Interactions of limonene and carvone on titanium dioxide surfaces

A Near-Explicit Reaction Mechanism of Chlorine-Initiated Limonene: Implications for Health Risks Associated with the Concurrent Use of Cleaning Agents and Disinfectants

Limonene, a key volatile chemical product (VCP) commonly found in personal care and cleaning agents, is emerging as a major indoor air pollutant. Recently, elevated levels of reactive chlorine species during bleach cleaning and disinfection have been reported to increase indoor oxidative capacity. However, incomplete knowledge of the indoor transformation of limonene, especially the missing chlorine chemistry, poses a barrier to evaluating the environmental implications associated with the concurrent use of cleaning agents and disinfectants. Here, we investigated the reaction mechanisms of chlorinated limonene peroxy radicals (Cl-lim-RO2•), key intermediates in determining the chlorine chemistry of limonene, and toxicity of transformation products (TPs) using quantum chemical calculations and toxicology modeling. The results indicate that Cl-lim-RO2• undergoes a concerted autoxidation process modulated by RO2• and alkoxy radicals (RO•), particularly emphasizing the importance of RO• isomerization. Following this generalized autoxidation mechanism, Cl-lim-RO2• can produce low-volatility precursors of secondary organic aerosols. Toxicological findings further indicate that the majority of TPs exhibit increased respiratory toxicity, mutagenicity, and eye/skin irritation compared to limonene, presenting an occupational hazard for indoor occupants. The proposed near-explicit reaction mechanism of chlorine-initiated limonene significantly enhances our current understanding of both RO2• and RO• chemistry while also highlighting the health risks associated with the concurrent use of cleaning agents and disinfectants.

Heterogeneous interactions and transformations of dibasic esters with indoor relevant surfaces

Dibasic esters (DBEs) have recently become emerging indoor air pollutants due to their usage as a solvent for mixtures of paints and coatings. In this study, we explored the adsorption/desorption kinetics, heterogeneous interactions, and chemical transformations of dimethyl succinate (DMS, C6H10O4), a component of commercial dibasic ester solvent mixtures, on indoor relevant surfaces using transmission Fourier-transform infrared (FTIR) spectroscopy and high-resolution mass spectrometry (HRMS). Silica (SiO2) and rutile (TiO2) were used as proxies for window glass, and an active component in paint and self-cleaning surfaces, respectively. FTIR spectroscopy of these surfaces shows that DMS can interact with SiO2 and TiO2 through hydrogen bonding between the carbonyl oxygen and surface hydroxyl groups. The kinetics show fast adsorption of DMS onto these surfaces followed by slow desorption. Furthermore, new products formed observed on TiO2 surfaces in addition to molecularly adsorbed DMS. In particular, succinate (C5H7O) was observed binding to the surface in a bidentate chelating coordination mode as indicated by the appearance of νas(COO-) and νs(COO-) bands in the FTIR spectra. These absorption bands grow in intensity over time and the resulting product remains strongly adsorbed on the surface. The formation of adsorbed succinate is a result of a reaction with DMS on Lewis acid sites of the TiO2 surface. Overall, the slow desorption of these adsorbed species indicates that indoor surfaces can become long term reservoirs for dibasic esters and their surface products. Moreover, in the presence of ∼50% relative humidity, water displaces outer layers of adsorbed DMS on SiO2 and TiO2, while having no impact on the more strongly bound surface species.

Gas-Phase and Surface-Initiated Reactions of Household Bleach and Terpene-Containing Cleaning Products Yield Chlorination and Oxidation Products Adsorbed onto Indoor Relevant Surfaces

The use of household bleach cleaning products results in emissions of highly oxidative gaseous species, such as hypochlorous acid (HOCl) and chlorine (Cl2). These species readily react with volatile organic compounds (VOCs), such as limonene, one of the most abundant compounds found in indoor enviroments. In this study, reactions of HOCl/Cl2 with limonene in the gas phase and on indoor relevant surfaces were investigated. Using an environmental Teflon chamber, we show that silica (SiO2), a proxy for window glass, and rutile (TiO2), a component of paint and self-cleaning surfaces, act as a reservoir for adsorption of gas-phase products formed between HOCl/Cl2 and limonene. Furthermore, high-resolution mass spectrometry (HRMS) shows that the gas-phase reaction products of HOCl/Cl2 and limonene readily adsorb on both SiO2 and TiO2. Surface-mediated reactions can also occur, leading to the formation of new chlorine- and oxygen-containing products. Transmission Fourier-transform infrared (FTIR) spectroscopy of adsorption and desorption of bleach and terpene oxidation products indicates that these chlorine- and oxygen-containing products strongly adsorb on both SiO2 and TiO2 surfaces for days, providing potential sources of human exposure and sinks for additional heterogeneous reactions.

Adsorption of 6-MHO on two indoor relevant surface materials: SiO2 and TiO2

The compound 6-methyl-5-hepten-2-one (6-MHO) is a product of skin oil ozonolysis and is of significance in understanding the role of human occupants in the indoor environment. We present a joint computational and experimental study investigating the adsorption of 6-MHO on two model indoor relevant surfaces, SiO₂, a model for a glass window, and TiO₂, a component of paint and self-cleaning surfaces. Our classical force field-based molecular dynamics, ab initio molecular dynamics simulations, and FTIR absorption spectra indicate 6-MHO can adsorb on to both of these surfaces via hydrogen and π-hydrogen bonds and is quite stable due to the linear geometry of 6-MHO. Detailed analysis of 6-MHO on the SiO₂ surface shows that relative humidity does not impact surface adsorption and adsorbed water does not displace 6-MHO from the hydroxylated SiO₂ surface. Additionally, the desorption kinetics of 6-MHO from the hydroxylated SiO₂ surface is compared to other compounds found in indoor environments and 6-MHO is shown to desorb with a first order rate constant that is approximately four times slower than that of limonene, but six times faster than that of carvone. In addition, our joint results indicate 6-MHO forms a stronger interaction with the TiO₂ surface compared to the SiO₂ surface. This study suggests that skin oil ozonolysis products can partition to indoor surfaces leading to the formation of organic films.