Adsorption of Aliphatic Alcohols, Alkanoic Acids and Acetone on the Silica Surface: Chemical and Steric Factors of Monolayer Formation; Apparent Fractal Dimension

Abstract

In this communication we made a detailed and critical analysis of the factors which may lead to mistakes in measuring the number of molecules in monolayer on the surface of different silica gels, when using aliphatic alcohols, alkanoic acids and acetone as adsorbates. The chemical factor is due to the complex interaction of alcohol (acid) molecules with the surface hydroxy groups of different reactivity. This results in a two-steped form of adsorption isotherms studied by IR technique. The steric factor is connected with the peculiarities of packing adsorbate molecules on the surface of micropores, if one takes into consideration the intrinsic volume of the molecules. As a result, the apparent fractal dimension (D_a >2) of the surface can be introduced, even if the adsorbent surface has no irregularities of the atomic scale size. This effect was shown by simulation of adsorption on the surface of Menger sponge. Based on the analysis mentioned above, an accurate method for measuring monolayer adsorption and surface fractal dimension was suggested.