Nanoscale chemical heterogeneity in aromatic polyamide membranes for reverse osmosis applications
File(s)
Supporting information
Accepted version
Author(s)
Type
Journal Article
Abstract
Reverse osmosis membranes are used within the oil and gas industry for seawater desalination on off-shore oilrigs. The membranes consist of three layers of material: a polyester backing layer, a polysulfone support and a polyamide (PA) thin film separating layer. It is generally thought that the PA layer controls ion selectivity within the membrane but little is understood about its structure or chemistry at the molecular scale. This active polyamide layer is synthesized by interfacial polymerization at an organic/aqueous interface between m-phenylenediamine and trimesoyl chloride, producing a highly cross-linked PA polymer. It has been speculated that the distribution of functional chemistry within this layer could play a role in solute filtration. The only technique potentially capable of probing the distribution of functional chemistry within the active PA layer with sufficient spatial and energy resolution is scanning transmission electron microscopy combined with electron energy-loss spectroscopy (STEM-EELS). Its use is a challenge because organic materials suffer beam-induced damage at relatively modest electron doses. Here we show that it is possible to use the N K-edge to map the active layer of a PA film using monochromated EELS spectrum imaging. The active PA layer is 12 nm thick, which supports previous neutron reflectivity data. Clear changes in the fine structure of the C K-edge across the PA films are measured and we use machine learning to assign fine structure at this edge. Using this method, we map highly heterogeneous intensity variations in functional chemistry attributed to N—C═C bonds within the PA. Similarities are found with previous molecular dynamics simulations of PA showing regions with a higher density of amide bonding as a result of the aggregation process at similar length scales. The chemical pathways that can be deduced may offer a clearer understanding of the transport mechanisms through the membrane.
Date Issued
2020-04-29
Date Acceptance
2020-04-07
Citation
ACS Applied Materials & Interfaces, 2020, 12 (17), pp.19890-19902
ISSN
1944-8244
Publisher
American Chemical Society (ACS)
Start Page
19890
End Page
19902
Journal / Book Title
ACS Applied Materials & Interfaces
Volume
12
Issue
17
Copyright Statement
© 2020 American Chemical Society. This document is the Accepted Manuscript version of a Published Work that appeared in final form in ACS Applied Materials and Interfaces, after peer review and technical editing by the publisher. To access the final edited and published work see https://doi.org/10.1021/acsami.0c01473
Sponsor
BP International Limited (0946)
Identifier
https://pubs.acs.org/doi/10.1021/acsami.0c01473
Grant Number
Order No. 75195/ICAM10 (IC)
Subjects
electron energy-loss spectroscopy
nanoscale chemical mapping
reverse osmosis membranes
water desalination
water permeation pathway
03 Chemical Sciences
09 Engineering
Nanoscience & Nanotechnology
Publication Status
Published
Article Number
acsami.0c01473
Date Publish Online
2020-04-07