Next-Generation Filtering Membranes

Thursday, December 12, 2019

Synthetic polymer membranes are widely used to purify water, but currently available membranes are ineffective when it comes to cleaning water, such as that used in oil and gas production, that is heavily contaminated with diverse pollutants and toxins. Better membranes are needed, but gaps in scientific understanding stand in the way. 

In 2018, the U.S. Department of Energy provided a four-year, $10.75 million grant to fund the Center for Materials for Water and Energy Systems (M-WET) as one of its Energy Frontiers Research Centers. Involving teams at UCSB, the University of Texas Austin (UT), and the UC Berkeley National Lab, the highly collaborative center is intended to address those gaps and develop transformative polymer materials for energy-water applications.

UCSB M-WET researchers include nine UCSB professors and numerous graduate students and postdoctoral researchers working in four collaborative research units — three “Gap Attack Platforms” (GAPs) and an Integrating Framework group. Here, three graduate students and a postdoctoral scholar share what each GAP is doing. 

Second-year PhD student Sally Jiao says that the challenge for her GAP 1 colleagues is to use simulations, while working closely with experimentalists, to understand “how to engineer, or ‘decorate,’ a membrane surface so that it resists gunk sticking to it and fouling it, which impairs performance.” 

Jiao explains that the entire M-WET Center is focused on studying a single nanoscale membrane platform called the universal membrane chemistry platform (UMCP). “It is a specific triblock copolymer used to create the membrane,” she says. “The polymer is composed of three different blocks, and you get a bunch of the self-assembling blocks together to create a porous structure. You can remove single blocks to create pores that allow solute to pass through, and other blocks can be functionalized with polymers called peptoids, to modulate the hydrophobicity and hydrophillicity of the surface.

“Another person in GAP 1 is working on modeling the part of the t-block copolymer that forms the surface adjacent to the water, and I’m working on testing models for the peptoids,” Jiao adds. “In both cases, we’re validating our models by comparing our results to those from experimentalists. Eventually, we’ll put those models together and simulate the actual realistic surface that the water is going to see.”

“In M-WET, we want to design membranes for the specific water and the specific purpose we’re interested in,” says Sam Warnock, a second-year PhD student on GAP 2. The group is working to design a thin separation layer that has selected chemical functionalities incorporated into it to target specifi c solutes that the researchers either want to remove from the water and save for use, such as lithium for batteries, or remove and discard, such as pollutants. “In this case, we hope to change the membrane’s transport characteristics so that it will interact specifically with lithium, ignoring other ions, such as sodium and magnesium,” Warnock says.

The work requires he and his postdoctoral research partner, Shou Zhao, to examine various aspects of the polymer, especially water uptake, which is the mass of water inside the membrane divided by the mass of the membrane itself. That ratio dictates the amount of free volume inside a membrane. “Too much free volume and things can pass right through, because they won’t meet any resistance,” Warnock notes. “Too little, and not enough water goes through the membrane, resulting in lower throughput.” 

Gap 2 researchers would like to mimic natural, biological membranes, which achieve both rapid transport and high selectivity. Says Warnock, “There’s an optimum value, and we’re trying to find that balance by adding these selective interactants.“

“The membranes commonly used in the water-purifi cation industry look a lot like Swiss cheese or a sponge, with the holes, called pores, having different sizes and shapes at the nanometer scale” says Ségolène Antoine, a postdoctoral researcher on GAP 3. 

“Understanding the relationship between the membrane’s pore structure and the flow of solvent versus solute [dissolved particles] is crucial for developing next-generation membranes,” she explains. “Our mission consists of understanding the relationship between the structure of a membrane and its properties.”

GAP 3 researchers seek to develop membranes having controllable and tunable pores with known and optimized geometry and pore-wall chemistry, in order to establish a relationship between the membrane architecture and the resulting transport of water and solute through the membrane. “In this project, we employ multiple experimental and computational tools, both to generate membranes of varying geometrical parameters and to characterize their performance,“ Antoine says.

The team is also developing a model for the transport of fluids in moderately sized, moderately complex cylindrical pores to understand how these parameters depend on pore geometry and on the chemistry of both pore wall and fluid. “After comparison with experimental results, this model will serve as the training set for the design of the ‘best’ pore,” says Antoine.

According to second-year PhD student Varun Hegde, the main job for him, his UCSB faculty colleagues in the mathematically oriented Integrating Framework Group — professors Michael Doherty and Todd Squires — and their University of Texas collaborators is twofold: “to make sure that all the groups can relate to each other’s work, and to help direct future research.”

In the former effort, standardization of an important central value is key. “When we look at a membrane, we want to know what its permeability is, and that is often dominated by the diffusion coefficient of the solvent that is moving through it,” Hegde says. “All the different GAPs, through all their different experiments, interact with this value in some way.”

Various important processes — simulations, macroscopic flux measurements, microscopic interferometry measurements, and nuclear magnetic resonance measurements to study water molecules moving in polymer membranes — can be used to determine diffusion coefficient values that are not necessarily the same.

“You can get a different form of the value depending on whether conditions change or you factor out a variable for a given experiment or simulation,” Hegde notes. “It is important that the value is either always the same or, if it isn’t, that everyone knows why not, so that the theoretical transport framework is standardized.”

That standardization is critical for allowing other researchers to use the right value in the context of their own research.