By ADSCIENTIS
The specific or polar interactions involve all type of interactions except the dispersive interaction or London interaction by instantaneous dipoles. They can be polar, ionic, magnetic, electric or acid-base. These acid-base interactions are largely predominant according to Mittal and Anderson (1991). Just as for the morphology indexes, the specific interaction parameters (ISP) will be determined with reference to the n-alkane straight line. This is achieved by considering that the gap of free energy change of adsorption between the representative point of the polar probe (ΔGa) and its projection on the alkanes straight line (ΔGad), will correspond to the contribution of the specific interactions (ΔGaSP). So the ISP of a probe is given, in kJ/mole, by the following expression: ISP = ΔGaSP = ΔGa - ΔGad The figure hereafter illustrates the ISP determination method, which allows the easy splitting of the free adsorption enthalpy, into a dispersive term (ΔGad) and a specific term (ISP). Here, the molecular descriptor (χT) is calculated like for the alkanes, by taking into account the geometry of the polar probes [Brendlé and Papirer (1997b)]. As for the alkane probes, the χT parameters of the polar probes correlate well with the literature data concerning their polarizabilities [Balard et al. (2000a)]. Historically, other molecular descriptors have been proposed. So Saint Flour and Papier (1983) retained as parameter the logarithm of the vapour pressure of the probe (log(P0)). Then Schultz and al. (1987) proposed the product of the molecular area of the probe (log(P0)) by the square root of the dispersive component of its superficial pressure (γsd) that is (a.√γsd). The first parameter suffers to be related to the cohesion energy of the liquid, hence to the interaction energy between two identical molecules. The second parameter overcomes partially this fault, in referring to the dispersive component of the liquid. But it supposes also the knowledge of the adsorption area, which cannot exactly be known, because it varies with the conformation of the molecule in the adsorbed state. Despite that, it is still used. These limitations justify the choice of a descriptor (χT), which is directly related to the polarisability of the probe and easily accessible by computation. By diversifying the functionality and the acid-base character of the injected molecule, it becomes possible to have access on the acid-base character of the studied solid.
