122 The solid-gas interfacePO POPO PO POFigure 5.5 Brunauer's classification of adsorption isotherms (pn ~ saturated vapour
pressure)containing a very fine pore structure (e.g. nitrogen on microporous
carbon at 77 K).
Type II isotherms (e.g. nitrogen on silica gel at 77 K) are
frequently encountered, and represent multilayer physical adsorption
on non-porous solids. They are often referred to as sigmoid
isotherms. For such solids, point B represents the formation of an
adsorbed monolayer. Physical adsorption on microporous solids can
also result in type II isotherms. In this case, point B represents both
monolayer adsorption on the surface as a whole and condensation in
the fine pores. The remainder of the curve represents multilayer
adsorption as for non-porous solids.
Type IV isotherms (e.g. benzene on iron(III) oxide gel at 320 K)
level off near the saturation vapour pressure and are considered to
reflect capillary condensation in porous solids, the effective pore