Wr. Bowen et al., CHARACTERIZATION OF NANOFILTRATION MEMBRANES FOR PREDICTIVE PURPOSES - USE OF SALTS, UNCHARGED SOLUTES AND ATOMIC-FORCE MICROSCOPY, Journal of membrane science, 126(1), 1997, pp. 91-105
An asymmetric nanofiltration membrane (Hoechst, PES5) has been charact
erised by three different techniques: modelling of the rejection of si
mple salts, modelling of the rejection of uncharged solutes and atomic
force microscopy. Interpretation of experimental data for the rejecti
on of three salts having common co-ion (LiCl, NaCl, KCl) with model ca
lculations allows a characterisation of the membrane in terms of three
parameters: an effective pore radius (r(p)), the ratio of effective t
hickness over porosity (Delta x/A(k)) and an effective charge density
(X). Interpretation of experimental data with model calculations for u
ncharged solutes (Vitamin B-12, raffinose, sucrose, glucose, glycerin)
allows a characterisation in terms of r(p) and Delta x/A(k). Atomic f
orce microscopy (AFM) allows direct determination of surface pore radi
us r(p)(s) and surface porosity A(k)(5). The AFM images provide direct
confirmation of the presence of discrete surface pores in such membra
nes. Further comparison of the characterisation obtained with salts an
d that obtained with uncharged solutes shows that it is better to desc
ribe the transport through such membranes as occurring through discret
e pores rather than using an homogenous description of the membrane st
ructure. It is also shown that the complexity of a ''space-charge'' de
scription of the electric field distribution in the nanometre dimensio
n pores of such membranes is not warranted. Direct experimental eviden
ce of the charging mechanism of the membranes is provided. Overall cha
racterisation parameters suitable for predictive purposes are suggeste
d.