Quantitative Characterization of a Desalination Membrane Model System by X-ray Photoelectron Spectroscopy

Abstract

Aromatic polyamide films form the active layer in reverse osmosis (RO) desalination membranes.Despite widespread use of this technology, it suffers from low rejection rates for certain water contaminants and from membrane fouling. Through a better understanding of the fundamental surface chemical processes during reverse osmosis desalination, advances in membrane and material design are expected. The recent invention of a molecular layer-by-layer (mLbL) preparation technique 1 yields films that are sufficiently smooth to warrant investigation with high resolution microscopy and spectroscopy methods. We present high-resolution, quantitative XPS data on the surface chemistry of ultrathin polyamide films that can serve as a model system for 2 desalination membranes. We show that a quantitative analysis of the XPS spectra gives information on the functional groups of the film as well as other compounds present due to the synthesis under ambient conditions. Unpolymerized functional groups are identified and aid in understanding the degree of crosslinking. Investigation of polymers with synchrotron-based XPS requires taking beam induced changes into account. We quantify X-ray beam damage and show that beam damage to the polyamide is limited, allowing long-term investigation of thin polyamide films. Characterizing mLbL grown films in high-resolution XPS is the basis for a better understanding of the chemical interplay of polyamide surface functional groups with the major components of desalination systems.

Water-polyamide chemical interplay in desalination membranes explored by ambient pressure X-ray photoelectron spectroscopy

“…The O 1s spectrum taken at the start of the experiment under dry conditions (bottom trace in Figure 1) exhibits two peaks, which have been discussed previously. 8 The low BE peak at 531.3 eV is consistent with literature values for carbonyl oxygen groups 16,18,19 and is assigned to the amide groups in the polyamide sample. The other peak at 532.7 eV contains contributions from a variety of chemical species.…”

“…groups, residual water, and potentially adventitious carbonhydroxyl species. 8 This peak grows considerably when water vapour is dosed into the vacuum chamber (middle trace in Fig. 1), indicating the adsorption of water onto the membrane.…”

“…The peak positions found for aromatic C-C groups (284.7 eV), C-N groups (286 eV), C=O groups (287.7 eV) and COOH groups (289 eV) are consistent with literature values. 8 The right panel in Figure 2 shows an enlargement of the high BE region of the C 1s spectrum before, during and after water exposure. In the presence of water, the COOH peak disappears, and the C=O peak increases in intensity compared to the initial spectrum taken under vacuum conditions.…”

“…The characterization of such films in ultrahigh vacuum (UHV) was previously reported and showed the presence of benzyl rings, amide groups, as well as unpolymerized acid groups and amine groups. 8 It was also shown that under UHV, X-ray beam damage to the polymer is not significant. Ambient pressure XPS data were recorded using two synchrotron-based APXPS endstations utilizing soft X-rays for core-level spectroscopy, namely APXPS-1 [Ref.…”

“…6 A simple thin film preparation routine for aromatic, cross-linked polyamide films was recently published by Stafford et al, 7 providing reproducible model systems that replicate the chemistry of commercial RO desalination membranes. 8 In this paper, we investigate the interplay of the surface chemical groups of thin film model polyamide membranes with water vapour. We observe a strong interaction of the free acid groups, amine groups and amide groups in contact with water, evidenced by a clear chemical shift that is reversible.…”