Abstract
Oil/water mixtures are a potentially major source of environmental pollution if efficient separation technology is not employed during processing. A large volume of oil/water mixtures is produced via many manufacturing operations in food, petrochemical, mining, and metal industries and can be exposed to water sources on a regular basis. To date, several techniques are used in practice to deal with industrial oil/water mixtures and oil spills such as in situ burning of oil, bioremediation, and solidifiers, which change the physical shape of oil as a result of chemical interaction. Physical separation of oil/water mixtures is in industrial practice; however, the existing technologies to do so often require either dissipation of large amounts of energy (such as in cyclones and hydrocyclones) or large residence times or inventories of fluids (such as in decanters). Recently, materials with selective wettability have gained attention for application in separation of oil/water mixtures and surfactant stabilized emulsions. For example, a superhydrophobic material is selectively wettable toward oil while having a poor affinity for the aqueous phase; therefore, a superhydrophobic porous material can easily adsorb the oil while completely rejecting the water from an oil/water mixture, thus physically separating the two components. The ease of separation, low cost, and low-energy requirements are some of the other advantages offered by these materials over existing practices of oil/water separation. The present review aims to focus on the surface engineering aspects to achieve selectively wettability in materials and its their relationship with the separation of oil/water mixtures with particular focus on emulsions, on factors contributing to their stability, and on how wettability can be helpful in their separation. Finally, the challenges in application of superwettable materials will be highlighted, and potential solutions to improve the application of these materials will be put forward.
Highlights
There has been a growing interest in utilizing smart bioinspired materials with selective functional wettability for separation of oil/water mixtures and emulsions
Indian canna plants contain wax platelets, which are randomly distributed on microscale rods (WCA ∼ 165◦), whereas the taro leaves are composed of a surface containing nanoscale elliptic features, which are uniformly distributed on micron-sized features (WCA ∼ 159◦) (Crawford and Ivanova, 2015)
There is recent trend in engineering microscale absorbents with superhydrophobic and superoleophilic properties, which can go into the emulsion under mechanical agitation and collect the dispersed oil phase from it (Duan et al, 2015; Guo et al, 2017a,b; Chen et al, 2018)
Summary
There has been a growing interest in utilizing smart bioinspired materials with selective functional wettability for separation of oil/water mixtures and emulsions. Surface Engineering of Ceramic Nanomaterials leaves of the lotus plant or water lily (Nelumbo nucifera) is due to a rough surface with microscale villi and is waxy and hydrophobic (Barthlott and Neinhuis, 1997) This hierarchical structure imparts a low surface energy to the leaf, making it extremely repellent to water and is said to be “superhydrophobic” [water contact angle (WCA) > 150◦]. Indian canna plants contain wax platelets, which are randomly distributed on microscale rods (WCA ∼ 165◦), whereas the taro leaves are composed of a surface containing nanoscale elliptic features, which are uniformly distributed on micron-sized features (WCA ∼ 159◦) (Crawford and Ivanova, 2015) Insects such as water striders (Gerridae), dragonflies (Anisoptera), cicadae (Cicadoidea), and butterflies (Papilionoidea) possess antiwetting and anti-fogging adaptations (Figure 1). This review covers different types of superwetting interfaces for separation of biphasic mixtures
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