Adsorption of polyacrylic acid on aluminium oxide: DRIFT spectroscopy and ab initio calculations

Diffuse reflectance Fourier transform infrared (DRIFT) spectroscopy was use d to study the adsorption process of the water-soluble polyacrylic acid (PA A) polymer on hydrous delta-Al2O3. Vibrational assignment of PAA, sodium po lyacrylate, (Na-PA) and the PA-oxide surface complex was achieved by compar ison of observed band position and intensity in the DRIFT spectra with wave numbers and intensities from ab initio quantum mechanical calculations. The presented data of polyacrylic acid suggest that IR data calculated ab init io on relatively short oligomers (quantum-mechanical oligomer approach) may provide valuable information regarding the interpretation of polyelectroly te infrared spectra. Batch adsorption experiments were performed to sorb PA A onto the delta-Al2O3 surface. The results obtained from DRIFT studies wer e compared with adsorption isotherm experiments in order to relate the leve l of PAA coverage to the nature of the surface complex. Ab initio molecular orbital calculations on PAA/Al2O3 clusters were used to model possible sur face complexes. Strong correlation were found between theoretical and obser ved DRIFT frequencies of the antisymmetric R-COO- vibration. A number of po ssible configurations of the polyacrylic acid/aluminate surface complex wer e tested via ab initio calculations. These possible configurations included different di-aluminium octahedral Al3+ surface models. Results obtained fr om adsorption isotherm experiments, DRIFT spectra and ab initio calculation s indicate that the carboxylate oxygens bridge an Al3+-octahedral dimer [Al -2(OH)(2)4(H2O)2(OH)] in a ligand-exchange inner sphere complex. (C) 2000 E lsevier Science B.V. All rights reserved.