Examining the electrochemical properties of a nanofiltration membrane with atomic force microscopy

In this investigation, two methods for characterizing membrane surface potential are investigated. Results from atomic force microscopy (AFM) analyses are compared with streaming potential measurements. In calculating surface potential from AFM force measurements, assumptions of constant charge and constant potential were both considered for modeling electrostatic interactions. For a ceramic mica surface, the constant charge assumption was found to be most appropriate while for a polymeric membrane surface, the constant potential assumption provided results that agreed better with theoretical expectations. For both the mica and membrane surfaces, results from AFM agreed with the measured values determined from streaming potential analysis. The advantage of AFM is that in addition to determining the mean surface potential value for membrane surfaces, this technique provides a spatially resolved measure of charge distribution. One drawback of the technique is that it is sensitive to surface roughness, as the measured charge distribution increased with increasing surface roughness.