Heterogeneous Interactions between Carvone and Hydroxylated SiO₂

Oxygenated semivolatile organic compounds are present in indoor air due to the oxidation of monoterpenes and other hydrocarbons. The partitioning of these compounds to indoor surfaces and the formation of organic thin films are of great interest as these surfaces become potential reservoirs for these compounds affecting indoor air composition. Here, we have investigated the interaction of carvone, a limonene oxidation product, with hydroxylated SiO2 surfaces at 297 ± 1 K with FTIR spectroscopy. The detailed surface interactions were understood through both force field-based molecular dynamics (MD), ab initio molecular dynamics (AIMD) simulations, and electronic structure calculations. The adsorption/desorption kinetics of carvone on hydroxylated SiO2 have been measured as a function of relative humidity. The desorption kinetics of carvone from the SiO2 surface are slow, approximately 30 times slower when compared to limonene under dry conditions. Molecular dynamics (MD) simulations reveal that carvone forms O–H hydrogen bonding (carbonyl O to HO–Si) with the isolated hydroxyl groups and oxygen silicon associations (carbonyl O to Si–O–Si) with siloxane bridges that are also on the SiO2 surface. A kinetic model was applied to further examine the carvone adsorption and desorption kinetics; the experimental measurements can be reproduced by assuming that some carvone molecules become trapped in pores between the SiO2 particles. Interestingly, relative humidity does not change the desorption kinetics of carvone from the SiO2 surface. Instead, water molecules adsorb onto the preadsorbed carvone, which hydrates the organic-coated surface. Overall, this study shows how carvone, an oxygenated organic compound, interacts with indoor relevant surfaces and the impact of relative humidity on these interactions. It also demonstrates how chemical transformations (e.g., oxidation) in indoor environments can play an important role in how molecules partition to surfaces and interact with condensed phase water.