UV Resonance Raman Spectra and Molecular Orbital Calculations of Salicylic and Phthalic Acids Complexed to Al3+ in Solution and on Mineral Surfaces

Abstract

Salicylic and phthalic acids have been used to study the complexation of aromatic carboxylic acids with Al(III) in solution and on mineral surfaces. A method for studying aqueous and surface carboxylic acids/metal complexes with UV Raman has been developed. The acids and Al(III) solutions were mixed in a 1:1 ratio at concentrations of 0.01 M. The mixtures were adjusted to pH 2.5 and 4 to be above and below the pKa1 values of the carboxylic acids. The pH values of the acid solutions were based on the pKa values for the different acids to examine the neutral and charged species. The carboxylic acids were also adsorbed onto Al2O3. Comparisons between solution and adsorbed complexes were examined. Various possible types of complexes for the metal/acid complexes were modeled with ab initio molecular orbital/density functional theory calculations. The assignments of Al(III)-acid complexes were based on the best fits between experimental and model vibrational frequencies. Results are compared to previous speciation models based on potentiometric data.