Effect of Preheating Treatment on the Measured Heats of Adsorption of Organic Probes on Clays with Different Surface Compositions

The effect of column preconditioning temperature (T-cp) on the measured heats of adsorption of selected aliphatic and aromatic compounds for kaolinite and illite, two clay minerals commonly encountered in the oil sands industry, was studied using the technique of inverse gas chromatography (IGC). T-cp was varied from 100 to 340 degrees C. Our results showed that, as T-cp was increased from 100 to 170 degrees C, the heats of adsorption (affinities) of the selected aliphatic solutes for kaolinite increased slightly, but those for illite stayed nearly the same. At higher T-cp values (170-230 degrees C), solute adsorption exhibited essentially no dependence on T-cp for both kaolinite and illite. This observation is attributed to the decreasing hydrophilicity for kaolinite from 100 to 170 degrees C, but not for kaolinite from 170 to 230 degrees C or for illite from 100 to 230 degrees C. Higher T-cp values (230-340 degrees C) were used for the two aromatic solutes (benzene and toluene), and it was found that their heats of adsorption for illite were significantly reduced (50%) whereas those for kaolinite were insensitive to T-cp. It is believed that this observation can be attributed to the loss of surface water as a result of increasing temperature. Here, departure of surface water would decrease the number of Lewis acid sites on the tetrahedral surfaces, thereby reducing the attractive interaction between the surfaces and the pi bonds of the aromatic solutes at elevated T-cp. Because illite contains only tetrahedral surfaces, the measured heats of adsorption of benzene and toluene decreased with increasing temperature. However, in the case of kaolinite, which contains 1:1 tetrahedral and octahedral surfaces, this effect was offset by the increasing attractive interaction between the exposed hydroxyl moieties on the octahedral surfaces and the pi bonds of the aromatic probes. As demonstrated in this work, the measured heat of adsorption depends on T-cp. Therefore, care must be taken when comparing solute affinity for various types of minerals based on IGC measurements.