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 SiO 2 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 SiO 2 have been measured as a function of relative humidity. The desorption kinetics of carvone from the SiO 2 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 SiO 2 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 SiO 2 particles. Interestingly, relative humidity does not change the desorption kinetics of carvone from the SiO 2 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.

The indoor environment is a dynamic one with many variables impacting indoor air quality and indoor air chemistry. These include relative humidity (RH) and the presence of different surfaces. Although it has been suggested that the indoor concentrations of gas-phase compounds increase at higher relative humidity, because of displacement of these compounds from indoor surfaces, little is known from a molecular perspective about how RH and adsorbed water impact the adsorption of indoor relevant organic compounds such as limonene with indoor relevant surfaces. Herein, we investigate the effects of RH on the adsorption of limonene, a hydrophobic molecule, on hydroxylated SiO 2 surfaces, a model for glass surfaces. Experimental data using infrared spectroscopy to directly measure limonene adsorption are combined with both force field-based molecular dynamics (MD) and ab initio molecular dynamics (AIMD) simulations to understand the competitive interactions between limonene, water, and the SiO 2 surface. The spectroscopic data provide evidence that adsorbed limonene is not completely displaced by adsorbed water, even at high RH (∼80%) when the water layer coverage is close to three monolayers (MLs). These experimental data are supported by AIMD and MD simulations, which indicate that limonene is present at the adsorbed water interface but displaced from direct interactions with SiO 2 . This study shows that although some limonene can desorb from the surface, even at the highest RH, more than half the limonene remains adsorbed on the surface that can undergo continued surface reactivity. A complex network of π-hydrogen bonds, water−water hydrogen bonds, and SiO 2 −water hydrogen bonds explains these interactions at the air/adsorbed water/SiO 2 interface that hold the hydrophobic limonene molecule at the interface. Importantly, these interactions are most likely present for a range of other systems involving organic compounds and solid surfaces at ambient relative humidity and may be important in a range of scientific areas, from sensor development to cultural heritage science.

Limonene, a monoterpene, found in cleaning products and air fresheners can interact with a variety of surfaces in indoor environments. An oxidation product of limonene, carvone, has been reported to...

Oxygenated organic compounds (OOCs) are widely found in indoor environments and come from either the direct emissions from indoor activities or the subsequent oxidation of nonoxygenated OCs. Adsorption and partitioning of OCs on surfaces are significant processes in indoor chemistry, yet these interactions specifically involving OOCs are still poorly understood. In this study, we investigate the interactions of three prevalent indoor OOCs (dihydromyrcenol, α-terpineol, and linalool) on an indoor surface proxy (hydroxylated SiO 2 ) by combining vibrational spectroscopy with ab initio molecular dynamics simulations. The adsorption of these compounds on the SiO 2 surface is driven by π hydrogen bonding and O−H hydrogen bonding interactions, with O−H hydrogen bonding interactions being stronger. The results of kinetic measurements suggest that indoor surfaces play a significant role in the removal of these OOCs, especially under moderate and low air exchange. Additionally, indoor surfaces can also serve as a reservoir of OOCs due to their much slower desorption kinetics when compared to other indoor relevant organic compounds such as limonene. Overall, the results gleaned by experiment and theoretical simulations provide a molecular representation of the interaction of OOCs on indoor relevant surfaces as well as implications of these interactions for indoor air chemistry.

We present a study of four monoterpene isomers (limonene, γ-terpinene, terpinolene, and α-pinene) that are prevalent in indoor environments and their interaction with the hydroxylated SiO2 surface, a model for the glass surface, by combining infrared spectroscopy and computational simulations. These isomers are molecularly adsorbed onto SiO2 through π-hydrogen bonds with surface hydroxyl groups. However, experimental results suggest that the strength of interaction of these compounds with the SiO2 surface varies for each isomer, with α-pinene showing the weakest interaction. This observation is supported by molecular dynamics simulations that α-pinene adsorbed on the SiO2 surface has lower free energy of desorption and a lower mass accommodation coefficient compared to other isomers. Additionally, our ab initio molecular dynamics simulations show lower π-hydrogen bonding probabilities for α-pinene compared to the other three constitutional isomers. Importantly, these interactions are most likely present for a range of other systems involving organic compounds and solid surfaces and, thus, provide a thorough framework for comparing the interactions of organic molecules on indoor relevant surfaces.