Janus Membranes with Asymmetric Wettability Applied in Oil/Water Emulsion Separations

Abstract

but these separation methods have the disadvantages of low separation efficiency, high energy consumption, complicated operation, and so on. [8] In order to solve these problems, a variety of original oil-water separation methods emerged, which usually separate oil/water mixtures by filtration and absorption. The materials for separating oil/water mixtures by filtration principle include meshes, [9,10] membranes, [11,12] filters, [13,14] film, [15,16] and so on; the materials for separating oil/water mixtures by absorption are porous materials, [17,18] powders/particles, [19,20] gels, [21,22] nanocomposites, [23,24] and so on. [3] The oil/water mixture can be divided into three categories according to the diameter size: free oil: the diameter of droplet is greater than 150 µm; dispersed oil: the diameter of droplet is less than 150 µm and greater than 20 µm; emulsified oil: the diameter of droplet is less than 20 µm. [25][26][27][28][29][30] Among them, emulsions consisting of micro-/nanoscale dispersed droplets within a continuous phase are the most difficult to separate, and the general oil/water separation method cannot effectively separate them. [31][32][33][34][35] Traditional separation methods of oil/water emulsion include thermal/chemical demulsification separation, electrochemical demulsification, centrifugation, etc. [36] However, these separation methods not only have complex operation, low separation efficiency, but also may cause secondary pollution. [4] The separation of oil/water emulsion also adopts the principle of filtration and absorption. [3] When oil/water emulsions are separated by absorption, materials with a 3D structure are often used to separate them, for example, Chen et al. [37] prepared 3D superhydrophobic foams by in situ pretreated by noncovalent surface decoration of plant polyphenol-glutaraldehyde cross-linkage that were pretreated by noncovalent surface decoration of plant polyphenolglutaraldehyde cross-linkage. Although this separation method is effective and does not clog the membrane, it cannot be reused for many times, which increases the cost. When the oil/water emulsion is separated through filtration, demulsification is usually used. Demulsification refers to the reduction of emulsion stability, resulting in immiscible phase separation, including chemical, biological, and physical treatment methods, which are commonly used in membrane separation demulsification methods including size sieving effect, charge-screening effect, With frequent oil spill accidents and the large amount of oily wastewater produced in various industrial and domestic processes, the separation of emulsified oil/water mixtures has become an important topic in environmental protection. Janus membranes with asymmetric wettability stand out among many separation materials for their switchable separation, high separation efficiency, low energy consumption, good repeatability, and durability. In recent years, Janus membranes have developed rapidly, but their separation principles and advantages and disadvan...

Asymmetric Robust Superhydrophobic/Superhydrophilic Janus Membranes for the Moisture Proofing of Oil and Purification of Water

“…Janus membranes will cause mutual influence between the two sides due to their asymmetric wettability. 45 After wetting, the liquid penetrated from the superhydrophilic side to the superhydrophobic side; thus, the hydrophobic effect would be affected. The Janus membrane was immersed in deionized water, and then, the CA change curve of both sides was obtained (Figure 7b) to show the damage to the Janus membrane due to the impact of asymmetric wettability.…”

“…Janus membranes will cause mutual influence between the two sides due to their asymmetric wettability . After wetting, the liquid penetrated from the superhydrophilic side to the superhydrophobic side; thus, the hydrophobic effect would be affected.…”

Bioinspired Superhydrophobic/Superhydrophilic Janus Copper Foam for On-Demand Oil/Water Separation

“…The pollution has resulted in a serious impact on the ecosystem and global economy. In recent decades, a series of oil–water separation materials with special wettability have been constructed through the synergy between the hierarchical surface structure and chemical composition of materials. − Superwetting mesh materials, ,− flexible polymeric nanofibrous membranes, − and three-dimensional (3D) porous materials − showed excellent separation performance toward a different density oil/water mixture. However, oily wastewater often contains heavy oil (HO, which has a density higher than that of water) and light oil (LO, which has a density lower than that of water), making the efficient separation of the complex mixture difficult.…”

Universal Rapid Demulsification by Vacuum Suction Using Superamphiphilic and Underliquid Superamphiphobic Polyurethane/Diatomite Composites

“…The use of superwetted materials to separate oil and water has attracted attention recently because it meets most of these requirements. − Oil and water can be effectively separated because of their completely different wetting behaviors and motion on the same superwetted surface. , Filtration, coalescence, and adsorption are typically used for oil/water emulsion separation. ,− Filtration exploits the screening effect of porous superwetted materials, which hinder the passage of tiny droplets owing to surface repellency but allow the penetration of the continuous phase which is attracted. Coalescence is similar to filtration, but the continuous and dispersed phases both penetrate the porous superwetted materials and tiny droplets coalesce because of compression and dragging in microchannels, forming large droplets that are easily separated.…”

“…The oil fouling of superwetted porous materials is inevitable in coalescence modes regardless of whether superhydrophobic or underwater superoleophobic materials are employed. , Oily substances passing through the microchannels of a porous superwetted material are attracted by the surface or hindered by the complicated structure, causing oil accumulation and increased penetration resistance. The negative effects of oil fouling can be decreased by accurately tuning the wettability to obtain membranes with an intermediate affinity for oil and water, , combining a catalytic process for the self-cleaning of the membrane, − or employing a Janus membrane by combining superhydrophobicity and superhydrophilicity. ,− However, these methods usually complicate the preparation procedure, decrease the applicability, and increase the operating cost.…”