Hydroxyphenol Interactions With Iron and Aluminum Oxide Colloids by Chemical Force Spectrometry
Abd. Rahman Azmi, Alyza Azzura
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Tannins and humic substances commonly referred to as natural organic matter (NOM), constitute an important component of natural water and soil systems. These species contain numerous phenol and carboxyl functional groups whose reactivity is strongly dependent on both the quantity and location of these moieties on the aromatic ring. In the realistic environmental conditions, both phenolic and carboxylic functional groups are adsorbed on a variety of colloidal metal oxide surfaces. Unfortunately, due to the complexity of humic-based substances, experimental data involving mineral-humate interactions are difficult to interpret. Here, we aim to develop a more detailed understanding of mineral-NOM interactions in aquatic systems, using self-assembled monolayers (SAMs) of simple organic acids having functional groups similar to those found in humic substances. SAMs of 4-(12-mercaptododecyl)benzene-1,2-diol (o-hydroxyphenol-terminated), 5-(12-mercaptododecyl)benzene-1,3-diol (m-hydroxyphenol-terminated), bis(11-thioundecyl) hydrogen phosphate (monoprotic phosphate) and 11-thioundecyl dihydrogen phosphate (diprotic phosphate) were prepared and deposited on a Au(111) surface. The composition of elements present on the surface were determined by X-ray Photoelectron Spectroscopy (XPS) and the orientation of monolayers on the Au(111) surface was explored by Attenuated Total Reflectance-Fourier Transform Infrared (ATR-FTIR) Chemical force spectrometry has been used to determine the surface pKa of the monolayers and further used to explore the role of phenolic groups in the surface complexation of NOM by monitoring adhesion forces between iron and aluminum oxide sample and hydroxyphenol-terminated Atomic Force Microscopy (AFM) modified tip. The results are discussed in the context of hydrogen bonding between corresponding species. The system in which there are multiple hydroxyl groups ortho to the carboxylic groups or adjacent to one another on the benzene ring results in significantly different force-distance profiles when interacting with the hydroxyphenol tip.