ATR-FTIR spectroscopy was used to investigate in situ the adsorption of simple compounds, such as dicarboxylic acids, hydroxybenzoic acids and resorcinol, onto the clay mineral kaolinite. Identification of the absorption bands was based on changes which occur in the spectra of the compounds on changing the pH value of the aqueous solution. The band shifts which occur in the spectra of the dicarboxylic acids as a result of the interaction with the kaolinite surface suggest inner-sphere complexes, where either one or both oxygen atoms of the carboxylic group are bound to the kaolinite surface. A comparison of the spectra of the dicarboxylic acids showed that small structural differences had a great influence on the kind of surface complex formed. In the case of the hydroxybenzoic acids and resorcinol, the band shifts were too small to make any statements regarding possible surface complexes.
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