Unanticipated Stickiness of α-Pinene

The adsorption of α-pinene to solid surfaces is an important primary step during the chemical conversion of this common terpene over mesoporous materials, as well as during the formation of atmospheric aerosols. We provide evidence of tight and loose physisorbed states of α-pinene bound on amorphous SiO₂ as determined by their adsorption entropy, enthalpy, and binding free energies characterized by computational modeling and vibrational sum frequency generation (SFG) spectroscopy. We find that adsorption is partially (40-60%) irreversible over days at 294-342 K and 1 ATM total pressure of helium, which is supported by molecular dynamics (MD) simulations. The distribution of α-pinene orientation remains invariant with temperature and partial pressure of α-pinene. Using the Redlich-Peterson adsorption model in conjunction with a van't Hoff analysis of adsorption isotherms recorded for up to 2.6 Torr α-pinene in 1 ATM total pressure of helium, we obtain ΔS°_ads, ΔH°_ads, and ΔG°_ads values of -57 (±7) J mol^-1 K^-1, -39 (±2) kJ mol^-1, and -22 (±5) kJ mol^-1, respectively, associated with the reversibly bound population of α-pinene. These values are in good agreement with density functional theory (DFT)-corrected force field calculations based on configurational sampling from MD simulations. Our findings are expected to have direct implications on the conversion of terpenes by silica-based catalysts and for the synthesis of secondary organic aerosol (SOA) in atmospheric chambers and flow tubes.