Comparison of the Adsorption–Desorption Kinetics of Limonene and Carvone on TiO₂ and SiO₂ Surfaces under Different Relative Humidity Conditions

Adsorption and desorption of volatile organic compounds (VOCs) on indoor material surfaces can impact VOC concentrations in indoor air. In this study, we investigate the adsorption and desorption kinetics of limonene and one of its oxidation products, carvone, at 297 ± 1 K using in situ Fourier transform infrared (FTIR) spectroscopy. In particular, the adsorption and desorption kinetics of limonene and carvone on two model indoor-relevant surfaces, TiO₂ (a component of paint and self-cleaning surfaces) and SiO₂ (a model for window glass), are compared at low relative humidity as well as at higher relative humidity. Carvone readily partitions to both surfaces with slower desorption kinetics compared to limonene. Under dry conditions, the desorption kinetics of limonene is about 40 times faster than that of carvone from TiO₂, and the desorption kinetics of limonene and carvone from TiO₂ are about two and four times slower than on SiO₂, respectively. Under high relative humidity conditions, the presence of adsorbed water on both TiO₂ and SiO₂ surfaces generally increases the desorption kinetics of limonene yet has no effect on the desorption kinetics of carvone. The K2-SURF kinetics model has been previously applied to limonene adsorption/desorption on SiO₂, and here, it is used to obtain a detailed mechanism for limonene interactions on TiO₂ as a function of relative humidity to explain the observed kinetics. Importantly, the combined experimental results with kinetic modeling and classical and <i>ab initio</i> molecular dynamics simulations provide mechanistic insights into the heterogeneous interactions of volatile organic compounds on indoor-relevant surfaces and implications for indoor air quality.