Surface treatment of calcite with fatty acids: Structure and properties of the organic monolayer

Thermogravimetric analysis was used to investigate the surface cleanliness of calcite fillers and to determine the optimal amount of fatty acid needed to coat the particles with an organic monolayer. The use of excess surfactant led to the formation of a bilayer as well as to the presence of free acid molecules. Both species could be detected by TGA. Optimal coating of calcite with stearic acid gave a monolayer of calcium stearate bicarbonate in which one acid molecule is attached to every Ca2+ present on the surface. The alkyl chains in the monolayer are vertically oriented to the surface and are restricted in their motion. C-13 NMR and IR spectroscopy revealed that the chain conformation shows a high trans population, which leads to an ordered solidlike phase. Monolayers of saturated fatty acids with long alkyl chains (greater than or equal toC(10)) showed similar behavior, while the lower homologues ( greater than or equal toC(10)) gave monolayers with dynamically disordered phases. The immobilization of oleic acid molecules by tethering them to the calcite surface rendered them liable to thermal polymerization at relatively low temperatures. In other words, calcite can be coated with a monolayer of oleic acid molecules, which is polymerized later to give a polymeric monolayer.