Dependence on composition of GC retention on a self-associating solvent/hydrocarbon mixture: Its interpretation by the computation of available sites for interaction with solutes

Diamides, e.g., N-lauroyl-L-valine-tert-butylamide, self-associate in appropriate media through hydrogen bonding. Such selectors form with apolar solvents binary phases, for which the contribution of the polar constituent to the overall retention is the product of the retention on the pure diamide, its weight fraction (x), and a concentration dependent factor (mu(x)). In this paper a model is presented to interpret the effect of the weight fraction on mu(x). The diamide in the apolar solvent forms a mixture of structures of association degree 1 to n, between which n - 1 equilibria can be written. Every associate has at its terminals a site capable of hydrogen bonding to appropriate solutes (e.g., alpha-amino acid derivatives). It is proposed that mu(x) = N-x/N-1, where N-x is the number of available sites for hydrogen bonding at weight fraction x and N-1 is that of the pure diamide (x = 1). On the assumption that the above equilibria have closely similar constants, equations were developed to compute the number of the available sites by both numerical and analytical procedures. A good fit between the computed and experimental mu(x) values was found. The equations developed also permit the estimation of the minimal value of the equilibria constants as well as various other features of the diamide association-dissociation system.