Adsorption isotherm and orientation of alcohols on hydrophilic SiO ₂ 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 adsórbate-substrate interactions as well as intermolecular interactions.