How water moves through soft materials
ChoongSze Lee

Hundreds of feet below Texas Hill Country lies the Edwards Aquifer, which supplies drinking water for nearly two million people in Austin and San Antonio, Texas. The Edwards Aquifer is one of the most unique groundwater systems with water weaving through a series of honeycomb, porous limestones. A few hundred feet above the aquifer, at the University of Texas at Austin, scientists are studying how water flows through similar channels, albeit at a nanometer length scale in soft materials. These scientists are at the Center for Materials for Water and Energy Systems (M-WET), one of the 46 Energy Frontier Research Centers that are part of the US Department of Energy Office of Basic Energy Sciences. Researchers at M-WET model water transport in porous polymeric materials, leading to insights about the behavior of water that can inform the design and optimization of membranes.

In water-purification applications such as desalination and reverse osmosis, the dynamics and structure of water in membranes can affect the efficiency of the removal of charged molecules, or ions, from water. Thus, it is imperative to understand how water molecules themselves are affected by pore morphology, size, and chemistry. The membrane studied here is a triblock copolymer composed of a continuous chain of styrene, ethylene oxide, and lactic acid beads. These beads are connected via bonds that act like a spring that can be stretched and compressed between the beads, or the monomer units. The configuration of the pores was tested in lamellae, cylinder, and gyroid shapes and the length of the bead chains were varied to assess whether the morphologies would affect water transport (Figure 1).

Figure 1. (Left) Cross section of the Edwards Aquifer in Texas, highlighting the honeycomb limestone through which water flows, similar to a water-purification membrane. (Right) Triblock copolymer systems with water in lamellae, cylinder, and gyroid configurations.

Commonly studied transport characteristics include properties such as flow velocity, flow path, and even interface interactions between water and the pore wall. Experimental techniques can be used to assess how fast water is moving in the membrane systems, but are not able to capture the flow path or its interactions with the membrane surface. The synthesis of various morphologies for the experiments is also a time-consuming process. Therefore, computational tools can be employed to simulate the environment that water encounters.

The scientists at M-WET modeling the membrane use a computational technique called the dissipative particle dynamics (DPD) method. In this method, how water interacts with the membrane is taken into consideration via forces, such as repulsive forces, velocity-dependent drag forces, and random force. Important descriptors in the DPD method for quantifying the dynamics of water are the global diffusion coefficient, i.e., how far a single water molecule has traveled from its origin, and the local diffusion coefficient, i.e., how far the molecule is from the pore walls. Another parameter called tortuosity can be described as the diffusion of particles through a porous media and the many curves the particle may encounter. One can visualize the tortuosity of a membrane by imagining a trickle of water through a sediment bed, such as through the porous limestone aquifer beneath Austin. From the simulation results, the authors found that the dimensions and tortuosity of the pore walls are the main factors affecting water mobility in the membrane.

To create pores in the system, the researchers selectively removed sections of the polymer membrane (Figure 2). Each section, or block, is designed to not mix, similar to water-oil interactions. When the polylactic acid (PLA) block is removed from the system in Figure 2b, water transport changes. This shift in water behavior is induced by the water-soluble nature of the polyethylene oxide (PEO) chains decorating the pore wall—water is able to “stretch out” into the pore. Specifically, the diffusivities of water change dramatically with respect to its distance from the PEO chains. Swelling of the pore wall can occur, where water molecules penetrate the PEO layers. The length of the PEO layers, i.e., the number of beads in the polymer chains, additionally affect the swollen characteristics of the pore walls.

Figure 2. Simulations for adding water particles in the pore in the (a) lamellae morphology, (b) pore created after removing polylactic acid (PLA) blocks, (c) initial system right after adding water particles inside the pore, and (d) final configuration of the system in water. Representative color: green for the polystyrene (PS) block, blue for the polyethylene oxide (PEO) block, red for the polylactic acid (PLA) block, and purple for water particles.

In conclusion, M-WET brought together scientists from the University of Texas at Austin and the University of California, Santa Barbara, to examine how water changes its interactions with membrane walls of various morphologies, such as in those of water- purification membranes and fuel cells. By employing computational tools based on the DPD method, the researchers simulated the behavior of water molecules in block copolymers to elucidate the role that nanopore morphology plays in this confining environment. These fundamental understandings can lead to less computationally expensive simulations, as well as direct future experimental studies for investigating the transport properties of water in polymer membranes.

More Information

Aryal, Dipak, Michael P. Howard, Rituparna Samanta, Segolene Antoine, Rachel Segalman, Thomas M. Truskett, and Venkat Ganesan, "Influence of Pore Morphology on the Diffusion of Water in Triblock Copolymer Membranes," J. Chem. Phys., 152, 014904 (2020).


The authors thank Professor Benny Freeman and other members of M-WET for many useful discussions. We also thank Ms. Pam Cook for suggestions and revisions that improved the stylistic aspects of the preprint. This work was supported by the Center for Materials for Water and Energy Systems, an Energy Frontier Research Center funded by the U. Department of Energy, Office of Science, Basic Energy Sciences under Award No. DE- SC0019272. Acknowledgment is also made to the Robert A. Welch Foundation (Grant No. F-1599 to VG and Grant No. F-1696 to TMT). We acknowledge the Texas Advanced Computing Center (TACC) at The University of Texas at Austin for providing HPC resources.

About the author(s):

ChoongSze Lee is a fourth-year Chemical Engineering graduate student at the University of Minnesota, Twin Cities. Her advisors, Paul Dauenhauer and Michael Tsapatsis, are in the Catalysis Center for Energy Innovation (CCEI), an Energy Frontier Research Center funded by the U.S. Department of Energy. ChoongSze is interested in the rational design of catalysts for the renewable production of fuels and chemicals.


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