Adsorption of ethylene glycol vapor on alpha-Al2O3(0001) and amorphous SiO2 surfaces: Observation of molecular orientation and surface hydroxyl groups as sorption sites

Vapor adsorption is an important process influencing the migration and the fates of many organic pollutants in the environment. In this study, adsorption of ethylene glycol (EG) vapor onto single crystal alpha-Al2O3 (0001) and fused SiO2 (amorphous) surfaces was studied with sum frequency generation spectroscopy, a well-suited surface specific technique for probing interfacial phenomena at the molecular scale. Air-aqueous EG solutions were also investigated to compare to the adsorption at the air-solid interface in the presence of water vapor. The gauche conformer of EG molecules dominates the air-aqueous EG solution interface, and EG molecules act as hydrogen acceptors at the air-liquid interface. Water and surface hydrophilic/ hydrophobic properties play important roles for the adsorption of EG onto silica and alumina surfaces. The adsorbed EG molecules interact in different ways at the two different oxide surfaces. EG molecules weakly physisorb onto the alpha-Al2O3 (0001) surface by forming relatively weak hydrogen bonds with surface water molecules. On the silica surface, the suppression of the silanol OH stretching peak indicates that EG molecules form hydrogen bonds with silanol OH groups.