GOALI: Charge Interactions in Transport of Mixed Solutes in Nanofiltration Membranes

Freshwater scarcity is growing worldwide, and nanofiltration (NF) membrane systems, which we propose to study here, will play an increasingly important role in water purification. NF membranes reject divalent ions and larger uncharged molecules, but operate at significantly lower energy cost than reverse osmosis (RO) membranes. The scientific challenge is in predicting how interactions between charged solutes and the charged membrane molecular nanostructure alter the structure and charge of the membrane itself, which subsequently alters the rejection of the contaminant solutes, particularly when multiple contaminants are present. This makes the application of NF membranes difficult in many situations that could benefit from their low energy usage or selective contaminant removal. The objective of the proposed research is to explore charge interactions between membranes and mixed solutes at the Ångström level using molecular dynamics (MD) simulations validated with experimental results for nanofiltration (NF) membranes. This GOALI research collaboration will utilize Ångström-scale molecular dynamics (MD) simulations to uncover the underlying membrane-solute charge interactions that alter transport and rejection for multi-solute mixtures in polymeric NF membranes. Molecular scale modeling at Northwestern University will be coordinated with experimental results from collaborators at Dow Water & Process Solutions to understand how multiple solutes interact with the membrane molecular structure to alter contaminant rejection. The focus is understanding how charge-based interactions between ionic solutes and carboxyl groups in the membrane alter contaminant rejection. Unlike RO, rejection in NF membranes is dominated by long-range charge-based interactions, which is dramatically impacted by ionic strength and the presence of divalent cations. While similar interactions take place in RO membranes, the more sensitive NF membranes provide an opportunity to explore selectivity and rejection in well-defined polymeric structures. Moreover, this research paves the way for designing NF and RO membranes at the molecular level in a virtual environment, tuning them for optimal solute selectivity or rejection and enhanced water permeability. Intellectual Merit: According to a 2017 Research Summary in the journal Science, "molecular-level design and insight, including advanced simulation and modeling, will be critical for breakthroughs" in membrane technology. The proposed research breaks new ground in the physics of solute and water transport in nanoporous polymeric membranes by using atomistic models to study molecular transport mechanisms at the nanoscale. The expertise of the Northwestern investigators in molecular simulations of nanoporous membranes will be blended with the experience and knowledge of the Dow investigators in membrane performance, design, and manufacture. Ultimately, the proposed research will illuminate atomic level details for membrane/solute charge interactions that govern membrane performance. Approaches developed in this research will lead to a toolset for virtual membranes design. Broader Impact: Membrane filtration technology is crucial for water purification in many applications. However, there are significant gaps in understanding the fundamental contaminant rejection mechanisms at the molecular level, limiting their application. NF membranes play increasingly important roles as the demand for water increases due to population growth, water scarcity, and decreasing raw water quality due to contamination by natura