Surface free energy (γsd) of active carbons determined by inverse gas chromatography:: influences of the origin of precursors, the burn off level and the chemical modification

The dispersive component of the surface free energies (y(s)(d)) of commercial active carbons (AC) from various origins were determined by inverse gas chromatography at infinite dilution (IGC-ID). This method discriminates clearly the AC produced from wood (and activated/carbonised with phosphoric acid) and those from coconut-shell (carbonised and steam activated at 850 degreesC). The values for the last AC (from coconut) are twice higher than the values for AC of wood origin. The structure and shape of the pores have to be considered to explain these values. It seems that for AC, IGC-ID globally characterises the most energetic micropores. This can be observed, in this work, by two ways: (i) washing of commercial AC (chemically activated) allows to liberate a part of the micropores blocked by soluble phosphate and consequently increases the y(s)(d) value; (ii) modifying coconuts AC by chemical treatment (formamide) results in a strong decrease of both microporosity and y(s)(d) value. On the contrary, thermal activation of the modified AC increases at the same time the microporosity and the surface free energy. Finally, we demonstrate that the IGC method is also an useful tool to monitor in situ the evolutions of the surface properties of carbonaceous materials. (C) 2001 Elsevier Science B.V. All rights reserved.