Adsorption of constitutional isomers of cyclic monoterpenes on hydroxylated silica surfaces

Abstract

We present a study of four monoterpene isomers (limonene, γ-terpinene, terpinolene, and α-pinene) that are prevalent in indoor environments and their interaction with the hydroxylated SiO2 surface, a model for the glass surface, by combining infrared spectroscopy and computational simulations. These isomers are molecularly adsorbed onto SiO2 through π-hydrogen bonds with surface hydroxyl groups. However, experimental results suggest that the strength of interaction of these compounds with the SiO2 surface varies for each isomer, with α-pinene showing the weakest interaction. This observation is supported by molecular dynamics simulations that α-pinene adsorbed on the SiO2 surface has lower free energy of desorption and a lower mass accommodation coefficient compared to other isomers. Additionally, our ab initio molecular dynamics simulations show lower π-hydrogen bonding probabilities for α-pinene compared to the other three constitutional isomers. Importantly, these interactions are most likely present for a range of other systems involving organic compounds and solid surfaces and, thus, provide a thorough framework for comparing the interactions of organic molecules on indoor relevant surfaces.

Heterogeneous Reactions of α-Pinene on Mineral Surfaces: Formation of Organonitrates and α-Pinene Oxidation Products

“…For hematite, the frequencies observed for the absorption bands found for adsorbed α-pinene corresponded closely to its gas-phase vibrational frequencies suggesting that α-pinene is molecularly adsorbed onto the hematite surface ( Figure 1 A). 30 , 48 With increasing α-pinene pressure, there is an increase in intensity of bands at 3605, 3632, and 3640 cm –1 with a concomitant loss of surface hydroxyl groups on hematite at 3670 and 3684 cm –1 . These can be attributed to the formation of π hydrogen bonds between α-pinene and the surface hydroxyl groups.…”

“…These can be attributed to the formation of π hydrogen bonds between α-pinene and the surface hydroxyl groups. 30 The spectral feature around 1650 cm –1 was assigned to the C=C bond stretching mode of adsorbed α-pinene. 47 Upon evacuation overnight, almost all of the adsorbed α-pinene is removed, thus suggesting reversible adsorption of α-pinene on hematite surfaces.…”

“…Most notable is the disappearance of the peak at 3021 cm –1 corresponding to the olefinic C—H stretch, indicating reactions of α-pinene ( Figures 1 A and 2 B) with new peaks appearing at 3074, unsaturated C—H stretch and 1771 and 1662 cm –1 suggesting surface oxidation and the formation of C=C and C=O bonds. 30 , 46 , 49 …”

“…New spectral features produced from the reaction of α-pinene with nitrated hematite and kaolinite primarily are observed in the spectral regions extending from 2800 to 3050 cm –1 and 1450 to 1475 cm –1 . 30 , 49 , 53 , 58 Furthermore, strong absorptions at ∼3500 cm –1 (3507, 3472, 3355 cm –1 ) corresponding to the presence of different oxygenated organic compounds and hydrogen bonding were observed on the hematite surface. Therefore, the reactions of α-pinene with nitrated hematite and kaolinite surfaces suggest the formation of new surface-bound products through heterogeneous reactions in the dark and under dry conditions.…”

“…For instance, adsorption of α-pinene on different types of silica surfaces in the absence of photochemistry showed reversible adsorption. 30 , 31 Additionally, the adsorption of NO 2 and HNO 3 on mineral dust has been extensively investigated. 32 − 35 NO 2 is emitted to the atmosphere primarily via fossil fuel combustion and vehicle exhausts.…”