Possible Effects of Ozone Chemistry on the Phase Behavior of Skin Oil and Cooking Oil Films and Particles Indoors

Abstract

Heterogeneous reactions occurring between ozone and films or particles containing skin oil or cooking oil can be a sink of ozone and a source of organic compounds indoors with negative health effects. To predict this heterogeneous chemistry, information on the phase behavior (number and types of phases) within these films and particles, and its evolution during ozone exposure, is needed. In this work, we used optical microscopy to directly observe the phase behavior of proxies for skin oil and cooking oil particles (diameters of 110–220 μm) during exposure to O3. As a proxy for skin oil, we used a multicomponent mixture of unsaturated lipids. For cooking oil, we used a commercially available canola oil. We also studied single-component unsaturated lipids (squalene, triolein, and oleic acid) and jojoba oil (mostly unsaturated monoesters) as simpler proxies for skin oil and cooking oil. Prior to reactions with ozone, the particles were all liquid. Exposure to ozone led to the formation of a new phase within the particles. For oleic acid, the new phase was most likely a high-molecular-weight liquid formed by liquid–liquid phase separation. For all other systems, the new phase was likely solid or semi-solid. Our results, together with the resistor model and the reaction extent, were then used to predict the time it takes for a new phase to form in skin oil and cooking oil films indoors. Based on our results, we predict that a new phase will form in skin oil films with a thickness of 0.1–1 μm after a timescale of 3 h to 7 days under typical indoor ozone concentrations (6 ppb) and relative humidity (RH) conditions ranging from 5 to 65%. In cooking oil films, we predict that a new phase will form after 1.5–16 h for films of 0.1–1 μm thickness under ozone concentrations of 6 ppb and RH conditions ranging from 5 to 65%. A new phase will likely impact diffusion rates and partitioning coefficients within the films and hence ozone loss rates indoors, production rates of respiratory irritants, and partitioning of these irritants between films, particles, and the gas phase.

Quantifying the impact of relative humidity on human exposure to gas phase squalene ozonolysis products

“…64,65 The formation of a new semisolid or solid phase has recently been observed for squalene and skin oil proxy particles that have been exposed to high concentrations of ozone. 66 However, due to uncertainties in the exact mechanism that causes the slower squalene decay, such potential surface crust formation and associated kinetic limitations are not treated in the model. We can also speculate that such a burial process may be affected by temperature and may be different on room surfaces compared to human skin.…”

“…AIOMFAC simulations also predicted that the addition of acids to the mixture, such as present in skin oil, could cause LLPS and lead to the formation of surface crusts that can limit partitioning and multiphase reaction kinetics. 81,82 Further experimental validations as well as simulations with more realistic skin oil compositions including other unsaturated fatty acids and triglycerides 83 are required to further improve our understanding on skin ozonolysis and its impacts on indoor air quality.…”

Unanticipated Hydrophobicity Increases of Squalene and Human Skin Oil Films Upon Ozone Exposure

“…We then considered recent work by the Bertram group, who reported that liquid squalene and skin oil exposed to ozone form a new condensed phase composed of oligomeric ozonolysis products . These higher-molecular-weight oligomers could show increased hydrophobicity compared to unreacted squalene and skin oil, but the optical microscopy data reported by the Bertram group indicate them to be inside of the imaged sample drops.…”

“…The role of “sorbed” or interfacial water may be important, as the heterogeneous oxidation of squalene has been shown to produce gas-phase ozonolysis products with compositions varying with relative humidity . Just like in an outdoor atmospheric secondary organic aerosol, which can transition from having honey-like viscosity at high humidity to marble-like viscosity at low humidity, ,− water molecules may also act as a plasticizer in indoor surface layers, films, and materials. Experiments geared toward quantifying the amount of water uptake, for instance, by quartz crystal microbalance mass estimations or linear and nonlinear spectroscopy are therefore needed to further understand how the heterogeneous ozonolysis of human skin oil and its constituents forms a material with increased hydrophobicity.…”

“…49 We then considered recent work by the Bertram group, who reported that liquid squalene and skin oil exposed to ozone form a new condensed phase composed of oligomeric ozonolysis products. 51 These higher-molecular-weight oligomers could show increased hydrophobicity compared to unreacted squalene and skin oil, but the optical microscopy data reported by the Bertram group indicate them to be inside of the imaged sample drops. If such a new phase is formed inside the films studied in our present work, then it would be unlikely to be accessible to the water drops used in our sessile contact angle measurements, except for water diffusion through the bulk.…”