The meaning of monolayer coverage

Hill has shown that when sufficient adsorption has occurred to cover the surface with exactly one layer of molecules, the fraction of surface, (0o)m not covered by any molecules is dependent on the BET C value and is given by 

It is evident from equation (4.44) that when sufficient adsorption has occurred to form a monolayer there is still always some fraction of surface unoccupied. Indeed, only for C values approaching infinity will 6q approach zero and in such cases the high adsorbate-surface interaction can only result from chemisorption. For nominal C values, say near 100, the fraction of surface unoccupied, when exactly sufficient adsorption has 

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Of the three equations available to interpret adsorption isotherms, the Langmuir and BET equations are based on general models of adsorption and are not derived from concepts of porosity. They could be described as curve-fitting equations. The Langmuir and BET equations, initially, were derived to interpret adsorption isotherms of non-porous carbons. Here, the concept of a surface area has a physical interpretation. Later, the equations were applied to isotherms derived from microporous materials and appeared to work, and values for monolayer coverage could be derived. However, the meaning of monolayer coverage in a microporous solid is an issue which has not been resolved to this day. 

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