Analysis of adsorption selectivity in a one‐dimensional model system

The adsorption selectivity, S i, j = (X i /y i )/(x j /y j ), is examined in a 1–D model of a mixture of hard rods adsorbing on a linear substrate. The adsorbed components are characterized by their length, l i , and equilibrium constant, k i . We derive an expression for the bulk‐phase pressure P o , for which there is no separation of components i and j, S, i, j = 1, in terms of these parameters. The selectivity of the smaller component always increases with the bulk pressure, which results in a selectivity reversal if, as is usually the case, the larger component is more strongly adsorbed at low surface coverages. In a binary mixture, when P < P o , the selectivity of the smaller component decreases with increasing bulk‐phase mole fraction, y 1 , while for P > P o , the reverse is true. The effect of varying the molecular parameters on the selectivity is also studied.