Bridged organosilica membranes incorporating carboxyl-functionalized cage silsesquioxanes for water desalination

Organosilica-based reverse osmosis (RO) membranes are beneficial for water desalination, owing to their robust properties such as chlorine resistance and thermal stability. However, their water permeance is lower than that of conventional membranes. In a first, carboxyl-functionalized polyhedral oligomeric silsesquioxane (POSS–COOH) was utilized in this work as a hydrophilic nanofiller to improve the water permeance characteristic of organosilica-based RO membranes. The monomer, 1,2-Bis(triethoxysilyl)ethane (BTESE), was subjected to acidic hydrolytic polycondensation in the presence of POSS–COOH in various amounts to produce films and membranes. The water contact angle of the films decreased with increasing POSS–COOH content, indicating that POSS–COOH can work as a hydrophilic agent in the membrane. Scanning electron microscope observations of the membrane surface revealed the formation of linear patterns due to phase separation, which was caused by POSS–COOH aggregation with a large interaction of the carboxyl group. The composite membrane BTESE/POSS–COOH exhibited water desalination properties. Their water permeance increased significantly, while salt rejection decreased gradually, with increasing POSS–COOH concentration. This phenomenon arises from increased hydrophilicity and the phase separation structure of the membrane.