Impact of Adsorbed Water on the Interaction of Limonene with Hydroxylated SiO2: Implications of pi-Hydrogen Bonding for Surfaces in Humid Environments

ES Frank and HY Fan and M Shrestha and S Riahi and DJ Tobias and VH Grassian, JOURNAL OF PHYSICAL CHEMISTRY A, 124, 10592-10599 (2020).

DOI: 10.1021/acs.jpca.0c08600

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 SiO2 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 SiO2 surface. The spectroscopic data provide evidence that adsorbed limonene is not completely displaced by adsorbed water, even at high RH (similar to 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 SiO2. 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 pi-hydrogen bonds, water-water hydrogen bonds, and SiO2 -water hydrogen bonds explains these interactions at the air/adsorbed water/SiO2 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.

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