MEMBRANES AND POLYMER/WATER INTERACTIONS

Water Purification Membranes

Synthetic polymer membranes are widely used to purify water, mainly because they are more energy efficient than competing technologies. However, water in energy applications is often heavily contaminated with diverse organic and inorganic components. Current membranes were not designed for such applications. The Center for Materials for Water and Energy Systems (M-WET) seeks to improve understanding of fluids and materials to catalyze design of novel surfaces, highly selective solute/fluid interactions, mesoscopic structures, and membranes for energy applications.

Molecular Design of Surfaces to Control and Tune Water Properties at Interfaces

Water near the interface affects membrane surface properties and mediates interactions between the surface and solutes in aqueous mixtures. Despite progress in characterizing water and dilute solutions at idealized interfaces, water’s interactions with and structure near complex heterogeneous surfaces remains poorly understood. Even less is known of interfacial water at surfaces in complex aqueous fluids (e.g., concentrated ionic solutions) as seen in membrane filtration applications. One of our primary aims is to understand and characterize hydration layer water, which is influenced or “programmed” in complex ways by surface chemical organization. In turn, hydration layer structure and properties directly impact solute/ion adsorption on membrane surfaces and pore walls. Thus, such knowledge is critical to understanding selective adsorption and fouling onto membranes.

Resolving properties of the hydration layer with high precision requires advanced characterization methods. Magnetic resonance technique Overhauser dynamic nuclear polarization (ODNP) probes the dynamics of water with nanometer scale spatial resolution by measuring the coupling between a local surface water molecule and a nitroxide spin label tethered to the surface site of interest. Double electron electron resonance (DEER) measures distances between 2 and 8 nanometers to provide full distributions of molecular distances between two spin labels in aqueous environments. Combined, these techniques provide molecular-scale dynamic and structural information on the nature of the hydration layer at complex interfaces. Ambient-pressure X-ray Photoelectron Spectroscopy, which detects atom chemistry at the surface in the presence of water, allows us to see membrane surface chemistry and water adsorption. These, paired with molecular dynamics simulations performed by other members of M-WET, vastly improve our understanding of hydrated surfaces.

Polymer Mesoscale Structures to Tailor Fluid Flow

Among the greatest challenges in membrane design is producing porous mesoscale materials with uniform pore sizes and well-defined pore size distributions (PSDs), which are critical for understanding and improving separation of complex mixtures. Water filtration membrane pores must necessarily be on the mesoscale (10–100 nm), but PSDs in commercial membranes are formed via highly non-equilibrium processes. These yield very broad PSDs, and always exhibit a permeability/selectivity trade-off. Thus, an urgent need is to disrupt this traditional permeability/selectivity tradeoff by employing novel block copolymer membranes to tune pore size and distribution and create functionality.

One of our key research thrusts is fundamentally focused on understanding the role of the mesostructure in a fluid traversing a pore. We leverage experiment, theory, spectroscopy, and modeling to study transport of fluids in porous structures by designing, developing, and modeling a library of pore-forming block copolymers with known and optimized porosity, geometry, and pore wall chemistry. This will allow us to understand how those parameters affect transport and separation of both simple solution and complex multi-component fluids. Experiment, theory, and modeling are combined to fundamentally understand the rules of water filtration membrane and design the “best” pores.

Molecular Diffusion in Membranes over Different Length Scales

A rich variety of experimental and simulation techniques exist which can be employed to elucidate diffusion processes and molecular dynamics in polymer membranes over different length and time scales.  Use of complementary experiments and simulations can enable unique access to the fundamental interplay between diffusion at different length scales.

Macroscopic techniques, such as gradient-driven salt and water permeation measurements, observe Fickian diffusion processes occurring on the order of the thickness of the polymer membrane.  Microscopic NMR techniques, such as Pulsed-Field Gradient (PFG) NMR, measure self-diffusion coefficients of NMR active species driven by Brownian motion at equilibrium.  Electrochemical techniques using alternating (AC) or direct currents (DC) often measure ionic conductivities arising from ion migration under an applied electric field gradient.  High frequency AC measurements can examine microscopic polarization and molecular orientations, while short-frequency AC or DC measurements can observe diffusion at microscopic or even macroscopic length scales.  Molecular scale techniques such as Electron Paramagnetic Resonance (EPR) and Overhauser Dynamic Nuclear Polarization (ODNP) examine the dynamics of single molecules over very short, sub-nanometer length scales in the vicinity of a spin probe.  Finally, a wealth of molecular simulation techniques can probe particle mean squared displacement and diffusion at equilibrium and under applied gradients.