Gap Attack Platform B

Project #1: Role of Ion-Ion and Ion-Polymer Interactions on Ion Transport and Separations in Dry to Wet Systems

Ion association has been widely studied by M-WET researchers Segalman and Clément in energy-focused applications and is being studied by M-WET researchers Katz and Freeman for water purification systems. Understanding ion hydration and ion association will provide profound insight for tuning membrane structures to control water and solute transport. We will combine Clément’s pulsed field gradient (PFG)-NMR and novel electrophoretic NMR (eNMR) techniques, which enable the determination of ion self-diffusion coefficients and electrophoretic mobilities, respectively, with ionic conductivity (Segalman, Freeman) and ion permeability (Katz, Freeman) to connect molecular-level interrogation of ion association and hydration with macroscopic measures of ion diffusion in UMCP-derived materials using MFP solutes that are both NMR-active and relevant for water/energy applications (e.g., Li+, K+ Na+, Mg2+, Al3+, PO4 3-, F-, and TFSI- ions).

We propose synthesizing a model system based on the UMCP in which we have tethered ligand groups (Segalman). These functionalities range from bulky moieties with delocalized charges based on ionic-liquid chemistries to denser charges more common in ion exchange membranes. Larger charged groups lead to weaker electrostatic interactions, which can be overcome by modest thermal energies such that temperatures at or near room temperature provide substantial ion mobility in dry systems. At UCSB and UT Austin, we have developed a detailed understanding of how segmental motion, ligand-bonding chemistry, backbone dielectric constant, and salt dissolution interplay in dry ion conduction in poly ionic-liquids (PILs).

We hypothesize that the strength of ligand binding to the metal cation will be influenced by water content and will in turn alter both salt dissociation (ion aggregation/pairing) and the dynamics of ion motion. We will probe the role of solute anion and cation identity on the permeability and selectivity of these dry, decorated membranes in mono- and multivalent salts (LiCl, LiTFSI, NaCl, NaTFSI, KCl, KTFSI, MgCl2, BaCl2, AlCl3) at a range of ion concentrations. Building on the results for dry polymers, we will investigate the role that water plays on these properties by exposing samples to humid air in a quartz crystal microbalance to observe water uptake and changes in ionic conductivity (Segalman). We will then correlate these observations with macroscopic changes in permeability and selectivity as a function of water uptake in the IF (Katz, Kumar, Freeman). The investigation of alkali salts paired with organic fluorinated ions enables PFG-NMR and eNMR characterization (Clément), while also serving as an important bridge between ion dynamics in water filtration and energy focused systems.

Calculation of ion diffusion coefficients from ion permeability and ionic conductivity values using the Nernst-Planck relationships assumes a lack of ion aggregation/pairing. However, ion aggregation (referred to as pairing or clustering in different communities) can be prevalent in such systems and is anticipated to depend sensitively on the extent of membrane hydration, as well as membrane structure, morphology and ion properties (charge, size, etc.). Yet, our quantitative understanding of the molecular mechanisms that govern ion sorption transport in water purification membranes is limited to rather crude, continuum level models for highly charged and highly hydrated IEMs assuming complete dissociation, with no systematic connection to less highly charged or neutral polymers. Establishing this connection will require pairing the above measurements with non-traditional approaches aimed at establishing a molecular level understanding of ion behavior.

We will leverage comparison of electrochemically measured conductivities, macroscopic ion sorption , and NMR based diffusion measurements to probe the extent of ion aggregation as a function of membrane chemistry and hydration levels. NMR chemical shift analysis of ions in solution will be used to quantify the extent of ion solvation with increasing membrane hydration. We will compliment this analysis with PFG-NMR measurements of anion (𝐷-), cation (𝐷+), and water (𝐷H2O) self diffusion coefficients, offering insight into the relationship between membrane hydration and ion/water transport mechanisms. Furthermore, ion self-diffusion measurements will be used to calculate the Nernst-Einstein conductivity, which reflects the average diffusion of all paired and unpaired ions within the membrane. Together with electrochemical impedance measurements, which account only for charged ionic species, we will quantify the extent of ion aggregation in membranes via the Haven ratio. This analysis, performed for model metal ions and their counter-ions, will bridge theoretical descriptions of ion diffusion in wet and dry systems (Ganesan), while informing transport dynamics beyond the present macroscopic continuum models. Freeman will complement these efforts with macroscopic membrane measurements (i.e., permeability, solubility, and diffusivity) in the IF.

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