How Does An Ultrathin Charged Layer Control Ion Rejection in Membranes?

A negatively charged surface layer on a nanofiltration membrane greatly enhances Na₂SO₄ rejection, whereas a positively charged layer may increase MgCl₂ rejection and greatly decrease Na₂SO₄ rejection.  How can such an ultrathin layer with minimal mass transport resistance affect ion transport so dramatically?  We hypothesize that the charged layer asymmetrically alters the partition coefficients of anions and cations in the underlying barrier layer to create a skin layer depleted of either anions or cations.  Due to the zero-current condition, the depleted ion controls the salt passage to give high salt rejection.  Unlike traditional modeling of reverse osmosis or nanofiltration, this hypothesis does not assume local electric neutrality at the scale of the membrane barrier layer.  Typical commercial thin-film composite membranes have skin layer thicknesses less than 10 nm and low ion concentrations, so the internal screening length may be large enough for a charged region to include the entire barrier layer.  A model of such transport qualitatively agrees with some published results including a decrease in the rejection of single salts with increasing concentration and an increase in the negative rejections of traces of single salts with increasing concentrations of dominant doubly charged salts.