Molecular Dynamics Study of Carbon Nanotubes/Polyamide Reverse Osmosis Membranes: Polymerization, Structure, and Hydration

Abstract

Carbon nanotubes/polyamide (PA) nanocomposite thin films have become very attractive as reverse osmosis (RO) membranes. In this work, we used molecular dynamics to simulate the influence of single walled carbon nanotubes (SWCNTs) in the polyamide molecular structure as a model case of a carbon nanotubes/polyamide nanocomposite RO membrane. It was found that the addition of SWCNTs decreases the pore size of the composite membrane and increases the Na and Cl ion rejection. Analysis of the radial distribution function of water confined in the pores of the membranes shows that SWCNT+PA nanocomposite membranes also exhibit smaller clusters of water molecules within the membrane, thus suggesting a dense membrane structure (SWCNT+PA composite membranes were 3.9% denser than bare PA). The results provide new insights into the fabrication of novel membranes reinforced with tubular structures for enhanced desalination performance.

The role of interaction between low molecular weight neutral organic compounds and a polyamide RO membrane in the rejection mechanism

“…Based on previous research, we built the base molecular models for the PA membrane active layer by connecting TMC and MPD monomers in a ratio of 1 : 1 to generate cross-linked ring and linear structures with four-units. 15,16,42 Models consisting of twoand six-unit TMC and MPD cross-linked monomers, elemental composition of the membrane model, and the impact of side chains resulting from larger PA membrane structures were also investigated (ESI Text S3 †). To maintain acceptable computational time consumption for the subsequent QM calculations and focus on the effects of the solute-membrane interactions on the critical membrane structure, we used the four-unit model for the following study.…”

“…To validate the theoretically calculated aqueous-phase free energy of interaction for the membrane-water system, we calculated the free energy of interaction using eqn (6) based on the literature-reported, experimentally obtained partition coef-cient of water in a PA membrane. 16 f ¼ ð1 À lÞ 2 exp À DG intr aq RT !…”

“…The radius of a water molecule was calculated (1.95Å) based on the volume of water (18.8 cm 3 mol À1 ) estimated using the LeBas method, 47 and we used a membrane pore radius of 3Å, which was calculated from a MD study. 48 A partition coefficient of 0.275 was obtained for the FT-30 membrane from Suzuki et al (2015), 16 which also used a membrane model with a TMC to MPD ratio of 1 : 1 to simulate PA membrane hydration. The experimental partition coefficient was reported as 0.29.…”

“…Classical molecular modeling and dynamics (MM/MD) have been used to construct RO membrane structures and characterize the membrane properties to engineer the design of membrane materials and simulate the rejection of solute. [15][16][17][18][19][20] Studies using MD techniques have been very successful at simulating cross-linked polyamide (PA) membranes and related polymer materials. Xiang et al (2013) found the critical role of carboxylate group of a polyamide membrane surface for binding with a foulant, alginate, through their MD study.…”

MembrFactory: A Force Field and composition Double Independent Universal Tool for Constructing Polyamide Reverse Osmosis Membranes

“…Up to now, molecular dynamics (MD) simulations have proved to be a powerful tool to help scientists to understand the transport and rejection mechanism in the PA layer of the TFC RO membrane . Since MD simulations are based on explicit descriptions of atomic level details, it is prerequisite to have a reliable three‐dimensional atomic model of the PA layer.…”