By ADSCIENTIS
This determination is performed with IGC-ID, i.e. Inverse Gas Chromatography at Infinite Dilution, and was first proposed by Dorris and Gray (1979). For that purpose, they measured the retention time (t_N) of a series of n-alkane molecular probes. With respect to the infinite dilution conditions, the measured retention times are directly related to their free energy change of adsorption (ΔG_a). With help of this linear behaviour, a free energy increment by methylene group (ΔG_a^CH2) is determined. It corresponds to the slope of the n-alkanes line. The n-alkane probes are non-polar and only able to exchange non-specific interactions with the surface (London Forces). Therefore, the work of adhesion of the methylene unit to a surface W_a^CH2, as described by Fowkes (1967), can be written. Since the adsorption and the work of adhesion are reversible, we can bring them closer. It is worth to underline that this approach supposes that the n-alkane probes adsorb flat on the surface and that the surface is homogeneous. The table below reports some range of values of the γ_s^d values measured on different types of solids: Polymer 25 to 60; Cellulosis >35 to 55; Calcium carbonate 33 to 110; Talcs 110 to 200; Carbon blacks 80 to 550. We can see that for a given type of solid, the dispersive component can vary in a large range. The dispersive interactions are strongly related to the polarizability of the surface. In the case of carbon blacks, a large variability of the measured γ_s^d values is observed. Especially, the highest values are measured on conducting carbon blacks. This can be explained taking into account the fact that, for these solids, the electrons are much mobile, i.e. more polarisable. But this explanation does not match for the calcium carbonates, neither for talc. Indeed, both solids are insulators and also show high variability of their γ_s^d values. One question arises: Are assumptions made for the computation of the γ_s^d value respected? More precisely: is the surface of the examined solid smooth and homogeneous? We will see that the IGC-ID is also able to give access to information about the surface morphology at the molecular probe scale.
