Measuring the Volume-Averaged Charge Density of Active Layers of Thin-Film Composite and Nanocomposite Membranes Using a Quartz Crystal Microbalance

Background and Motivation

Membrane-based separation processes such as reverse osmosis (RO) and nanofiltration (NF) typically make use of polyamide-based thin film composite (TFC) membranes as the separation barrier.  TFC membranes are commonly comprised of three layers: an ultrathin polyamide top layer (~20-100 nm), an intermediate porous polysulfone layer (~30 mm) and a non-woven polyester backing (~200 mm). A recent development in membrane technology, zeolite thin-film nanocomposite (TFN) membranes, have the same structure as TFC membranes with the difference that the active layer of TFNs have zeolite nanoparticles dispersed throughout the active layer.

The key component controlling the diffusion of water and solutes through both TFC and TFN membranes are their ultra thin polyamide-based active layers. A key physiochemical property dictating active layer performance is charge density, which is directly related to the degree of crosslinking of the polyamide structure.  Charge density is also important for membrane performance because it affects the electrostatic exclusion of contaminants and other properties such as active layer hydrophilicity.  As a result, there is a need for development of bench-top methods for charge density characterization in active layers.

Project Description

Accordingly, in this study we developed a user friendly and highly reliable method to quantify the volume-averaged charge density as a function of pH in polyamide active layers of TFC and TFN membranes using a quartz crystal microbalance (QCM). We present the experimental procedures, results obtained for various TFC and TFN membranes, and comparison between results obtained with the QCM method and the previously developed (non bench-top) method based on Rutherford backscattering spectrometry (RBS).

 

Results: For measurement of the volume-averaged charge density in a TFC/TFN active layer, the active layer was first isolated onto microbalance sensors. Next, to measure the volume-averaged negative charge density in the isolated active layer we measured the areal mass (ng/cm²) of cesium ions that saturated the negative charge sites in the active layer at the pH of interest.  By assuming a 1:1 correspondence between charged sites and Cs+ ions the mass of cesium uptake by the active layer was converted to concentration of charged sites (i.e., charge density). 

Measurements of the volume-averaged negative charge density (~0.5M) in isolated active layers of TFC membranes using the QCM method were found to be repeatable within 3% for measurements obtained with the same sample over several weeks and reproducible within 3.8% for three different samples of the same TFC membrane.  Also, results obtained for the volume-averaged negative charge density of a TFC membrane were consistent (within 2.1% at pH.10.5) with results obtained using the RBS method.  The QCM method was also successfully used with TFN samples.  The charge density of the TFN samples tested was similar (~0.5 M) to the charge densities in TFC samples.

 

Significance to Industry: This method provides a friendly, bench-top procedure to membrane scientists to measure the important property of charge density in active layers, and thus provides a tool to membrane developers  for future developments of improved membranes.