Thin-film composite nanofiltration membranes prepared by electropolymerization

Abstract

A novel approach to preparation of composite asymmetric nanofiltration membranes is reported based on a thin selective layer deposited by electropolymerization (EP) on top of an asymmetrically porous and electronically conductive porous support. Support films with ultrafiltration characteristics were cast from a concentrated dispersion of carbon black particles, a few tens of nanometers large, in a solution of polysulfone followed by precipitation in a non-solvent bath (phase inversion). Composite membranes with poly(phenylene oxide) and polyaniline thin top layers were prepared by EP deposition from solutions of phenol and aniline, respectively, of which polyaniline film demonstrated a dense uniform structure and water flux and rejection to sucrose and magnesium sulfate in the nanofiltration range.

Appendix. Derivation of Eq. 1 for circular and rectangular geometries

Given the electrochemical cell design, i.e., a circular film of diameter D clamped at the entire edge and collecting a uniform current density i, the change in potential in the radial direction can be described by a system of differential Eqs. A1 and A2:

$$ I(r) = - 2\pi rh\sigma \frac{{d\varphi }} {{dr}}. $$

(A1)

$$ dI = - 2\pi ridr $$

(A2)

where φ is the potential, I(r) the current flowing through the film in the radial direction at radial position r, h the film thickness, and σ is the specific conductivity of the film. From the radial symmetry I(0) = 0, then at r = D/2, we solve Eqs. A1 and A2 to obtain

$$ \Updelta \varphi = \varphi (r = D/2) - \varphi (r = 0) = \frac{{iD^2 }} {{16\sigma h}} $$

(A3)

where Δφ is the potential drop from the current collector to the center of the circular anode and D is the diameter of the anode. For a rectangular film of length L clamped (connected to the same current collector) at the opposite edges, r and 2πr in Eqs. A1 and A2 are replaced with x (length coordinate) and a constant width W, respectively, and, after integrating from x = 0 (the center of the film) to x = L/2 (the edge), Eq. A3 is replaced with

$$ \Updelta \varphi = \frac{{iL^2 }} {{8\sigma h}}, $$

(A4)