Air-water interfaces are common and auspicious reaction environments in the natural world. Organic molecules partition preferentially to water surfaces, such as the sea surface microlayer or aqueous aerosol. Recent studies in the literature have highlighted the possible role that the interfaces can play in promoting reactivity and enhancing reaction rates compared to the bulk. One key form of reactivity in these natural environments is the direct photochemistry of organic molecules. The photochemistry of organic molecules is known to be highly condition dependent, leading to the synthesis of differing photochemical products under different environmental conditions. It has been suggested that adsorption to an interface can also affect observed photochemical reactivity of model species; however, few studies have examined the photochemistry of monolayer films directly. In order to examine differences between bulk and interfacial photochemistry, we have chosen α-keto acids as a class of model compounds whose bulk photochemistry is understood in mechanistic detail. Here, we describe the synthesis of insoluble surfactants with α-keto acid functionality. Specifically, we focus on the synthesis of 2-oxotetradecanoic acid and 2-oxooctadecanoic acid, which are 14 and 18 carbon molecules, respectively. The synthesis primarily used Grignard chemistry under a positive pressure of nitrogen gas to optimize reaction conditions. The surface thermodynamics of these α-keto acids were then characterized on the air-water interface using a Langmuir trough. Following characterization of the surface films of these precursors, photolysis is initiated using UV LEDs and products are analyzed offline via ESI mass spectrometry. Characterizing these model α-keto acids on the surface of aqueous environments, helps understand potential changes in reactivity and any environmental implications of photochemistry on air-water interfaces.

Synthesis and characterization of surfactant α-keto acids: Towards photochemistry at the air-water interface

ACS Spring 2022

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