Continuous molecular separation in a polymer-introduced circular tube via diffusion, forced convection and adsorption

Membrane filtration is commonly used for the separation of biomacromolecules; however, fouling often leads to a loss in the recovery of the target materials. As a separation material capable of isolating molecules and polymers on the order of several tens of nanometers, we have newly prepared a tubular device whose inner surface was modified with a cationic polymer, poly(2‑(dimethylamino)ethyl methacrylate) (polyDMAEMA). Small molecules and particles diffuse in the radial direction and are adsorbed onto the polymer layer, whereas larger particles are transported axially by forced convection and elute from the tube before reaching the adsorption layer, size‑based separation becoming feasible. Compared with gel-permeation chromatography (GPC), this method allows continuous flow through the tube, enabling higher processing throughput, and clogging does not occur because the separation unit is a simple cylindrical tube. The number density of polyDMAEMA introduced into the polyimide tube was assumed to be 0.57 chains/nm², indicating the formation of a dense polymer layer. When solutions of bromophenol blue (BPB, Mw 669) and poly(styrene sulfonic acid) (PSS, Mw 70 kDa) were permeated through the tube, BPB was adsorbed whereas PSS eluted immediately, demonstrating that the polymer‑modified tube can achieve separation. The breakthrough curves were fitted using partial differential equations to obtain the mass‑transfer coefficient, maximum adsorption capacity, and dispersion coefficient, which were then correlated with the Peclet and Damköhler numbers. Using these correlations, the separation efficiency of BPB and PSS at different flow rates was predicted, demonstrating that efficient separation is achieved at low flow rates. This proposed tube shows high potential for application in the separation of biomolecules and particles.