On the critical use of molar absorption coefficients for adsorbed species: The methanol/silica system

The ambient temperature adsorption of methanol vapour on two preparations of non-porous silica (Aerosil 50 and Aerosil 300) activated at two different temperatures (∼300 and 1073 K) has been studied by gas-volumetric and in situ transmission FTIR spectroscopic methods, in order to check the validity of equations of the Beer-Lambert type in the case of heterogeneous systems. The results indicate that, for both physically adsorbed and chemisorbed methanol, the calculated integral molar absorption coefficients are of the right order of magnitude (as compared with those typical of methanol in homogeneous phase), but the actual values vary much from one sample to another, depending on several factors. Among the factors, very important turn out to be: the specific surface area of the adsorbent, the sample thickness, the surface dehydration level of the adsorbing system and, most probably, the scattering properties of the adsorbent. In particular, the first two factors of variability imply that, when dealing with the spectral response of adsorbed species, an adequate normalization of the observed band intensities against sample surface area as well as against sample weight is difficult to achieve. The conclusion is that: (i) in the case of either physically adsorbed or chemisorbed species, the concept of molar absorption coefficient is virtually impossible to apply in general terms; (ii) the values calculated as "apparent" absorption coefficients cannot be transferred from one system to another, even if the ε figures calculated for adsorbed species do fall in the range of ε values determined for homogeneous systems.