Adsorption Isotherm and Orientation of Alcohols on Hydrophilic SiO₂ under Ambient Conditions

The adsorption isotherm and orientation of small alcohol molecules ( n -propanol and n -pentanol) on clean, hydrophilic silicon oxide surfaces under ambient conditions were studied with attenuated total reflectance infrared (ATR-IR) spectroscopy and density functional theory (DFT). The ATR-IR study reveals that as the alcohol partial pressure relative to its saturation vapor pressure ( P / P sat ) increases from 0 to ∼10%, the isotherm thickness of n -propanol and n -pentanol increases rapidly to ∼0.3 nm and ∼0.6 nm, respectively. Upon further increase in P / P sat, the isotherm thickness increases only slightly until the condensation occurs at the near saturation vapor condition. The alkyl chains of the alcohol molecules adsorbed at 5% P / P sat appear to be tilted toward the surface, while the hydroxyl groups are oriented toward the surface normal direction. As P / P sat increases further, the molecular orientation of adsorbed molecules drastically changes to a structure that is either random or oriented 40−50° from the surface normal. The DFT calculations for n -pentanol adsorbed on SiO 2 support the molecular structure determined from the ATR-IR experiment. The similar transition in molecular orientation with increase of the adsorbed thickness was observed for longer chain alcohols ( n -decanol and n -octadecanol). The alkyl chain packing into a self-assembly like structure was observed only when the chain length is long enough and the substrate is heated. These observations are discussed in terms of the adsorbate−substrate interactions as well as intermolecular interactions.