A Critical Review of Membrane Wettability in Membrane Distillation from the Perspective of Interfacial Interactions

Abstract

Hydrophobic membranes used in membrane distillation (MD) systems are often subject to wetting during long-term operation. Thus, it is of great importance to fully understand factors that influence the wettability of hydrophobic membranes and their impact on the overall separation efficiency that can be achieved in MD systems. This Critical Review summarizes both fundamental and applied aspects of membrane wetting with particular emphasis on interfacial interaction between the membrane and solutes in the feed solution. First, the theoretical background of surface wetting, including the relationship between wettability and interfacial interaction, definition and measurement of contact angle, surface tension, surface free energy, adhesion force, and liquid entry pressure, is described. Second, the nature of wettability, membrane wetting mechanisms, influence of membrane properties, feed characteristics and operating conditions on membrane wetting, and evolution of membrane wetting are reviewed in the context of an MD process. Third, specific membrane features that increase resistance to wetting (e.g., superhydrophobic, omniphobic, and Janus membranes) are discussed briefly followed by the comparison of various cleaning approaches to restore membrane hydrophobicity. Finally, challenges with the prevention of membrane wetting are summarized, and future work is proposed to improve the use of MD technology in a variety of applications.

Membranes for the Gas/Liquid Phase Separation at Elevated Temperatures: Characterization of the Liquid Entry Pressure

“…According to Equation (1) and Figure 1, the LEP depends on the surface tension of the liquid , the largest size and geometric factor B of the membrane pores and the contact angle of the liquid at the pore entrance ( ): The liquid surface tension here described as the capillary force responsible for forming a convex meniscus in the membrane pore that prevents liquid breakthrough [11]. An increase in temperature, and therefore, in molecular thermal activity, results in the decrease of the surface tension, as the cohesive interaction between the phases falls with the temperature [18,19]. Preventing wetting at higher temperatures is, therefore, a difficult task [11].…”

Janus Membrane with a Dense Hydrophilic Surface Layer for Robust Fouling and Wetting Resistance in Membrane Distillation: New Insights into Wetting Resistance

“…To overcome membrane wetting and fouling, novel MD membranes with special wettability have been employed. − Specifically, omniphobic MD membranes that are resistant to wetting by low-surface-tension liquids have been proposed for wetting mitigation, − and composite MD membranes with a superhydrophilic surface layer and a hydrophobic substrate have been developed for fouling mitigation. − Since the compositions of industrial brines are usually complex, membrane wetting and fouling could occur simultaneously. However, based on previous studies, , neither omniphobic nor composite MD membranes can resist membrane wetting and fouling at the same time.…”

NaOH-Induced Fabrication of a Superhydrophilic and Underwater Superoleophobic Styrene-Acrylate Copolymer Filtration Membrane for Effective Separation of Emulsified Light Oil-Polluted Water Mixtures

“…Generally, depending on the surface geometry, the wetting behavior of a filtration membrane is explained using the Young, the Wenzel, and the Cassie−Baxter models, as it is observed in Scheme 2. 9,11,49 The Young equation suggests the correlation between the contact angle and interfacial tension for a smooth flat surface (i.e., ideal solid surface)…”

“…Generally, depending on the surface geometry, the wetting behavior of a filtration membrane is explained using the Young, the Wenzel, and the Cassie–Baxter models, as it is observed in Scheme . ,, The Young equation suggests the correlation between the contact angle and interfacial tension for a smooth flat surface (i.e., ideal solid surface) where θ Y is the Young contact angle (the liquid contact angle for a smooth flat surface) and γ sv , γ sl , and γ lv represent the interfacial energies between the solid–vapor, solid–liquid, and liquid–vapor interfaces, respectively. The liquid contact angle (θ*) is measured experimentally on a macroscopic scale.…”