Modelling the amphoteric behaviour of polyamide nanofiltration membranes

The "adsorption-amphoteric" model is studied with the aim of predicting the amphoteric behaviour of polyamide Desal DK membranes, in the simple case of NaCl-water solutions. The membrane is schematized through a site-representation as the sum of hydrophobic as well as of hydrophilic groups which can support charges derived from various chemical-physical mechanisms. The pseudo-chemical reactions approach is applied to describe separately acid/base ionization, ions adsorption and counter-ions site-binding. In the case in which site-binding is negligible with respect to competitive adsorption on hydrophobic sites, the model contains five adjustable parameters which are calculated through a fitting procedure. The role of the electrostatic potential existing at the adsorption surface close to the feed/membrane interface is relevant in determining the pH dependence of adsorption charge on hydrophobic groups. At high pH values, chloride adsorption is dominant with respect to sodium adsorption, whereas, at low pH's, sodium competitive adsorption remarkably affects the amphoteric behaviour.