In-situ-FTIR-spectroscopic studies of organic acid monolayers on metal oxides in humid environments

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A new in-situ setup combining ultra-high vacuum (UHV), plasma application, photoelastic-modulated-infrared-reflection-absorption-spectroscopy (PM-IRRAS), and quartz crystal microbalance (QCM) was developed to analyze self-assembled monolayers (SAMs) on engineering metals. The adsorption behavior of various organophosphonic acids on nickel-titanium (NiTi) shape memory alloy (SMA) was first examined. PM-IRRAS demonstrated that the binding mechanism involves mono- or bidental bonds. It was found that a chain length of 17 CH2 groups is necessary for a stable SAM in an aqueous environment. In-situ measurements assessed the formation, barrier properties, and stability of octadecylphosphonic acid (ODPA) and nonadecanoic acid (NDA) SAMs under high water activities. PM-IRRAS confirmed the self-assembly of both molecules on oxyhydroxide-covered aluminum surfaces, showing stability in humid air. A significantly reduced amount of water was observed due to the low energy of the aliphatic surfaces preventing hydrogen bonding. However, ODPA monolayers did not significantly inhibit the H2O/D2O isotope exchange reaction with aluminum hydroxyl groups. The stability of the phosphonate group and the ordering of the SAM are attributed to strong acid-base interactions with Al ions and molecular interactions of the aliphatic chains. For NDA monolayers, the kinetics of surface hydroxylation by H2O dissociative adsorption was notably reduced, while th