Abstract
Indoor surfaces are often coated with organic compounds yet a molecular understanding of what drives these interactions is poorly understood. Herein, the adsorption and desorption of limonene, an organic compound found in indoor environments, on hydroxylated silica (SiO2) surfaces, used to mimic indoor glass surfaces, is investigated by combining vibrational spectroscopy, atomistic computer simulations and kinetic modeling. Infrared spectroscopy shows the interaction involves hydrogen-bonding between limonene and surface O-H groups. Atomistic molecular dynamics (MD) simulations confirm the existence of π-hydrogen bonding interactions, with one or two hydrogen bonds between the silica O-H groups and the carbon-carbon double bonds, roughly one third of the time. The concentration and temperature dependent adsorption/desorption kinetics as measured by infrared spectroscopy were reproduced with a kinetic model, yielding the adsorption enthalpy of ∼55 kJ mol-1, which is consistent with the value derived from the MD simulations. Importantly, this integrated experimental, theoretical and kinetic modeling study constitutes a conceptual framework for understanding the interaction of organic compounds with indoor relevant surfaces and thus provides important insights into our understanding of indoor air chemistry and indoor air quality.
Highlights
Organic compounds are highly prevalent in the indoor environment.[1]
Among the various surface chemical processes occurring on indoor surfaces, here we have focused our attention on elucidating the fundamental interactions, a detailed molecular picture, and the kinetics of the adsorption/desorption process of a prevalent indoor gas, D-limonene, on hydroxylated SiO2, a model for glass surfaces which are prominent in indoors
SiO2 was chosen as the model indoor surface in this study since it can represent the chemistry occurring on glass surfaces, which are abundant in the indoor environment
Summary
Among the various surface chemical processes occurring on indoor surfaces, here we have focused our attention on elucidating the fundamental interactions, a detailed molecular picture, and the kinetics of the adsorption/desorption process of a prevalent indoor gas, D-limonene, on hydroxylated SiO2, a model for glass surfaces which are prominent in indoors. The SiO2 sample used in our study is in powder form, having a high surface area, to allow us to obtain detailed information on surface adsorption mechanisms To our knowledge, this is the rst investigation of the interaction type, strength, and kinetics of limonene adsorption/ desorption on SiO2 surface by integrating surface adsorption measurements obtained from vibrational spectroscopy with theoretical calculations and kinetic modeling. Prior to and a er the exposure of limonene, the single-beam spectra of surface- and gas- phases (300 scans) were acquired at 296 K, as well as from 298 K to 308 K for temperature dependence experiments. During and following exposure to limonene, single-beam spectra (10 scans) of the respective SiO2 and gas phase surfaces were automatically acquired using a Macro (OMNIC Macro Basics so ware) to study the kinetics of limonene adsorption until equilibrium was reached. Absorption bands attributed to gas phase limonene (measured through the blank half of the tungsten grid) were subtracted from the surface absorbance spectra to obtain the FTIR spectra of the adsorbed particle species loaded on the tungsten grid
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