Separation of C6 cyclic hydrocarbons before the inert peak on microporous silica: a case of negative adsorption

A new equation for the retention of solutes in liquid–solid adsorption chromatography with mixed mobile phases

“…Eqn ( 12) is based on the concept of an adsorbed phase in equilibrium with bulk solution, without any specific assumptions about the nature of the adsorbed phase. To express the activity coefficients rP in terms of the concentrations in the two phases we now introduce the parallel-layer model in which the adsorbed phase is assumed to consist of a single monolayer of molecules arranged parallel to the surface.13 For a binary system of similar-sized molecules the activity coefficients of the two components in the liquid and adsorbed phases are expressed in the form (i = 1,2): (15) (16) where ai is the molecular size (a,/a, = r,,), 4; is the volume fraction of i in the mobile phase and I @ is the corresponding area fraction of i in the adsorbed monolayer; x12 is a Flory-type interaction parameter for 1-2 contacts. The above expression for yf results from the assumption that molecules in the adsorbed monolayer interact not only with their neighbours in the surface layer but also with molecules in the adjacent layer of the liquid phase (Ap and Av denote fractions of nearest neighbours to a given lattice site in the same layer and in each of the adjacent layers parallel to the surface).…”