Reliable, bench-top measurements of charge density in the active layers of thin-film composite and nanocomposite membranes using quartz crystal microbalance technology

Abstract

A reliable, user-friendly, bench-top method was developed and evaluated for the measurement of negative charge density in the active layers of thin-film composite and thin-film nanocomposite membranes. The method consists of isolating the active layer on a quartz crystal microbalance (QCM) sensor (i.e., AL+sensor sample), exposing the AL+sensor sample to an aqueous cesium solution at any pH of interest, and measuring with a QCM the mass of cesium ion that associates with the negative sites of the active layer. Results showed that QCM measurements of charge density in active layers were: (1) repeatable within 3% for tests performed with the same AL+sensor sample under the same experimental conditions; (2) reproducible within 3.8% for tests performed with the same AL+sensor sample when the ionic strength of cesium solutions was varied by 300%; (3) reproducible within 4% for active layers isolated from nearby locations of 2 the same membrane sheet; and (4) consistent within 2.1% at pH = 10.5 with results obtained using the previously reported Rutherford backscattering spectrometry method on non-isolated active layers. The results therefore demonstrate the robustness, repeatability, reproducibility, and accuracy of the QCM method. We also demonstrated that the ionization behaviors of the polyamide-based thin-film composite and nanocomposite membranes tested were similar: both membranes had bimodal pK a distributions and negative charge densities of |0.5 M at full ionization.

Investigating the void structure of the polyamide active layers of thin-film composite membranes

“…The active layer isolation procedure was similar to that described in our previous study [31], is based on a protocol previously reported by Freger [32], has been successfully used by various research groups [7,17,22], and has been shown not to affect physico-chemical and transport properties of the active layer [31,33]. Briefly, the membrane polyester backing was peeled off by hand leaving behind a composite of the polyamide active layer and polysulfone support layer.…”

“…For each membrane, we tested two samples, each with an area of 1.54 cm 2 , and for each sample we conducted duplicate measurements of water sorption in each liquid and vapor environments. The mass of active layer isolated on a sensor (m AL ) was obtained as described in our previous study [31] from the difference in QCM response between the uncoated sensor and the sensor coated with the active layer. Active layer mass measurements were performed with the sensors placed in Q-Sense flow modules (Biolin Scientific, Lithicum Heights, MD), which are depicted in Fig.…”

“…In all cases, an uncoated control sensor was exposed in parallel to the same liquids or gases as the coated sensors to account for changes in the response of the QCM due to variations in the viscosity and density of the fluid to which the sensor was exposed, as well as to surface adsorption to the sensor and isolated active layer [31,34,35]. In other words, the measurements obtained with the control bare sensor were always subtracted from those obtained with the isolated active layer sample (e.g., m l or m v ¼apparent mass increase in active layer sample -apparent mass increase in control bare sensor).…”

Ion transport controlled by nanoparticle-functionalized membranes

“…4) and rescaling by the pore diameter, we find s ¼ 0.20±0.08 mC cm À 2 . Extrapolating to pore sizes on the order of a nanometre, these measurements indicate that carboxylterminated NP membranes exhibit a charge density that compares well with the polyamide-active layer of thin-film composite membranes 11 , while at larger length scales (for example, 100 nm), the corresponding rejections exceed that of carbon nanotubes 13,14 .…”

“…In industrial-based applications, such as reverse osmosis (RO) membranes for desalination, detailed interfacial polymerization techniques and a combination of reactants for the active layer of thin-film composite membranes are required to simultaneously achieve a particular membrane morphology or pore size with a desired set of chemical interactions to influence transport 6,7 . Such efforts have reached a considerable level of maturity 8,9 ; however, recent efforts to further functionalize these systems using nanoparticles (NPs) 10,11 highlight the underlying fact that further improvements can be made.…”