The Application of the Tóth and Unilan Equations for Testing the Adsorption Properties of Active Carbons Employed in Sample Preparation

Active carbon is generally considered to consist of rigid clusters of microcrystallites, with each microcrystallite made up of a stack of graphene planes (2D, the sp-type of hybridisation of carbon valence orbitals). Free spaces between the planes create a pore network (i.e. pores of different sizes). Strongly activated carbons have a wider distribution of the adsorption space of the micropores as a function of their dimensions which may be accurately described by the superposition of two distributions. A means of mutual access of these types of pore is one of the factors of particular relevance to the kinetics of adsorption/desorption and adsorbate elution. There are a number of different even totally divergent hypotheses on this subject. For example, according to one of them (developed on the basis of a tree-like porous structure), micropores represent branching from mesopores, while another hypothesis views the three types of pores as having direct connection to the surface of the carbon grains [1]. Hence, the investigations of gas-active carbon equilibria, i.e. the adsorption properties are very difficult. The specific physical and chemical properties of activated carbons render these materials to be difficult for testing by chromatographic method, because the process of chromatographying the test substance takes place at the conditions rather difficult for an exact description: on the two-dimensional surface of pore walls, in the three-dimensional space of their pore structure. According to Erickson active carbon has to characterise a very specific adsorption properties to be employed for sample preparation [2]. For example, it is an adsorbent which is employed to separate those polychlorinated biphenyls for which a planar conformation can be employed [2]. In this case a commonly employed characteristics of adsorbents on the base of specific surface area is even irresponsible because some important adsorbents features are completely ommited, e.g. structural and surface heterogeneity, concentration of surface species, and so on. The inverse gas chromatography (IGC) seems to be a suitable and highly sensitive technique for studying the adsorption of very small concentrations of gases or vapours on different adsorbents employed in sample preparation. Therefore, the general equation for the net retention volume, VN , can be written after Rudziński in the following form [3]: