Evaluation of the dispersive component of the surface energy of active carbons as determined by inverse gas chromatography at zero surface coverage.

The methods to obtain the dispersive component of the surface energy (gamma(s)(d)) of active carbons (AC) from inverse gas chromatography (IGC) measurements usually render values much higher than those obtained by other techniques. In this paper this is ascribed to two factors: (i) the high temperatures at that IGC measurements are carried out and (ii) the microporosity of the AC. It is shown that the temperature dependence of the area of the methylene group is an important factor in the high gamma(s)(d) values. Thus, corrections for this dependence should be considered in the calculations. In relation to microporosity, the cooperative effect of the pore walls is also an important factor to be considered in the evaluation of gamma(s)(d). The values gamma(s)(d) obtained after these corrections have their own physical meaning related to ideal flat carbon surfaces. Critical comments are made about some reported relationships between gamma(s)(d), obtained from IGC, and the BET surface area or pore volume of AC as determined from nitrogen adsorption at 77K. These are based on the very different experimental conditions at which nitrogen and IGC measurements are carried out.

Use of specific surface areas in inverse gas chromatography studies at zero surface coverage.

Inverse gas chromatography (IGC) is frequently used to study adsorption processes at zero surface coverage on microporous activated carbons. This allows to determine the thermodynamic adsorption parameters as equilibrium constants, V(S), standard enthalpies of adsorption, Delta HA degrees, standard free energy of adsorption, Delta GA degrees, and so on. Nevertheless, the surface areas of the adsorbents (microporous carbons in this case) are needed for this purpose. The experimental determination of the surface areas of microporous solids is not univocal and the results depend on the adsorbate employed in the measurements, usually N2 or CO2. This means that the thermodynamic parameters obtained by IGC are subjected to a degree of uncertainty depending on whether N2 or CO2 is used to determine the surface area values. The aim of this paper is to discuss which of the two surface area values is more appropriate to be used in IGC measurements at zero surface coverage. Experimental and theoretical considerations are supplied in a thorough discussion which supports that CO2 surface area value is more appropriate. Thus, it is proposed that this should be used instead of the more generally extended nitrogen specific surface area obtained by the BET equation.