A B. E. T.-like three sorption stage isotherm

Abstract

The statistical B.E.T.-treatment of multimolecular sorption is extended to include three distinct sorption stages, viz.(I) the strongly sorbed monolayer, (II) the following h–1 layers, much less strongly sorbed, and (III) the remaining layers, up to infinity, of ‘pure liquid’ characteristics. The new three sorption stages (t.s.s.) isotherm embodies the classical two-parameter B.E.T. equation [stages 1(h= 1)–III] as well as the three-parameter Guggenheim–Anderson–De Boer (G.A.B.) equation [stages I–II (h=∞)].

The t.s.s. isotherm, which necessarily uses a fourth parameter, retains the mathematical form of the older isotherms with two correction functions determined by the new constant h(extension of stages I and II), functions which become important only at high relative activities of the sorbate. The sorption stage III is evidenced by experimental data in this region : here the G.A.B. isotherm demands that the inverse of the mass sorbed per unit mass of sorbent should be linear in p/p°, while experimentally a deviation downwards is found, which is explained by the t.s.s. isotherm.

The four parameters, determined by versatile, easy to apply, graphical methods, show a certain correlation with the type of interactions characterizing the experimental systems. With longer ranged interactions the extent of the sorption stage II increases from h= 8–15 (gas/solids) to h= 15–20 (water/biopolymers) and to h= 20–25 (water/electrolytes–polyelectrolytes). The applicability range of the t.s.s. isotherm extends up to at least p/p°= 0.95.