Fundamental Investigation of Surfactant-Assembly-Regulated Interfacial Polymerization (SARIP) for Fabricating Next-Generation Membranes for Precise Solute-Solute Separation

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Membranes are used in industry to separate components from a mixture (a solution). Most membranes are used to remove solutes (dissolved molecules) from the solution. For example, wastewater treatment processes sometimes use membranes to remove contaminants, and seawater desalination often involves using a reverse osmosis membrane to remove salts from seawater. Membranes can also be used to separate solutes from each other, which is called "molecular sieving." Molecular sieving is an important separation process for the pharmaceutical, petrochemical, and energy industries. For example, membrane-based extraction of lithium (an important material for batteries) from seawater or brine requires high-precision molecular sieving. Membranes capable of selectively separating chemically similar solutes are challenging to manufacture in large volumes and at low cost. A primary challenge is that molecular sieve membranes must have exceedingly small pores, less than a nanometer, that are uniform in size. The investigators plan to develop an adaptable method for manufacturing molecular sieve membranes by chemically controlling pore size uniformity. The method combines readily available chemicals (surfactants and monomers) at an oil-water interface above a layer of support material. The interactions of the surfactants with both the oil and water causes the surfactants to accumulate in between the two. The accumulated surfactant spontaneously forms a network of flexible pores that allow the monomers to pass through to the support material. The monomers subsequently form a homogenous layer of pores comprising the membrane. The membrane structure and chemical composition, as well as the processes by which they are formed, will be characterized to understand how the approach works. This method for forming uniform, sub-nanometer polymer pores is expected to be both inexpensive, efficient, and compatible with existing infrastructure for membrane manufacturing. The outcomes of this project will rapidly advance the current state-of-the-art in membrane technology and separation science, serving to displace more energy-intensive and costly industrial separation methods. The project will also provide educational opportunities including undergraduate research experiences, graduate and undergraduate course-based instruction, and hands-on workshops for high school students through the Vanderbilt Summer Academy.The goal of this project is to develop a universal approach for forming highly uniform pores (free volume) in the polyamide active layer of nanofiltration membranes formed via interfacial polymerization. The investigators hypothesize that the interfacial polymerization process can be regulated by introducing a dynamically self-assembled, highly organized "network" of surfactants at the hexane/water interface, which is expected to result in the formation of a polyamide active layer with enhanced pore-size homogeneity. The uniformity in pore size is further expected to result in membranes with ultra-sharp selectivity for precise solute-solute separations. The investigators call this novel approach surfactant-assembly-regulated interfacial polymerization (SARIP). The investigators will test the hypothesis and uncover the attendant mechanisms through three research objectives. The first objective focuses on demonstrating that selected surfactants can sharpen the selectivity of thin film composite polyamide (TFC-PA) membranes. Surfactants with different hydrophilic charge groups or hydrophobic tails of varying length will be evaluated. Second, the investigators will assess the impact the surfactant functional groups and structures have on the SARIP-formed TFC-PA nanofiltration membrane material properties and performance. Finally, the impact of surfactant self-assembly on the kinetics of amine diffusion across the hexane/water interface in the absence and presence of polymerization (with acid chloride) will be determined. The development of a universal, scalable approach for fabricating membranes with highly uniform free volumes is expected to advance membrane technologies for sub-nanometer scale solute separations.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.