Distribution and diffusion coefficients of NaCl in polyamide (nylon‐6,6 and polyxylyleneadipamide) membranes

Thin membranes of an aliphatic polyamide (nylon‐6,6) and an aromatic polyamide (polyxylyleneadipamide) (PXAP) were prepared, and their distribution ( K ) and overall diffusion ( D ) coefficients of sodium chloride were measured with the unsteady‐state and steady‐state dialysis method. The overall diffusion coefficients at a zero concentration [ D (0) ] of sodium chloride for nylon‐6,6 and PXAP were 1.3–0.8 μm 2 /s (from 2 min of interfacial polymerization to 4 min) and 0.078, respectively. D (0) for PXAP was about 3 times greater than that of a cellulose acetate (CA) membrane (0.024 μm 2 /s). The K values for nylon‐6,6 and PXAP were 0.7–0.5 from 2 to 4 min and 0.05, respectively. K for PXAP was almost the same as K for CA (0.06). A two‐part (dense and porous) model of the membrane structure was applied to obtain D d (the diffusion coefficient in the dense part of the membrane) and D p (the diffusion coefficient in the porous part of the membrane) for CA, PXAP, and nylon‐6,6 thin membranes. The values of D d were almost the same for both nylon‐6,6 and PXAP (0.05–0.061 μm 2 /s) and about 10 times greater than the value for the CA membrane (5.6 × 10 −3 μm 2 /s). D p for PXAP was almost the same as D p for CA. However, D p for the nylon‐6,6 membrane was 10–16 times greater than D p for the PXAP membrane. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 2605–2612, 2002