Version 2 2023-09-19, 17:36Version 2 2023-09-19, 17:36
Version 1 2020-04-16, 20:44Version 1 2020-04-16, 20:44
journal contribution
posted on 2023-09-19, 17:36authored byCatriona M. McGilvery, Patricia Abellan, Michał M. Kłosowski, Andrew G. Livingston, João T. Cabral, Quentin M. Ramasse, Alexandra E. Porter
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 NCC 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.