TANGENTIAL FLOW STREAMING POTENTIAL MEASUREMENTS - HYDRODYNAMIC CELL CHARACTERIZATION AND ZETA-POTENTIALS OF CARBOXYLATED POLYSULFONE MEMBRANES

Computational fluid dynamics calculations were carried out to ensure t hat a self-made tangential flow mode streaming potential measurement c ell meets the hydrodynamic stipulations of laminar, steady and establi shed electrolyte flow necessary for reproducible electrokinetic measur ements. The calculations show that the cell design meets all of these conditions. Six carboxylated polysulfones with a range of different de grees of substitution (DS) from 0.26 to 1.74 carboxyl groups per polym er repeat unit were synthesized in a two-stage process of lithiation a nd carboxylation, Ultrafiltration membranes were made from both the un modified polysulfone and these hydrophilic materials. The zeta potenti als of these membrane surfaces were determined in 0.001 M KCl solution as a function of pH. The curves show the theoretically expected profi les for nonionic and weakly acidic materials. The growing influence of the COOH dissociation on the surface charge formation is indicated by the flattening of the curves at low pH values. The magnitude of the n egative zeta potentials plateau values ranged from -52 to -20 mV, Whil e unmodified PSU has a plateau value of -52 mV this value decreases co ntinuously with increasing DS to -20 mV for the PSU-COOH 1.74 material . It is suggested that this arises from a shift of the electrokinetic shear plane into the bulk electrolyte solution due to an extended swel ling layer reflecting the enhanced hydrophilicity of these membrane su rfaces. (C) 1998 Elsevier Science B.V.