The use of atomic force microscopy to quantify membrane surface electrical properties

An atomic force microscope has been used to quantify the electrical double layer interactions between a silica particle and a Desal nanofiltration membrane in electrolyte solutions of various ionic strengths. The experimental data has been compared to DLVO predictions incorporating a numerical solution of the non-linear Poisson–Boltzmann equation. At the lowest ionic strength studied (10−3 M) there was good agreement between the experimental data and the theoretical calculations. The membrane surface potential calculated from the best-fit membrane charge under these conditions was in good agreement with the zeta potential obtained from streaming potential data. The discrepancy between DLVO calculations and experimental data increased as the ionic strength increased and in particular as the membrane surface roughness became greater in magnitude than the solution Debye length. Under such conditions, colloid–membrane electrical double layer interactions may at present only be quantified by direct experimental measurement. Atomic force microscopy measurement of pull-off (adhesion) forces provides a further direct evaluation of fouling propensity.