On-Water and In-Water Effects: May Natural Organic Matter Serve as an Interfacial Reaction Facilitator?

Abstract This perspective builds on the concept of reaction facilitation, where certain complex reactions can be accelerated up to 700 times at interfaces between water and hydrophobic substances. These effects include the “on-water” phenomenon observed in emulsions formed by mixing hydrophobic reactants with water, and the “in-water” effect observed predominantly in the aqueous phase at organic-water interfaces, such as micelles. We suggest that similar reaction enhancements might also occur at the interfaces of natural organic matter (NOM) fractions that form supramolecular assemblies. Such interfaces could transform contaminants and potentially detoxify them in the environment. Initially, we review documented on-water and in-water effects described in the synthetic chemistry literature. Next, we examine evidence that some NOM fractions can form supramolecular assemblies under appropriate conditions. We then discuss experimental observations of reaction enhancements with NOM amendments─suggesting potential on-water- or in-water-like effects. Using these insights, we propose two mechanisms by which NOM could enhance reactions: 1) by stabilizing hydrogen-bonding transition states and 2) through hydrophobically driven interactions and/or a multisite functional group model. Finally, we outline future research directions to determine if such on-water- or in-water-like effects occur at NOM-supramolecular interfaces and discuss their potential environmental implications if confirmed.