Probing polyamide membrane surface charge, zeta potential, wettability, and hydrophilicity with contact angle measurements

Herein, we present the findings of a study employing contact angle measurements to systematically probe the impacts of solution chemistry (pH, ionic strength, ion valence, etc.) on surface charge, zeta potential, wettability, and hydrophilicity of a commercial polyamide RO membrane. Surface charge density, ionization fraction, and apparent dissociation constants were determined from contact angle titrations and converted to surface (zeta) potentials via the Grahame equation. Contact angle derived zeta potentials qualitatively agreed with zeta potentials independently determined from streaming potential measurements across a range of ionic strengths and pH values, although quantitative differences were significant. Contact angles of polar and apolar non-aqueous liquids were combined with aqueous contact angle titrations to quantify the influence of electrolyte pH, ionic strength, and ionic composition on surface acid–base functionality, wettability, and hydrophilicity. The polyamide membrane used in this study became more electron donor functionalized, more wetting, and less hydrophobic with increasing pH, salinity, and divalent cation content in the electrolyte. These results demonstrate, for the first time, a unified approach to characterize interfacial charge, electro-kinetic, and acid–base properties of polyamide composite RO membranes using a single analytical instrument.