Manufacture of a Hydrophobic Silica Nanoparticle Composite Membrane for Oil-Water Emulsion Separation

The superhydrophobic composite membrane was successfully manufactured by a sol-gel method by drying the surrounding pressure. Tetraethylorthosilicate (TEOS) was used as a hydrophobic agent, while waterglass was used as a source of silica. The effect of the water to waterglass ratio (noted at 16:1 and 19:1) was evaluated to study the hydrophobic properties of the silica film coated composite membrane surface. By measuring the water contact angle on the film surface, the highest contact angle was found to occur at the ratio of 19:1, which is 143.86°. The stability of the composite membrane was also investigated by immersing the membrane in water until day 6. The results show that the synthesized composite membrane has good stability until day 6. The hydrophobicity of the surface of the silica film membrane was found to be unaffected by immersion time. Furthermore, the hydrophobicity increased after 6 days due to the interaction of alkyl groups with the humidified environment, and the surface was more stable in hydrophobicity (i.e., the contact angle of water is 153.79°). In addition, hydrophobic properties were obtained, confirming that this film has the potential to be applied to the separation of oil-water emulsions.

Composite membrane; Hydrophobic membrane; Oil-water emulsions; Silica film; Sol-gel

    The separation of oil-water emulsions has become a major concern since the increase in oil-producing industries, which generate liquid waste and oil-contaminated water bodies due to oil spills. Oil pollution has caused severe environmental problems. The technology of oil-water separation is also important for the purification of biodiesel production (Atadashi et al., 2012). One of the most developed methods for oil-water separation is the absorption of oil using porous adsorbent materials, such as aerogels (Xue et al., 2014), sponges (Su et al., 2017), and sawdust (Jung et al., 2008). These adsorbents must have special properties, namely hydrophobicity. This property allows adsorbents to absorb oil without absorbing water. Besides its use for oil-water separation, the hydrophobic membrane is also applicable for other separation systems (Kartohardjono et al., 2017; Kartohardjono et al., 2019). Recently, it was found that hydrophobic properties can be formed using chemicals by means of surface modification of adsorbents with hydrophobic material such as organosilanes, thus forming hydrophobic films.

Various methods have been developed to prepare these hydrophobic films, such as sol-gel processes (Xiu et al., 2008; Xue et al., 2010), layer by layer of self-assembly (Ismijan et al., 2012), etching (Guo et al., 2005; Dong et al., 2011), chemistry (Rezaei et al., 2014), and electrochemical deposition (Benoit et al., 2013; Khorsand et al., 2014). Among these methods, the sol-gel method is a relatively simple and inexpensive option for hydrophobic film preparation. In addition, this method does not require high temperatures for the preparation process, so it can be applied to large-scale production.

Silica is widely known as a basic ingredient in making porous adsorbents (Mahadik et al., 2010; Bhagat et al., 2008), and the synthesis of hydrophobic silica films using the sol-gel method has also been reported (Bois et al., 2003; Rao et al., 2009; Liu et al., 2014). The hydrophobic properties are obtained during the silylation process by modifying the surface of the silica film in a hydrophobic material, a process that increases the hydrophobic properties through the replacement of silanol groups on the surface of silica with alkyl groups (Roach et al., 2008; Celia et al., 2013; Prihandana et al., 2015;). Three alkoxysilanes are common hydrophobic agents, including tri methyl chloro silicate (TMCS), which is widely used in a variety of applications to improve adhesion between organic matter and inorganic substrates. This alkoxysilane molecule has two key clusters. One of these clusters is an organic compound that can be hydrolyzed and can react with other chemicals or groups that are not reactive and hydrolyzed (Corriu, 2003; Lung and Matinlinna, 2012) and the other is silica. The drawback is that most of these hydrophobic agents are expensive and limited in supply.

In a previous study, tetraethylorthosilicate (TEOS) was generally preferred as a precursor to synthesize silica (Jyoti et al., 2009). TEOS is alkoxysilane that has four alkyl groups, one of which is C₂H₅, meaning that it is possible to use TEOS as a hydrophobic agent. TEOS is more economical than most of the other available hydrophobic agents, and its abundance is greater than silylation agents with three functional groups. Concerning silica sources, waterglass is the cheapest source of silica; there is also an abundance of large raw waterglass sources and it does not endanger the environment, which gives it good potential for industrial scale applications. On the other hand, the drying process of the silylation process is usually carried out through supercritical methods that require large amounts of energy, which limits commercial applications for this material. To reduce production costs, drying at atmospheric pressure can be applied.

    A composite membrane consisting of cotton cloth coated with silica film was successfully manufactured. It was found that this composite membrane with the support of cotton cloth combined with coating with silica film using TEOS as a surface modification agent was effective for delivering hydrophobic properties. The optimal ratio of the water to waterglass in the solution occurred at 19:1—that is, at a contact angle with water of 143.86°—which provided the best hydrophobicity. The hydrophobicity test results on day 6 increased to 153.79°, and this membrane was stable until day 6. This hydrophobicity provides the main factor for a more efficient oil-water emulsion separation. In addition, the method we propose can be used to prepare superhydrophobic silica-coated composite membranes with relatively easy production for oil-water separation.

       This work was supported by INSINAS (INSENTIF RISET SISTEM INOVASI NASIONAL) 2017 through grant Number of 733.1.2/UN10.C10/PN/2017 and 2018 through grant Number of 338.169/UN10.C10/PN/2018. The authors thank Mila Baarik Imansari and Nadia Sjavira Mahardana for their assistance in conducting the experiment.

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