Permeation and Responsiveness of a pH-Responsive Polyacrylic Acid-Coated Porous Alumina Tube Gate Membrane for Various Substances

A pH-responsive polymer coated gate membrane based on inorganic materials with excellent mechanical durability enables pH-dependent substance separation with promising applications in various biochemical and industrial processes. The membrane was fabricated by coating on a tubular α-alumina porous support 6 mm in diameter by graft polymerization with polyacrylic acid (PAA) that swells at high pH. The permeances of acetic acid, caffeine, phenol, and 2-butanone, which have different molecular sizes and negative charges, were evaluated in aqueous solution over a tenfold concentration range using a circulation system. The permeation rate remained almost constant regardless of concentration. The permeance of acetic acid, caffeine, and phenol decreased with increasing pH, and the permeance magnitude was in the order caffeine < acetic acid < phenol. The permeance of 2-butanone was the highest of the compounds and almost constant regardless of pH. At high pH, permeance was decreased by the electrical repulsion between the negative PAA chain and compound ions and by the decrease in pore size due to the swelling of the PAA caused by the electrical repulsion between the negative PAA chain charges. The reversible pH response performance of the membrane was confirmed by a pH swing experiment using phenol. These results demonstrate that the PAA-coated porous membrane functioned as a pH-responsive membrane.