Fresh water has become a valuable resource not only for consumption in our personal lives but also for use industrially. Approximately 3% of the Earth’s water supply is fresh water with the overwhelming majority present in the oceans.

The best current approach to produce fresh water is a technique called reverse osmosis (RO). In this process, a stream of contaminated water is forced through a polymer membrane under pressure. A cross flow method is used to trap the contaminants within the membrane while allowing fresh water to pass through. 

This type of filtration also is used industrially in water treatment to remove contaminants from effluent streams so the end-user is compliant with regulatory authorities. In a previous TLT article (1), researchers prepared water-treatment membranes using block polymers that self-assemble into nanostructures with pores that are well-defined and have the same size. The block polymers evaluated were prepared with a hydrophobic matrix containing polystyrene, a rigid but brittle resin and polyisoprene, an elastic material similar to rubber. A third more hydrophilic polymer, poly (dimethylacrylamide) was used to line the pore walls.

Traditional RO membranes contain a polyamide barrier layer prepared through an interfacial polymerization procedure. Dr. Benjamin Ocko, senior scientist at the U.S. Department of Energy’s Brookhaven National Laboratory in Upton, N.Y., says, “Interfacial polymerization is a process where the two reactants are soluble in incompatible solvents such as water and hexane. The polyamide is produced when water soluble, m-phenylenediamine is exposed to hexane soluble, trimesoyl chloride. This polymer barrier layer is supported by a thicker, porous, polysulfone layer.”

The resulting polyamide barrier is a heterogeneous, rough barrier layer with a complex amorphous structure and morphology. This surface layer is typically 20-200 nanometers thick. 

One challenge faced by the RO industry has been to determine the structure of this barrier layer. Ocko says, “Analytical techniques such as atomic force microscopy (AFM) were used in an effort to examine the molecular structure of the polyamide. Unfortunately an AFM probe is very useful at evaluating the local surface structures but ineffective in measuring the internal molecular structure. Molecular dynamic computer simulations also have been used, but the results have not been tested against experimental structural data in the case of polyamide-based barrier layer.”

A new analytical technique has now been used to provide further insight on the structure of the RO barrier layer.

GIWAXS
Ocko and his colleagues successfully analyzed the structure of the polyamide barrier layer using a technique known as grazing incidence wide-angle X-ray scattering (GIWAXS, see Figure 2). He says, “The advantage of using GIWAXS is that X-rays glance off the barrier layer at a small angle known as the grazing angle spreading out across the sample. This movement leads to a bigger scattering volume generating a larger X-ray pattern that is critical in identifying specific molecular packing motifs.”


Figure 2. Polyamide barrier membrane samples were prepared and evaluated using a technique known as GIWAXS to determine their molecular structure. (Figure courtesy of the U.S. Department of Energy’s Brookhaven National Laboratory.)

The researchers used the standard oil/water interfacial polymerization process to produce the polyamide barrier layer from phenylenediamine and trimesoyl chloride. A polished silicon wafer placed on the bottom of the beaker used in the polymerization reaction served as a support for the newly formed polyamide barrier layer. The film had a thickness of 18 nanometers. 

Citric acid was added as a post-treatment to assist the researchers with developing a clearer view of the polyamide barrier layer structure. Ocko says, “The rinse acid is needed to react with m-phenylenediamine monomer that did not polymerize. This helps to remove sharp crystalline peaks associated with the unreacted monomer that can obscure the results. Citric acid also protonates unreacted carboxylic acid groups present on the barrier layer to eliminate the possibility of hydrogen bonding within the membrane causing structural changes that might obscure the true membrane structure.”

The main structural feature affecting molecular structure is the aromatic rings because they represent the largest component in the polyamide. Two molecular packing motifs were identified by the GIWAXS analysis. 

Ocko says, “The first packing motif involves a parallel π-π stacking of the aromatic cores that leads to molecular spacing between 3.5 and 4 angstroms. In the second packing motif, a perpendicular stacking of neighboring aromatic cores produces a T-shaped configuration with an average spacing of 5 angstroms.”

The researchers suggest that the perpendicular stacking is more conducive to facilitating the diffusion of water through the membrane. Ocko says, “The perpendicular orientation is preferred because it is the lowest energy configuration for the polyamide barrier layer.”

A commercial RO membrane also was evaluated using GIWAXS and found to have similar molecular features to the membrane prepared by the researchers. Ocko says, “We do not have a good understanding of the molecular packing of polyamides that give rise to optimal RO properties. Our next series of studies will be done with additional commercial membranes to gain a better understanding of their molecular structure. Our hope is that this and future work can be used to develop more energy efficient RO membranes that can operate as effectively while using less energy.”

Additional information on this work can be found in a recent article (2) or by contacting Ocko at ocko@bnl.gov or his co-author, Benjamin Hsiao, Distinguished Professor in the department of chemistry at Stony Brook University in Stony Brook, N.Y., at benjamin.hsiao@stonybrook.edu. 

REFERENCES
1. Canter, N. (2018), “New Type of Polymer Membrane for Water Treatment,” TLT, 74 (8), pp. 16-17.
2. Fu, Q., Verma, N., Ma, H., Medellin-Rodriguez, F., Li, R., Fukuto, M., Stafford, C., Hsiao, B. and Ocko, B. (2019), “Molecular Structure of Aromatic Reverse Osmosis Polyamide Barrier Layers,” ACS Macro Letters, 8 (4), pp. 352-356.