The Role of Templating Agents in the Membrane Distillation Performance of Superhydrophobic PVDF Membranes

Membrane distillation has emerged in the past few years as an attractive alternative to current desalination systems, particularly where waste or low value heat is available. However, degradation of performance can arise due to progressive pore wetting in the membrane, compromising product quality and flux. Superhydrophobic modification of the membrane surface has been shown to reduce liquid penetration into the membrane pores. Recently, superhydrophobic (>150° contact angle) polyvinylidene fluoride (PVDF) membranes were successfully fabricated by combining low surface energy of fluorinated silanes with a nanostructured surface created by TiO₂ nanoparticles. During this process, the templating agent was believed to play an active role in influencing the TiO₂ nanostructure. The objective of this study is therefore to establish the relationship between templating agent properties, nano-architecture and wettability of the membrane modifications for membrane distillation applications. 

Polyethylene glycol (PEG), EO₁₀₀PO₆₅EO₁₀₀ (Pluronic F127), EO₁₅PDMS₁₅EO₁₅(Wacker IM-22) and cetyltrimethylammonium bromide (CTAB) were used as templating agents in this study. The sols were prepared via the hydrolysis of titanium (IV) isopropoxide, and subsequently mixed with the templating agent. Flat sheet PVDF membranes were dip-coated into the sol-gel and post-treated at low temperatures. This was followed by the filtration of low surface energy 1H,1H,2H,2H-perfluorododecyltrichlorosilane (FTCS) to synthesize a superhydrophobic surface. Desalination performance of modified membranes was tested using a direct contact membrane distillation (DCMD) configuration. Membranes were characterized by field emission scanning electron microscopy (FE-SEM), energy-dispersive x-ray spectroscopy (EDX), BET surface area measurements, and contact angle goniometry.

The use of templating agents resulted in porous coatings, with multi-level micro- and nanostructures after several coating cycles. Coatings which did not use a templating agent were found to have a higher loading of TiO₂, but lacked the hierarchical structure required for improved hydrophobicity. While similar microstructures were seen for templated coatings, different nanostructures were observed for each templating agent. This was attributed the difference in how the templating agent directed the formation of the crystallites. A denser structure was seen when using templating agents with shorter chains, consequently increasing the available bonding sites for superhydrophobic modification.

Coated membranes were initially hydrophilic due to capillary effects created by the coating structure. Superhydrophobicity was achieved for all modified membranes after the deposition of FTCS. However, contact angles varied depending on the templating agent used and the number of coating cycles. Wacker IM-22 and CTAB displayed the least improvement in hydrophobicity due to residual templating agent in the coating which obstructed bonding sites for FTCS. Among the templating agents, polyethylene glycol gave the most promising properties, with optimum hydrophobicity  The desalination performance of these superhydrophobic membranes was challenged using hypersaline solutions (100g/L NaCl  alone and with other salts) as the feed.

While the overall permeate flux observed during membrane distillation was similar to virgin membranes, modified superhydrophobic membranes significantly improved salt rejection, and consequently extended the operating time. Salt crystal nucleation was observed below the saturation limit without significant wetting when superhydrophobic membrane were used, indicating potential applications for membrane crystallisation.