Chemisorption and physisorption of alcohols on iron(III) oxide-terminated surfaces from nonpolar solvents

The adsorption isotherm of alkanols at the haematite|hydrocarbon interface should reflect both chemisorption (chemically bonded fraction) and physisorption (hydrogen bonded fraction). Quartz crystal microbalance (QCM) and X-ray photoelectron spectroscopy (XPS) have been used for characterization of Fe2O3|hydrocarbon interfaces with absorbed alcohol. A range of Fe2O3-terminated surfaces, alkanols, hydrocarbons and temperatures have been investigated. A chemisorption-physisorption Langmuir model (CPL) has been developed to interpret the data. All data show simultaneous physisorption and chemisorption of the alcohol. The CPL analysis reveals variability of the surface chemistry from one sample to another - different fractions of surface sites have been determined for Fe2O3-coated crystals, Fe2O3 powder, stainless steel-coated crystals etc. However, for a given surface, the fraction of chemisorption sites is stable - it does not change with the alcohol, and changes only slightly with the solvent. The physisorbed alcohol molecules assume configuration normal to the surface, while the chemisorbed molecules assume configuration that is either parallel (high homologues) or normal (low homologues). A transition from normal to parallel orientation has been observed at hexyl. The effects from branching of the alcohol are captured well by CPL. The temperature has been shown to alter both the strength of the physisorption and the fraction of available chemisorption sites.