Determination of the surface energetics of surface modified calcium carbonate using inverse gas chromatography

Stearic acid and several silane surface coating agents, including propyltriethoxysilane, aminopropyltriethoxysilane, and diphenyldimethoxysilane, were applied to uncoated calcium carbonate samples at coating levels ranging from 0 to 2.0% by weight. The surface energy of these coated calcium carbonate samples, as determined by inverse gas chromatography, decreased relative to that of the native mineral with increasing coating level for both the stearic acid and the diphenyldimethoxysilane. This decrease in energy arises from the nonpolar functionality of the coating masking the high energy surface of the mineral. The propyltriethoxysilane generated a decreasing surface energy with increase in coating level up to a theoretical monolayer coating. Above this coating level, an inverted bilayer formed that increased the surface energy of the mineral. Aminopropyltriethoxysilane caused an initial decrease in surface energy relative to that of the native mineral but then retained an approximately constant surface energy at all coating levels. This observation arises from the energetic groups presented at the surface of the mineral regardless of the orientation of the silane.