Adsorption of surfactants on CaCO₃ and its effect on surface free energy

Abstract

A CaCO₃ filler was coated with various mono- and dicarboxylic acids in a dry-blending process. The coated fillers were characterized by various techniques, including dissolution experiments, thermal analysis (differential scanning calorimetry) and inverse gas chromatography (IGC) to determine the amount of surfactant needed to achieve monolayer coverage; IGC proved to be the most convenient, reliable and universal method for this purpose. The dispersion component of the surface tension and the specific interaction potential of the coated filler can be derived from the results, but indirect conclusions can be also drawn from them about the orientation of the molecules on the filler surface and the structure of the layer formed. The coverage of the filler with an organic compound leads to a decrease both in the dispersion component of the surface tension and in the acid–base character of the filler surface. In the case of monocarboxylic acids the amount needed for monolayer coverage depends on the chemical structure of the surfactant. Linear chains are assumed to orientate vertically to the surface, while increasing chain length, branches as well as unsaturations lead to a less regular arrangement of the molecules, to a looser structure. At the same surface coverage, the surface tension of the filler covered by aliphatic monocarboxylic acids is the same for all compounds. The adsorption of dicarboxylic acids is more complicated and the decrease in surface tension is significantly smaller than for monocarboxylic compounds.

Acknowledgements The National Scientific Research Fund of Hungary (grants nos. T 030579 and T 029719), the Institute of Chemistry of the Hungarian Academy of Sciences, as well as the Bolyai Scholarship are acknowledged for their financial support of this study.