How To Functionalize Ceramics by Perfluoroalkylsilanes for Membrane Separation Process? Properties and Application of Hydrophobized Ceramic Membranes

Abstract

The combination of microscopic (atomic force microscopy and scanning electron microscopy) and goniometric (static and dynamic measurements) techniques, and surface characterization (surface free energy determination, critical surface tension, liquid entry pressure, hydraulic permeability) was implemented to discuss the influence of perfluoroalkylsilanes structure and grafting time on the physicochemistry of the created hydrophobic surfaces on the titania ceramic membranes of 5 kD and 300 kD. The impact of molecular structure of perfluoroalkylsilanes modifiers (possessing from 6 to 12 carbon atoms in the fluorinated part of the alkyl chain) and the time of the functionalization process in the range of 5 to 35 h was studied. Based on the scanning electron microscopy with energy-dispersive X-ray spectroscopy, it was found that the localization of grafting molecules depends on the membrane pore size (5 kD or 300 kD). In the case of 5 kD titania membranes, modifiers are attached mainly on the surface and only partially inside the membrane pores, whereas, for 300 kD membranes, the perfluoroalkylsilanes molecules are present within the whole porous structure of the membranes. The application of 4 various types of PFAS molecules enabled for interesting observations and remarks. It was explained how to obtain ceramic membrane surfaces with controlled material (contact angle, roughness, contact angle hysteresis) and separation properties. Highly hydrophobic surfaces with low values of contact angle hysteresis and low roughness were obtained. These surfaces possessed also low values of critical surface tension, which means that surfaces are highly resistant to wetting. This finding is crucial in membrane applicability in separation processes. The obtained and characterized hydrophobic membranes were subsequently applied in air-gap membrane distillation processes. All membranes were very efficient in MD processes, showing good transport and selective properties (∼99% of NaCl salt rejection). Depending on the membrane pore size and used modifiers, the permeate flux was in the range of 0.5-4.5 kg·m(-2)·h(-1) and 0.3-4.2 kg·m(-2)·h(-1) for 5 kD and 300 kD membranes, respectively.

Hydrophobic Ceramic Membranes for Water Desalination

“…CAs were determined for pristine and grafted membranes, based on sessile drop (static CA) and the tilting plate (dynamic CA) methods described elsewhere [44,45]. The apparent CA values were calculated by ImageJ software (ImageJ, NIH-freeware version), with an accuracy of ±2 • .…”

“…The AGMD experimental setup and the detailed experimental protocol are described elsewhere [42,45]. AGMD process was realized at temperate of feed equal to Tf = 90 °C and permeate equal to Tp = 5 °C.…”

“…It was found that polar component (γ p ) in overall SFE is very small (Table 1). Small contribution of the polar part compared to the dispersive component (γ d ) of SFE is characteristic for hydrophobic and highly hydrophobic materials [42,45,58,61]. Hydrophobization process also influences the surface roughness.…”

“…Chemically attached coupling agents with low-surface-energy can be also efficiently used to turn the hydrophilic character materials to hydrophobic one [38][39][40][41]. Within that group of compounds, perfluoroalkylsilane (PFAS) molecules [42][43][44][45], hydrophobic polymers [46], as well as Grignard compounds [47,48] have been found to generate hydrophobic surfaces. The aforementioned grafting agents were efficiently modified by the chemical attachment to the ceramic support.…”

“…For that particular reason, the modification of membrane surface is required to change the surface character from hydrophilic to hydrophobic one. Considering the high effectiveness of the grafting process of ceramic materials and good performance in membrane-based separation techniques (e.g., vacuum pervaporation [42,43,54,55], vacuum membrane distillation [56], air-gap and direct membrane distillation [42,45], and organic solvent nanofiltration [47,48,57]) it should be pointed out that such modification also possessed disadvantages. Namely, a significant drawback is the presence of fluorine atoms in coupling agents.…”