Scaling resistance by fluoro-treatments: the importance of wetting states

Membrane distillation is a thermally driven separation process using hydrophobic, porous membranes. Among various problems faced by membrane distillation, scaling remains an unresolved challenge in treating streams of high salinity. Development of superhydrophobic membranes has been a central approach to address this, with CF4 plasma treatment or fluorochemical modification commonly used. However, contradictory observations often occur where some membranes are scaling resistant, but others are not. For the first time, we examine this issue by systematic comparison of the impacts of commonly used fluoro-treatments on scaling resistance. A state-of-the-art surface patterned micro-pillared poly (vinylidene fluoride) membrane (MP-PVDF) was used and both CF4 plasma and fluorosilane reagents were utilized to enhance membrane hydrophobicity. The resulting membranes CF4-MP-PVDF (by CF4 plasma) and FAS-MP-PVDF (via fluorosilane) were systematically characterized and their anti-scaling performance was evaluated using a supersaturated CaSO4 solution. Although both modified membranes showed an increased water contact angle, reduced sliding angle and surface energy, CF4-MP-PVDF demonstrated better scaling resistance than FAS-MP-PVDF. Conventional thermodynamic nucleation models dictate similar nucleation energy barriers for both, in discrepancy to experimental observations. Instead, the wetting states and hydraulic surface slippage were identified as the determinant factors. The CF4-MP-PVDF in a suspended-wetting state with slippage resisted scaling robustly, while FAS-MP-PVDF in an unstable transition state and pristine MP-PVDF in a pinned state were suspectable to scaling. These results unravel, for the first time, the fundamental mechanism behind the differences in scaling resistance by CF4 plasma treatment and fluorosilane surface modification.