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

Aromatic polyamide films form the active layer in reverse osmosis 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 [ Johnson , P. M. ; Molecular Layer-by-Layer Deposition of Highly Crosslinked Polyamide Films . J. Polym. Sci., Part B: Polym. Phys. 2012 , 50 ( 3 ), 168 - 173 ] yields films that are sufficiently smooth to warrant investigation with high-resolution microscopy and spectroscopy methods. We present high-resolution, quantitative X-ray photoelectron spectroscopy (XPS) data on the surface chemistry of ultrathin polyamide films that can serve as a model system for desalination membranes. We show that a quantitative analysis of the XPS spectra gives information about 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 cross-linking. 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 via 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.

Dimethyl methylphosphonate adsorption and decomposition on MoO2 as studied by ambient pressure x-ray photoelectron spectroscopy and DFT calculations.

Organophosphonates range in their toxicity and are used as pesticides, herbicides, and chemical warfare agents (CWAs). Few laboratories are equipped to handle the most toxic molecules, thus simulants such as dimethyl methylphosphonate (DMMP), are used as a first step in studying adsorption and reactivity on materials. Benchmarked by combined experimental and theoretical studies of simulants, calculations offer an opportunity to understand how molecular interactions with a surface changes upon using a CWA. However, most calculations of DMMP and CWAs on surfaces are limited to adsorption studies on clusters of atoms, which may differ markedly from the behavior on bulk solid-state materials with extended surfaces. We have benchmarked our solid-state periodic calculations of DMMP adsorption and reactivity on MoO2 with ambient pressure x-ray photoelectron spectroscopy studies (APXPS). DMMP is found to interact strongly with a MoO2 film, a model system for the MoO x component in the ASZM-TEDA© gas filtration material. Density functional theory modeling of several adsorption and decomposition mechanisms assist the assignment of APXPS peaks. Our results show that some of the adsorbed DMMP decomposes, with all the products remaining on the surface. The rigorous calculations benchmarked with experiments pave a path to reliable and predictive theoretical studies of CWA interactions with surfaces.

Water Adsorption and Dissociation on Polycrystalline Copper Oxides: Effects of Environmental Contamination and Experimental Protocol.

We use ambient-pressure X-ray photoelectron spectroscopy (APXPS) to study chemical changes, including hydroxylation and water adsorption, at copper oxide surfaces from ultrahigh vacuum to ambient relative humidities of ∼5%. Polycrystalline CuO and Cu2O surfaces were prepared by selective oxidation of metallic copper foils. For both oxides, hydroxylation occurs readily, even at high-vacuum conditions. Hydroxylation on both oxides plateaus near ∼0.01% relative humidity (RH) at a coverage of ∼1 monolayer. In contrast to previous studies, neither oxide shows significant accumulation of molecular water; rather, both surfaces show a high affinity for adventitious carbon contaminants. Results of isobaric and isothermic experiments are compared, and the strengths and potential drawbacks of each method are discussed. We also provide critical evaluations of the effects of the hot filament of the ion pressure gauge on the reactivity of gas-phase species, the peak fitting procedure on the quantitative analysis of spectra, and rigorous accounting of carbon contamination on data analysis and interpretation. This work underscores the importance of considering experimental design and data analysis protocols during APXPS experiments with water vapor in order to minimize misinterpretations arising from these factors.