Synthesis of thin film composite polyamide membranes: Effect of monohydric and polyhydric alcohol additives in aqueous solution

- Monohydric and polyhydric alcohols were used as additives in IP reaction.
- Polyhydric xylitol was used for the first time to modify the TFC membranes.
- The trade-off relationship between flux and rejection was overcome by using alcohols.
- The effect of size and functionalities of alcohols was studied systematically.
- Complex interaction between the number of -OH groups and chain length was observed.

Thin film composite (TFC) polyamide (PA) membranes are the most widely used membranes in current water filtration processes. Over the past decade, extensive efforts have been made to improve the permeation properties of the TFC PA membranes via different techniques. In the present work, the effect of incorporating monohydric and polyhydric alcohols into the aqueous solution on surface morphology and permeation properties of resulting TFC PA membranes was studied. The TFC PA membranes were prepared by interfacial polymerization (IP) reaction between m-phenylenediamine (MPD)-aqueous and trimesoyl chloride (TMC)-hexane solutions over polyethersulfone (PES) support. Different concentrations of ethanol (C2H6O), ethylene glycol (C2H6O2), and xylitol (C5H12O5) were added into the MPD-aqueous solution. The results showed that addition of alcohols with low molar volumes such as ethanol and ethylene glycol significantly improved water flux of the synthesized membranes. However, in the case of alcohols with larger molar volumes, like polyhydric xylitol, the water flux enhanced by increasing the alcohol concentration up to ~1.0 wt%, and then decreased, suggesting the presence of an optimum concentration of xylitol. This observation indicated the presence of a complex interaction between the hydrophilic hydroxyl groups and hydrophobic aliphatic carbon chain in the structure of large alcohol molecules. Furthermore, all the modified membranes showed higher rejection percentage than the original membrane which is highly desirable for fabrication of high-performance TFC membranes. The findings of this study provide insight into the synergistic effects of structural properties and surface characteristics, which are altered by the addition of alcohols, on the permeation properties of the TFC membranes.