhydrophobic surface on a copper wafer, achieving a water contact angle of 155.4° The process involved etching the copper surface in an acid solution to enhance roughness, followed by a coating of a sol-gel solution composed of vinyl trimethoxylsilane, ethanol, water, and ammonia The resulting sample exhibited stability in a 3.5% NaCl solution, indicating its potential for anticorrosion and self-cleaning applications.
Wang et al (2008) developed superhydrophobic surfaces on various substrates, including polyester, wool, cotton textiles, filter papers, glass slides, electrospun nanofiber mats, and silicon wafers, achieving water contact angles exceeding 170° and sliding angles below 7° This was accomplished by preparing a sol solution containing silica nanoparticles through the co-hydrolysis and condensation of tetraethyl orthosilicate (TEOS) and tridecafluorooctyl triethoxysilane (FAS) in an ammonia-water-ethanol mixture, which was then applied to the substrates to create a transparent film.
In a 2018 study by Satapathy et al., a novel method for preparing superhydrophobic glass surfaces was introduced This involved immersing the glass in a solution containing sol-gel SiO2 nanoparticles within a linear low-density polyethylene (LLDPE) polymer matrix The porosity of the SiO2 nanoparticles embedded in LLDPE was modified using ethanol as a non-solvent, enhancing the properties of the resulting superhydrophobic surface.
The LLDPE matrix exhibited an impressive water contact angle of 170° and a sliding angle of just 3.8°, highlighting its exceptional surface properties Furthermore, the sample demonstrated excellent thermal, mechanical, and chemical stability, along with effective self-cleaning capabilities.
The dip coating method involves immersing a substrate into a solution containing the desired coating material, allowing for the formation of a film as the sample is withdrawn Once removed, the solvent evaporates, leaving a durable coating on the substrate's surface This technique offers several advantages, such as the ability to coat both sides of the substrate simultaneously, minimal material waste, compatibility with various materials, high production output, and the creation of a uniform and stable coating However, it is essential that all components are submersible to successfully achieve the coating layer.
Mahadik et al (2013) developed an innovative and cost-effective dip coating technique to create superhydrophobic surfaces using the organic and inorganic silica precursor methyltrimethoxysilane (MTMS) on quartz substrates The resulting coatings exhibited impressive water contact angles of 168 ± 2° and water sliding angles of 3 ± 1°, showcasing exceptional superhydrophobicity and superoleophilicity Additionally, these surfaces demonstrated high durability and optical transparency, making them suitable for various applications.
In a study by Sun et al (2014), superhydrophobic surfaces were created on zinc substrates through electrochemical processing under an electric field The researchers utilized an electrolyte mixture of NaNO3 and NaCl to increase surface roughness, followed by a dip-coating with a fluoroalkylsilane-ethanol solution to alter the surface chemistry This two-step process resulted in a zinc surface exhibiting a maximum water contact angle of 165.3° and a tilting angle of 2°.
Sriram et al (2020) introduced a novel fabrication method for superhydrophobic filter paper, utilizing a combination of sol-gel and dip-coating techniques The process began with the preparation of a sol-gel solution, which involved mixing poly(methyl methacrylate-co-ethyl acrylate) polymer and silicon dioxide nanoparticles in toluene while stirring This was subsequently followed by the incorporation of PFOTS silane to enhance the hydrophobic properties of the filter paper.
Techniques for superhydrophobic surface analysis
A scanning electron microscope (SEM) utilizes an accelerated beam of electrons to create detailed images of a sample's surface, making it particularly effective for analyzing micro and nano-sized particles This high-resolution imaging technique offers advantages such as straightforward sample preparation and rapid image acquisition, as noted by Vladár & Hodoroaba (2020).
Scanning Electron Microscopes (SEMs) utilize various signals, such as secondary electrons (SE), back-scattered electrons (BSE), characteristic X-rays, absorbed current, and transmitted electrons, to generate detailed images by analyzing the interactions between the electron beam and atoms at different depths within the sample While secondary electron detectors are a standard feature in all SEMs, it is uncommon for a single device to be equipped with detectors for all types of signals.
Secondary electrons (SEs), possessing low energy around 50 eV, have limited free paths in solid materials, allowing them to escape only from the top few nanometers of a sample's surface As a result, the signals generated by SEs are often localized at the point of impact of the primary electron beam, enabling the acquisition of high-resolution images of the sample surface with resolutions below 1 nm.
Back-scattered electrons (BSE) are electrons that are reflected from a sample due to elastic scattering, and they possess significantly higher energy than secondary electrons (SE), allowing them to originate from deeper within the specimen Consequently, BSE images typically have lower resolution compared to SE images However, BSE is frequently utilized in analytical scanning electron microscopy (SEM) alongside spectra from characteristic X-rays, as the intensity of the BSE signal correlates closely with the atomic number (Z) of the elements present While BSE images provide valuable insights into the distribution of various elements within a sample, they do not facilitate element identification.
Figure 2.3 Back-scattered electron image showing the difference in contrast due to the atomic number: (A) observed material and (B) image of back-scattered electrons
Characteristic X-rays are generated when the electron beam ejects an inner shell electron from the sample, causing outer-shell electron with higher energy to fill the shell and release energy The characteristic X-rays energy and wavelength can be
Energy-dispersive X-ray spectroscopy and wavelength-dispersive X-ray spectroscopy are utilized to identify, quantify, and map the distribution of elements within a sample (Assumpão Pereira-da-Silva & Ferri, 2017).
The accuracy of Scanning Electron Microscopy (SEM) measurements, particularly regarding particle shape and size, is significantly influenced by sample preparation To ensure optimal results, SEM samples must be small enough to fit the specimen stage and often require special treatment to enhance their electrical conductivity and stability This preparation is crucial for the samples to endure high vacuum conditions and the impact of high-energy electron beams Typically, SEM samples consist of powders or suspensions, which often need to be diluted to capture clear images of individual particles and reduce particle accumulation.
To ensure accurate imaging in scanning electron microscopy (SEM), samples must be conductive and electrically grounded to prevent charge buildup, especially in secondary electron mode Non-conductive specimens tend to accumulate charge, leading to scanning errors and imaging artifacts While metal objects typically require minimal preparation beyond cleaning and conductive attachment to a specimen stub, non-conductive materials often need an ultra-thin conductive coating This coating can be applied using low-vacuum sputtering or high-vacuum evaporation methods, with common materials including gold, gold/palladium alloys, tungsten, iridium, platinum, chromium, osmium, and graphite (Suzuki, 2002).
2.3.2 Energy-dispersive X-ray spectroscopy (EDX or EDS)
Energy-dispersive X-ray spectroscopy (EDX) is a nondestructive analytical method utilized for determining the elemental composition of samples via scanning electron microscopy This technique effectively detects elements with atomic numbers greater than boron, provided their concentrations are at least 0.1% (Abd Mutalib et al., 2017) EDX is a crucial tool in material science and finds applications across various scientific disciplines, including physics, chemistry, and geology.
EDX analysis utilizes the interaction between X-ray excitation and a sample, where a scanning electron microscope (SEM) collides an electron beam with the sample, resulting in the emission of characteristic X-rays Each element possesses a unique X-ray emission spectrum, allowing for precise discrimination and measurement of their concentrations within the sample This process occurs as the primary electron beam interacts with the atomic nucleus, ejecting an electron and creating an electron hole An outer shell electron then fills this vacancy, releasing X-rays in the process The emitted X-rays consist of both a continuum, generated by electron deceleration, and characteristic X-rays, produced by the transition of outer shell electrons into the electron hole in the nucleus shell (Abd Mutalib et al., 2017).
Each EDX spectrum is formed by different elements and composition existing in the sample However, some components represent the overlapping peaks, for instance, Ti-
The overlapping spectra of Kβ, V-Kα, Mn-Kβ, and Fe-Kα present a significant challenge in analysis Characteristic X-rays are generated when incident beams interact with the sample surface, enabling the determination of the sample's composition (Figure 2.4) The energy measurement of these characteristic X-rays is influenced by the material's quantity, density, spectrum accuracy, and detector quality Additionally, sample inhomogeneity and roughness can adversely affect the reliability of the analysis (Mishra et al., 2017).
Figure 2.4 Schematic representation of the types of X-ray spectrum emitted upon bombardment of fast electron (Reichelt, 2007)
Recent advancements in EDX technology allow for the simultaneous analysis of surface morphology and chemical composition of materials by integrating scanning electron microscopy with energy-dispersive X-ray spectroscopy, making it increasingly popular in the field of materials science.
2.3.3 Fourier-transform infrared spectroscopy (FTIR)
Fourier-transform infrared spectroscopy (FTIR) is a powerful analytical technique used to obtain the absorption or emission infrared spectrum of various states of matter, including solids, liquids, and gases This method effectively identifies both organic and inorganic compounds while detecting different functional groups within a sample The key instrument used in this process is a spectrophotometer, which measures the absorption spectrum of the compound A diagram of a simple FTIR spectrometer's main components is illustrated in Figure 2.5.
Figure 2.5 Basic component in Fourier transform infrared spectrometer (Mohamed et al., 2017)
The equipment generates a beam of infrared (IR) radiation from a glowing black body source, which then passes through an interferometer for spectral coding In the interferometer, beams with varying path lengths recombine to create an interferogram through constructive and destructive interference This beam subsequently enters the sample compartment, where the sample absorbs specific energy frequencies, defining its unique interferogram pattern The detector measures the interference signal across all frequencies simultaneously, while a stacked beam provides a background for operation Ultimately, the desired spectrum is obtained as the interferogram program processes the data automatically.
18 subtracts the background spectrum from the sample spectrum using Fourier transform computer software
To accurately determine the FTIR spectrum of a compound, selecting the appropriate sample cell or holder is crucial Different states of samples necessitate specific preparation techniques; for liquids, a thin film can be created by placing a drop between polished sodium chloride or potassium bromide plates, which are then mounted on a spectrophotometer holder For solid samples, three main preparation methods exist, one of which involves grinding the sample into a fine powder, mixing it with potassium bromide (KBr) in a 1:100 ratio, and pressing the mixture under high pressure (approximately 12,000 psi) to form a homogeneous KBr pellet for FTIR analysis.
Previous studies about application of superhydrophobic material in oil-water
Oil pollution has emerged as a significant environmental challenge, adversely affecting marine life, human health, and ecosystems While traditional treatment methods such as combustion, chemical treatment, and bioremediation often lead to secondary pollution, the use of superhydrophobic materials presents a promising solution This innovative approach effectively separates oil from water, restoring polluted water to its original state without causing additional environmental harm.
Recent studies have focused on the development of superhydrophobic surfaces for effective oil-water separation Yeom and Kim (2016) introduced a dip-coating method utilizing silica nanoparticles and hexadecyltrimethoxysilane to create superhydrophobic surfaces on steel mesh and sponge The resulting water contact angles were recorded at 151.9° ± 1.6° for steel mesh and 152.4° ± 3.2° for the sponge, demonstrating remarkable oil-water separation capabilities.
Crude oil and kerosene are frequently utilized in oil-water separation due to their connection with oil spill pollutants, posing significant risks to the environment and society In 2013, Xue et al developed a sol-gel method using tetraethoxysilane and 1,1,1,3,3,3-hexamethyl disilazane as precursors to enhance this process.
The preparation of superhydrophobic textiles involves a fabrication process that results in materials exhibiting both superhydrophobic and superoleophilic properties These advanced textiles are capable of effectively separating crude oil from water in oil-water mixtures.
Yin et al (2020) developed a hydrothermal method to create superhydrophobic nickel mesh for effective oil-water separation, utilizing nickel sulfide, thioacetamide, sodium hydroxide, and cetyltrimethylammonium bromide The modified mesh exhibited a water contact angle (WCA) of 158°, demonstrating excellent superhydrophobic properties In separation experiments with a kerosene-water mixture, the mesh achieved an impressive average recovery rate exceeding 95%.
Filter paper has emerged as a promising substrate for the fabrication of superhydrophobic surfaces due to its high flexibility, biodegradability, and low cost Research by Zhang et al (2012) demonstrated the successful creation of a superhydrophobic surface on filter paper using TEOS, octadecyltrichlorosilane, and polystyrene This process transformed the paper's wettability from superhydrophilic to superhydrophobic, achieving a remarkable water contact angle of 156° Additionally, the modified filter paper exhibited excellent performance in hexane-water separation applications.
Teng et al (2020) introduced an innovative coating method using nano TiO2, γ-aminopropyltriethoxysilane, and polydimethylsiloxane to create superhydrophobic filter paper This advanced material demonstrated impressive water contact and rolling angles of 154.5° and 3.5°, respectively Additionally, the filter paper showcased excellent anti-fouling properties, self-cleaning capabilities, and effective oil-water separation, as confirmed by filtration experiments with a dichloromethane-water mixture.
Many studies utilize hazardous chemicals like hexadecyltrimethoxysilane, aminopropyltriethoxysilane, and octadecyltrichlorosilane in the fabrication of superhydrophobic surfaces Additionally, these procedures often take more than a day to complete, making them time-consuming Furthermore, there is a lack of explicit reporting on the durability and reusability of the created surfaces Therefore, this study aims to present a fast and environmentally friendly method for creating superhydrophobic surfaces on filter paper.
The study will assess the effectiveness of the fabricated filter papers in oil-water separation, specifically focusing on kerosene as the oil component Additionally, the durability and reusability of the filters will be evaluated post-fabrication to determine their overall potential.
MATERIALS AND METHODOLOGIES
Objectives and contents of the study
This research aims to develop a superhydrophobic surface on filter paper using a dip-coating method The fabricated surface is characterized through various analytical techniques, including SEM, EDX, XRD, FTIR, and WCA The superhydrophobic filter papers are then tested for their effectiveness in separating oil-water mixtures, highlighting their potential for environmental treatment applications Additionally, the durability and reusability of the created surface are assessed to ensure its viability for future use.
Fabrication of superhydrophobic surface on filter paper
Characterization of fabricated filter paper surface
Evaluation of applicability of fabricated filter paper in environmental treatment
Evaluation of durability and reusability of fabricated filter paper
Materials
In this study, commercial filter papers with a diameter of 12.5 cm and a pore size of 10-15 μm, sourced from Newstar (China), were utilized To serve as substrates for subsequent processes, these filter papers were cut into small circular pieces measuring 5 cm in diameter The fabrication involved chemicals including Zn(NO 3 ) 2 6H 2 O, NaOH, and stearic acid, all procured from Xilong Chemical Company (China), along with ethanol from Duc Giang Chemical Company (Vietnam), with all chemicals exhibiting a purity greater than 99%.
Investigation of fabrication parameters
In this study, the dip-coating method was selected for fabrication due to insights from various published research and the available laboratory equipment Zinc oxide (ZnO) was utilized as a roughness-increasing agent, while stearic acid served as the chemical modifier The crystalline structure of the coating component is crucial for enhancing surface roughness, highlighting the importance of ZnO in this process.
A metal oxide with a controllable crystalline structure through pH adjustment was chosen as the roughness-increasing agent, while stearic acid was selected as the environmentally friendly and cost-effective chemically modified agent.
The procedure involved two coating steps (Figure 3.1), focusing on five key parameters: the ZnO coating procedure, the ZnO coating solvent, the pH of the ZnO coating, the number of ZnO coating cycles, and the number of stearic acid coating cycles This investigation aimed to identify the optimal fabrication method.
Figure 3.1 Coating steps in fabrication of superhydrophobic surface
3.3.1 Investigation of ZnO coating procedure
The first coating step with ZnO was carried out via two different procedures to find the more effective one to apply in the study
In the initial procedure, 0.4g of NaOH and 0.6g of Zn(NO3)2·6H2O were combined in 25mL of distilled water and stirred for 30 minutes using a magnetic stirrer to create a ZnO suspension The formation of ZnO on the filter paper surface is explained by specific chemical reactions.
2NaOH + Zn(NO 3 ) 2 → 2NaNO 3 + Zn(OH) 2
The bare filter paper was immersed in a solution for 5 minutes and subsequently dried at 100°C for another 5 minutes It was then dip-coated in 0.1M stearic acid for 30 minutes and dried at 70°C until its mass stabilized, resulting in the creation of hydrophobic filter paper.
In the second procedure, ZnO was synthesized by combining 0.4g of NaOH with 0.6g of Zn(NO3)2·6H2O in 25mL of distilled water, followed by vigorous stirring for 30 minutes The mixture was then placed in a falcon tube and heated at 100°C in a laboratory dryer for 3 hours After cooling, the solution was centrifuged to separate the solid ZnO, which was dried until a constant mass was achieved Subsequently, the solid ZnO was diluted in 25mL of distilled water, and bare filter paper was immersed in this solution for 5 minutes before being dried at 100°C for an additional 5 minutes Finally, a second coating with stearic acid was performed using the same method as in the initial procedure.
The hydrophobicity of the two fabricated papers was assessed by examining the shape of water droplets on their surfaces and observing changes over time This evaluation will help determine the most effective procedure for the subsequent phases of the study.
3.3.2 Investigation of ZnO coating solvent
In the initial fabrication process using ZnO, distilled water as the solvent led to issues with filter paper, including tearing and wrinkling after high-temperature drying To improve the quality of the fabricated filter paper, five alternative solvents—30%, 50%, 70%, 90%, and absolute ethanol—were tested for the first coating step Each solvent was applied to a separate filter paper coated with ZnO, and the stability post-immersion and appearance after drying were evaluated to identify the optimal solvent for the fabrication process.
3.3.3 Comparison between one-step coating and two-step coating
To illustrate the importance of a two-step coating process for creating superhydrophobic surfaces on filter paper, three experiments were conducted: one with a single coating of ZnO, another with a single coating of stearic acid, and a third utilizing a two-step coating of both ZnO and stearic acid The varying water droplet behavior on the coated papers serves as evidence for the necessity of the two-step coating method.
3.3.4 Investigation of ZnO coating pH
pH is a crucial factor in the fabrication of superhydrophobic surfaces and the crystalline structure of ZnO Therefore, examining the coating pH is vital for optimizing the process This study focuses on three distinct pH levels: pH 1, pH 7, and pH 13, representing acidic, neutral, and basic environments, respectively.
In the fabrication process, a solution was created by mixing 0.4g of NaOH and 0.6g of Zn(NO3)2.6H2O in 25mL of distilled water, resulting in a basic pH of 13 Subsequently, 1M HCl was utilized to adjust the pH to 1 and 7 The resulting changes in pH and the performance of the fabricated paper, using these modified solutions as ZnO coating solutions, were analyzed to determine the optimal pH for creating a superhydrophobic surface.
3.3.5 Investigation of ZnO coating cycle number
Surface roughness and surface chemistry are crucial for determining the wetting properties of a surface To achieve a superhydrophobic state in filter paper, modifications to both factors are necessary However, after a single coating of ZnO and stearic acid, the filter paper reached only hydrophobicity, with a water contact angle (WCA) between 90° and 150° To improve surface roughness and attain better superhydrophobicity, it is essential to repeat the ZnO coating process multiple times.
The dip-coating process of bare filter paper in a ZnO suspension for 5 minutes, followed by a 5-minute drying period, constitutes one cycle of ZnO coating To assess the enhancement of surface roughness, various coating cycles—specifically 1, 2, 4, and 6—were tested The wettability of the filter papers with different ZnO coating cycles was analyzed to determine the optimal number of cycles for achieving the best performance.
3.3.6 Investigation of stearic acid coating cycle number
Surface roughness and surface chemistry significantly impact a surface's superhydrophobicity Increasing the number of ZnO coating cycles enhances surface roughness, while adding more stearic cycles chemically modifies filter paper properties by introducing an additional layer.
Two types of hydrophobic filter papers were created by applying stearic acid to their surfaces, using one and two coating cycles, respectively For this experiment, six cycles of ZnO coating were applied The wettability of the filter papers was assessed to determine the optimal number of stearic acid coating cycles for enhanced hydrophobicity.
Fabrication of superhydrophobic surface on filter paper
The investigation results led to the documentation of optimized parameters, which were integrated to formulate a comprehensive procedure for creating superhydrophobic surfaces on filter paper Subsequently, the filter papers produced through this method were employed for characterization, as well as assessments of their applicability, durability, and reusability in the following phases.
Characterization of fabricated filter paper surface
Following fabrication, the fabricated filter papers, bare filter papers, and those with only a ZnO coating were analyzed using various material characterization techniques to evaluate their surface morphology, chemical composition, and wetting properties The results provided insights into the changes occurring throughout the fabrication process.
The surface morphology and chemical composition of the fabricated papers were examined using a JEOL JSM-IT100 scanning electron microscope (SEM) at an acceleration voltage of 10kV, in conjunction with a JEOL JED-2300 energy dispersive X-ray (EDX) analyzer The crystal structures of the sample components were analyzed with a Rigaku MiniFlex 600 X-ray diffractometer (XRD) utilizing CuKα radiation (λ 0.154 nm) and a scan step of 0.02° across the 2θ range of 10° to 80° Additionally, the functional groups present on the material's surface were identified using a Fourier Jasco FTIR 4600 infrared spectrophotometer Based on these characterization results, a mechanism for the formation of superhydrophobic coatings on the paper surface was proposed.
The wettability of filter papers was evaluated through static water contact angle and water shedding angle measurements Static water contact angles were measured using a SmartDrop water contact angle meter from Femtofab Co Ltd in Korea Additionally, the water shedding angle was determined through a specially designed experimental setup, as illustrated in the accompanying figure.
3.2 At first, a fabricated filter paper is placed on a rigid inclined surface which makes an angle α with the horizontal plane Drip water perpendicularly to the horizontal plane by a syringe and reduce the tilting angle α until the does not roll off the surface of the filter paper The recorded tilting angle α is the water shedding angle.
Applicability of fabricated filter paper in environmental treatment
The applicability of the fabricated filter paper for environmental treatment was assessed through its water-oil separation performance In the experiment, a mixture of 50 mL of water, dyed blue, and 50 mL of kerosene, dyed yellow, was filtered using a vacuum filter holder The volume of oil separated after filtration was measured to calculate the recovery rate This procedure was repeated three additional times with different samples of the fabricated filter paper to evaluate the consistency of the water-oil separation performance.
Durability and reusability of fabricated filter paper
3.7.1 Durability of fabricated filter paper
The durability of fabricated filter paper was examined through 3 different durability tests:
(i) The first test was storing the fabricated papers in normal condition and checking the wettability monthly to assess the durability of the paper after long-time period
The adhesive tape test involved applying adhesive tape to the fabricated paper surfaces and stripping it off repeatedly for a total of ten times Following this process, the wettability of the surfaces was assessed to verify the durability of the coating layers.
The third test assessed the durability of the fabricated paper in varying pH environments The papers were immersed in three solutions with pH levels of 1, 7, and 13 for two hours After immersion, the papers were dried until their mass remained constant, and their wettability was subsequently evaluated.
Finally, all of the examined filter papers in durability tests were used to filter a solution of 50mL water and 50mL kerosene in order to estimate their applicability
31 after being affected by different factors For each durability tests, 4 fabricated papers were employed to evaluate the precision of the experimental results
3.7.2 Reusability of fabricated filter paper
In this experiment, four custom filter papers were utilized to separate identical solutions of 50mL water and 50mL kerosene The recovery rates of the oil were meticulously recorded, and the filter papers were dried after each use This process was repeated nine additional times for each filter paper to evaluate their water-oil separation performance and assess their reusability for environmental treatment applications.
RESULTS AND DISCUSSIONS
Investigation of fabrication parameters
Both filter papers produced through distinct methods exhibited hydrophobic characteristics, with water contact angles exceeding 90° However, their wetting properties varied significantly over time The filter paper made using the first method showed no changes after a water droplet was placed on its surface for several minutes, indicating strong hydrophobicity Conversely, the second method's filter paper allowed the droplet to slowly permeate, demonstrating poor hydrophobicity Additionally, the first method proved to be more time-efficient, leading to its selection for subsequent research phases.
The study evaluated the effects of five different ethanol concentrations (30%, 50%, 70%, 90%, and absolute ethanol) on the stability and appearance of coated filter papers The 30% ethanol-coated paper exhibited poor durability, being easily torn and wrinkled after drying, while papers coated with higher concentrations (50%, 70%, 90%, and absolute ethanol) demonstrated improved stability and a smooth surface However, the 90% and absolute ethanol-coated papers developed a yellow tint, whereas the 50% and 70% ethanol-coated papers retained their white color Consequently, 50% and 70% ethanol emerged as the most effective coating solutions, with 50% ethanol being preferred for its cost-effectiveness and environmental benefits Images of the coated filter papers are shown in Figure 4.1.
Figure 4.1 The filter papers after coated using different solvents
4.1.3 Comparison between one-step coating and two-step coating
The wettability of filter papers coated with ZnO, stearic acid, and a combination of ZnO and stearic acid exhibited significant differences As illustrated in Figure 4.2, the water droplet behavior on these coated surfaces varied markedly.
Figure 4.2 The WCAs of paper with (a) ZnO coating, (b) stearic acid coating and (c)
Filter paper coated solely with ZnO exhibited immediate water absorption, indicating a water contact angle (WCA) of 0° (Figure 4.2a) In contrast, filter paper coated only with stearic acid showed no absorption of water drops, maintaining a hydrophilic nature with a WCA of less than 90° (Figure 4.2b) However, when filter paper was coated with both ZnO and stearic acid, it achieved a hydrophobic surface with a WCA exceeding 90° (Figure 4.2c) This demonstrates that a two-step coating process is essential for effectively creating a superhydrophobic surface on filter paper.
During ZnO coating step, 3 different pH coating was examined, including pH =1, 7 and 13, corresponding to acidic, neutral and basic environment After mixing 0.4g a b c
A solution of NaOH and 0.6g Zn(NO3)2·6H2O in 25mL distilled water resulted in a white precipitate and a pH of 13, facilitating the formation of hydrophobic paper during the ZnO coating process However, lowering the pH to 1 caused the white precipitate to disappear, indicating the absence of Zn(OH)2 and ZnO, which hindered surface roughness enhancement and made acidic conditions unsuitable At a neutral pH of 7, the white precipitate persisted, yet the resulting filter paper still exhibited slow water absorption in a drop test Consequently, a pH of 13 was identified as optimal for the ZnO coating step in the fabrication process.
4.1.5 Investigation of ZnO coating cycle number
Observation of water contact angles (WCAs) reveals that filter papers with varying ZnO coating cycle numbers exhibit distinct wettability characteristics Specifically, filter papers coated with 1 and 2 cycles of ZnO show similar WCAs, which are significantly lower than those of filter papers with 4 cycles.
6 ZnO coating cycles The WCAs of filter papers with 2 and 4 coating cycles of ZnO are demonstrated in Figure 4.3
Figure 4.3 The WCAs of filter paper with (a) 2 coating cycles and (b) 4 coating cycles of ZnO a b
The water contact angles (WCAs) of filter papers with 4 and 6 ZnO coating cycles appeared similar visually To identify the optimal coating cycle, a test was conducted where the papers were inclined at a 10° angle, and water was dropped onto their surfaces The filter paper with 4 coating cycles retained all the water droplets, while the paper with 6 coating cycles exhibited some droplets rolling off, indicating a lower water shedding angle Consequently, the 6 coating cycles were selected for the ZnO coating process.
4.1.6 Investigation of stearic acid coating cycle number
The distinction in water contact angles (WCAs) between filter papers coated with one and two cycles of stearic acid was not easily discernible visually, necessitating the use of a water contact angle meter for accurate measurement The results of the WCA measurements are illustrated in Figure 4.4.
The filter paper coated with one cycle of stearic acid exhibited an average water contact angle (WCA) of 144.3°, indicating good hydrophobicity, but it did not reach superhydrophobicity In contrast, the paper with two coating cycles achieved an average WCA of 154.1°, successfully attaining a superhydrophobic state Therefore, the optimal number of stearic acid coating cycles for enhanced hydrophobic properties is determined to be two.
Figure 4.4 WCA measurement of filter paper with (a) 1 coating cycle and (b) 2 coating cycles of stearic acid a b
Fabrication of superhydrophobic surface on filter paper
The fabrication procedure began with mixing 0.4g of NaOH and 0.6g of Zn(NO3)2·6H2O in 25mL of 50% ethanol, stirring for 30 minutes to create a ZnO suspension The bare filter paper was then immersed in this solution for 5 minutes and dried at 100°C for an additional 5 minutes, with this coating-drying process repeated five times to enhance surface roughness Subsequently, the filter paper was dip-coated in 0.1M stearic acid for 30 minutes and dried at 70°C until its mass stabilized, with this second coating-drying process repeated once more The final products were obtained after completing these procedures.
Characterization of fabricated filter paper surface
4.3.1 Scanning electron microscope (SEM) analysis
The surface morphology of three samples—bare filter paper, filter paper coated with ZnO, and filter paper coated with ZnO-stearic acid—was analyzed using SEM The results of the SEM analysis are illustrated in Figure 4.5.
Figure 4.5 SEM pictures of (a) bare filter paper, (b) filter paper with ZnO coating and
(c) filter paper with ZnO and stearic acid coating
The initial filter paper exhibited a smooth texture with abundant fiber structures (Figure 4.5a) However, after the application of ZnO, there was a notable increase in surface roughness due to the presence of ZnO micro and nano structures (Figure 4.5b) Furthermore, the SEM image of the filter paper coated with both ZnO and stearic acid reveals the distinct appearance of stearic acid following the coating process (Figure 4.5c).
4.3.2 Energy-dispersive X-ray spectroscopy (EDX) analysis
EDX analysis is employed to identify the chemical elements present on the surface of materials The results from three samples—bare filter paper, paper coated solely with ZnO, and paper coated with ZnO and stearic acid—are illustrated in Figure 4.6 The bare filter paper revealed only carbon (C) and oxygen (O) on its surface (Figure 4.6a) Following the application of the ZnO coating, the detected elements shifted to zinc (Zn) and oxygen (O) (Figure 4.6b), as the six-layer ZnO application effectively covered the entire surface, eliminating the presence of carbon In the EDX results for the filter paper coated with both ZnO and stearic acid, the analysis showed the presence of carbon, oxygen, and zinc (Figure 4.6c), indicating the inclusion of stearic acid in the coating composition.
Figure 4.6 EDX analysis of (a) bare filter paper, (b) filter paper with ZnO coating and
(c) filter paper with ZnO and stearic acid coating
The crystal structures of three filter paper samples were analyzed using X-ray diffraction (XRD) technique, with results illustrated in Figure 4.7 The bare filter paper, primarily composed of cellulose, displayed peaks at 2θ = 15.1°, 16.5°, and 22.9°, corresponding to the (010), (001), and (011) crystal structures of cellulose (COD 4114382) In contrast, the ZnO-coated and ZnO-stearic acid-coated papers exhibited additional peaks at 2θ = 31.8°, 34.6°, 36.5°, 47.7°, 56.8°, 63.0°, and 68.1°, indicating the presence of ZnO crystal structures (COD 2107059) The emergence of these diffraction peaks, along with SEM results, confirms the successful coating of ZnO on the filter paper surfaces.
Figure 4.7 XRD analysis of bare and coated papers
4.3.4 Fourier-transform infrared spectroscopy (FTIR) analysis
The functional groups present on the surface of three filter paper samples were analyzed using the FTIR technique, as shown in Figure 4.8 The bare filter paper exhibited peaks at 3335 cm -1, 2913 cm -1, and 1028 cm -1, indicating the presence of –OH, –CH2–, and –COC– functional groups, which are characteristic of cellulose, the primary component of filter paper (Kim & Peppas, 2003; Skenderidis et al., 2019) Following fabrication, a significant increase in peak intensity at 2913 cm -1 was observed, along with the emergence of new peaks at 2847 cm -1, associated with –CH2– groups (Skenderidis et al., 2019), and at 1552 cm -1 and 1420 cm -1, linked to –COO– functional groups (Kim & Peppas, 2003) These changes indicate the incorporation of stearic acid into the modified filter paper, as evidenced by the alterations in peak intensity and the appearance of new peaks.
Figure 4.8 FTIR analysis of bare and coated papers
Figure 4.9 Formula structure of (a) cellulose and (b) stearic acid
4.3.5 Proposal of superhydrophobic coating formation mechanism
The characterization results indicate the development of a superhydrophobic coating on the surface of filter paper, as illustrated in Figure 4.10 Following the initial coating step with ZnO, –OH functional groups were found to be attached to the micro and nano structures of ZnO, evidenced by a peak at 335 nm corresponding to the –OH functional group.
In the subsequent step, stearic acid was applied to create a superhydrophobic coating on the filter paper surface, as the CH3(CH2)16COO– groups replaced the hydroxyl (–OH) groups.
Figure 4.10 Schematic illustration of the superhydrophobic coating
4.3.6 Water contact angle (WCA) and water shedding angle (WSA)
The wettability of the fabricated filter paper was assessed through static water contact angle (WCA) and water shedding angle (WSA) measurements The filter paper exhibited an average WCA of 154° (left WCA = 154.7°, right WCA = 153.5°) and a WSA of 8°± 1° These results indicate the successful creation of a superhydrophobic surface, as the WCA exceeded 150° while the WSA remained below 10° Figure 4.11 illustrates the water shape observed during the WCA measurement.
Figure 4.11 Water shape in WCA measurement
Applicability of fabricated filter paper in environmental treatment
The separation of a 50 mL water and 50 mL kerosene mixture was achieved using a specially fabricated filter paper, effectively allowing the oil to permeate through while the water remained on the surface The volume of the separated oil was measured to determine the recovery rate, and this procedure was subsequently repeated with three additional types of fabricated papers.
Table 4.1 displays recovery rates exceeding 92%, aligning closely with findings from previous studies Notably, Lathe et al (2020) reported a separation efficiency of 95±2% for a kerosene-water mixture using a superhydrophobic pellet, while Yin et al (2020) achieved a recovery rate of approximately 96% with a steel mesh filter This study demonstrates that the fabricated filter paper exhibits satisfactory separation efficiency, underscoring its promising application in environmental treatment as an oil-water separating filter.
Figure 4.12 The mixture of oil and water (a) before filtration and (b) after filtration a b
Table 4.1 Recovery rates in oil-water separation experiments
No Oil in mixture Oil separated Recovery
Durability and reusability of fabricated filter paper
4.5.1 Durability of fabricated filter paper
In the initial durability test, the fabricated papers were stored under normal conditions, and their wettability was assessed monthly for four months As shown in Figure 4.13, the water droplet shape on the filter paper remained largely unchanged throughout the testing period This indicates that there was no significant alteration in the wettability of the filter paper, demonstrating its stability even after prolonged storage.
Figure 4.13 Water drop shape on fabricated paper (a) in the beginning and (b) after 4 months
In the second durability test – adhesive tape test, the adhesive tape was attached to the fabricated paper surfaces and then stripped 10 times continuously After that, the a b
The wettability of the papers was evaluated, with results illustrated in Figures 4.14 and 4.15, showcasing the water droplet shape on the fabricated filter paper before and after the adhesive tape test The images reveal that minimal coating agents, specifically ZnO and stearic acid, remained on the adhesive tape post-stripping Additionally, the water contact angle (WCA) on the paper surface showed only a slight decrease after the test, suggesting that the coating layers were securely adhered to the filter paper surface.
Figure 4.14 The tape after adhesive tape test
Figure 4.15 Water drop shape on fabricated paper (a) in the beginning and (b) after adhesive tape test
In the third durability test, fabricated papers were immersed in solutions with varying pH levels (1, 7, and 13) for two hours and subsequently dried The wettability of these papers was then assessed, with water droplet shapes shown in Figure 4.16 The results indicated that the wettability of the fabricated filter papers remained relatively unchanged after exposure to acidic and neutral solutions (Figures 4.16b and 4.16c) However, a significant decrease in water contact angle (WCA) was observed after immersion in a basic solution (Figure 4.16d), leading to the conclusion that pH levels notably affect the wettability of the filter papers.
45 the fabricated filter papers were durable to acidic and neutral solutions but affected by basic solutions
Figure 4.16 Water drop shape on fabricated paper (a) in the beginning and after immersion in solution with (b) pH = 1, (c) pH = 7 and (d) pH = 13
After conducting durability tests, the examined filter papers were utilized to filter a mixture of 50mL water and 50mL kerosene, with recovery rates recorded to assess their applicability under various conditions Four fabricated papers were tested for each durability assessment to ensure the accuracy of the experimental results, which are detailed in Table 4.2 and Figure 4.17 The findings indicate that all recovery rates in the oil-water separation experiments exceeded 90%, even for papers subjected to a solution with a pH of 13 Overall, the fabricated papers demonstrated durability against physical impacts, varying pH environments, and prolonged storage, maintaining their oil-water separation capabilities under these conditions.
Table 4.2 Recovery rates in oil-water separation after durability tests
After immersion in solution with pH = 1 94 92 96 94 After immersion in solution with pH = 7 94 96 94 96 After immersion in solution with pH = 13 92 92 94 94
Figure 4.17 Recovery rates in oil-water separation after durability tests
4.5.2 Reusability of fabricated filter paper
In this study, four custom-designed filter papers were utilized to separate a mixture of 50mL water and 50mL kerosene After each filtration, the papers were dried and reused up to nine times to assess their reusability for oil-water separation The recovery rates for each filtration cycle are detailed in Table 4.3 and illustrated in Figure 4.18, demonstrating that all tested filter papers maintained effective performance in oil-water separation throughout the multiple uses.
47 reused 9 times, with the recovery rates higher than 90% Therefore, it can be concluded that the fabricated filter papers have high reusability
Table 4.3 Recovery rates in reusability tests
Figure 4.18 Recovery rates in reusability tests