Journal of Membrane Science 276 (2006) 286–294 Examining the electrochemical properties of a nanofiltration membrane with atomic force microscopy Jonathan A. Brant a,∗ , Kelly M. Johnson b , Amy E. Childress b a Rice University, Department of Civil and Environmental Engineering, MS 317, Houston, TX 77251, USA b University of Nevada, Reno Department of Civil and Environmental Engineering, MS 258, Reno, NV 89557, USA Received 21 May 2005; received in revised form 23 September 2005; accepted 3 October 2005 Available online 2 November 2005 Abstract 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. © 2005 Elsevier B.V. All rights reserved. Keywords: AFM; Zeta potential; Membrane; Streaming potential; Nanofiltration 1. Introduction Nanofiltration (NF) membranes are used in a wide range of drinking water, wastewater, and industrial applications [1,2]. Separation by NF membranes occurs primarily due to size exclusion and electrostatic interactions [1–3,4]. For colloids and uncharged molecules, sieving or size exclusion is most respon- sible for separation; for ions and charged organics, electrostatic interactions are responsible for separation [2,5,6]. For all appli- cations, membrane charge characteristics play a significant role in the transport of water and solute molecules through the mem- brane. Additionally, the interaction of colloids and charged macromolecules with the membrane, and subsequent fouling of the membrane, is dependent on the charge properties [7]. Because of this, the availability of a simple, reproducible, stan- dardized method of measuring membrane charge properties is of critical importance. In experimental investigations of membrane charge, stream- ing potential measurements have typically been used to calcu- ∗ Corresponding author. Tel.: +1 713 348 3374; fax: +1 713 348 5203. E-mail address: brantjon@rice.edu (J.A. Brant). late zeta potential. Streaming potential is the potential induced when an electrolyte solution is pumped across a stationary, charged surface. Streaming potential can be used to calcu- late zeta potential using the Helmholtz–Smoluchowski equa- tion. Several works on streaming potential measurements of NF membranes (e.g. [1,4,8–12]) have appeared in the literature. Although streaming potential measurements are the most fre- quently used method for evaluating charge properties, they have also been criticized. Results from prior studies reveal uncertainty in individual measurements as well as data scatter [13]. The Helmholtz–Smoluchowski relationship used to calculate zeta potential breaks down at very high or low ionic strengths [14]. Differences in instrument design and the lack of a calibration standard for streaming potential analyzers makes comparison of data among laboratories challenging. An additional concern is that membrane surfaces are heterogeneous—both physically and chemically and for rough or chemically heterogeneous surfaces, surface potential calculated from streaming potential measure- ments may provide an incomplete description of the surface’s charge characteristics [15]. For example, because zeta potential is an average value of the potential at some distance away from the surface, hydrodynamic effects due to surface roughness may distort zeta potential results [16,17]. Furthermore, streaming 0376-7388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2005.10.002