Adsorption behavior of salicylic, benzoic, and 2-methyl-2-hexenoic acid on alumina: an in situ modulation excitation PM-IRRAS study

Adsorption and desorption of acid molecules (10⁻⁴ M in cyclohexane) with different functional groups, namely salicylic acid (SA), benzoic acid (BA) and 2-methyl-2-hexenoic acid (MHA), on an Al₂O₃ thin film was studied by in situ polarization-modulation infrared reflection–absorption spectroscopy (PM-IRRAS). We used a flow-through PM-IRRAS cell to induce adsorption and desorption processes by alternating acid concentration. Simultaneous but separate detection of liquid-phase and surface species was achieved in a time-resolved manner by PM-IRRAS providing insight into the adsorption–desorption behavior and their kinetics. The reliability and sensitivity of recorded surface and liquid-phase spectra were assured by comparative measurements using attenuated total reflection infrared spectroscopy (ATR-IRS) and density functional theory (DFT) calculations. All three acids irreversibly adsorbed on the basic alumina sites in a bridging mode. Remarkably, reversibly adsorbing surface species were only detected for SA, whereas BA and MHA adsorption were irreversible. To enhance the sensitivity and to obtain kinetic information on adsorption and desorption processes, PM-IRRAS was combined with modulation excitation spectroscopy (MES) applying periodic acid concentration changes. The MES experiments revealed distinct kinetic responses of liquid-phase SA and two different types of surface adsorbed SA species. One of the adsorbed SA was a dimer-like species loosely bound to the alumina surface and the other was probably strongly bound and interacting with other adsorbed SA molecules by hydrogen-bonding. The formation of the two surface species was induced by the presence of the hydroxyl group in SA which most likely enhances the intermolecular interaction via hydrogen-bonding near and on the alumina surface.