Measuring Charge Density In Thin-Film Composite and Thin-Film Nanocomposite Membranes

Motivation and background:  Thin-film composite (TFC) and thin-film nanocomposite (TFN) membranes for reverse osmosis (RO) and nanofiltration (NF) applications are important technologies towards the production of desalinated, reclaimed and industrial ultrapure water.  The top ultrathin (»25-200 nm) active layer in the multilayer structure of TFC and TFN membranes constitutes the main barrier to the permeation of water and solutes [1].  Among the different properties of the active layers, charge density plays an important role in ion rejection not only through electrostatic interactions [2], but also because it is intrinsically related to the degree of polymerization and crosslinking of the active layer polymer [1,3,4] and the charge of nanoparticles in TFN membranes. As a result, it is important to develop easy-to-use methods for the quantification of charge density in active layers that can serve as tools for membrane researchers to accelerate membrane development.

Project description: The objective of this project was to develop a bench-scale method for the quantification of charge density in membrane active layers.  The method was developed using a TFC membrane (ESPA3, Hydranautics, Oceanside, CA) as a model membrane, and was also used to characterize a TFN membrane.  The method developed was validated via comparison of charge density results with results obtained using the previously reported [3,5] ion-probing+RBS (Rutherford backscattering spectrometry) method which is known to be precise and reliable [4], but which unfortunately also requires highly specialized equipment.  The repeatability, reliability and accuracy of the bench-scale method developed were evaluated.    

Results: A bench-scale method that successfully measures charge density in membrane active layers was developed.  The method is based on the use of microbalance technology and successfully characterized charge density in TFC and TFN membrane samples.  A standard deviation of 5% was obtained for charge density measurements in the active layer of a TFC membrane sample analyzed repeatedly during the span of 131 days.  The charge density results obtained using the microbalance method were in agreement with results obtained using the ion-probing+RBS method (~0.55 M).  The relative standard deviation between different TFC samples analyzed by the microbalance method was 6.5%, which was similar to the relative standard deviation of 7.2% found using the ion-probing+RBS method.  

Significance of the study: The microbalance method developed is an easy-to-use bench-scale method for the quantification of charge density in active layers, and can serve membrane researchers as a tool to accelerate membrane development.

References

1.         Petersen, R. J., Composite reverse osmosis and nanofiltration membranes. J. Mem. Sci. 1993, 83, 81-150.

2.         Childress, A. E.; Elimelech, M., Relating nanofiltration membrane performance to membrane charge (electrokinetic) characteristics. Environ. Sci. Technol. 2000, 34, 3710-3716.

3.         Coronell, O.; González, M. I.; Mariñas, B. J.; Cahill, D. G., Ionization behavior, stoichiometry of association and accessibility of functional groups in the active layers of reverse osmosis and nanofiltration membranes. Environ. Sci. Technol. 2010, 44, 6808-6814.

4.         Tiraferri, A.; Elimelech, M., Direct quantification of negatively charged functional groups on membrane surfaces. J. Mem. Sci. 2012, 389, 499-508.

5.         Coronell, O.; Mariñas, B. J.; Zhang, X.; Cahill, D. G., Quantification of functional groups and modeling of their ionization behavior in the active layer of FT30 reverse osmosis membrane. Environ. Sci. Technol. 2008, 42, 5260-5266.