Multilayer adsorption on fractal surfaces: Effect of the adsorbate-adsorbate interactions on the sorptional equilibrium

An analytical isotherm equation that can be applied to water vapor-biopolym er systems, was developed. The model describes the multilayer adsorption on fractal surfaces with energetic interactions adsorbate-adsorbate (measure in terms of free energy) different from that of bulk water. Assuming explic it mathematical functionalities for the variation of the free energy with t he number of adsorbed layers, it is possible to evaluate the effect of the adsorbate-adsorbate interactions on the adsorption capacity of solids of hi gh degree of surface irregularity. For those surfaces with relatively low d egree of irregularity (D values slightly larger than 2), it results that th e free energy variation with the layer number in the multilayer region affe cts considerably the sorption capacity of the adsorbent, even for water act ivities lower than those corresponding to the monolayer moisture content. T his effect becomes less marked as the fractal dimension increases (D approa ching to 3), being relevant for water activities much larger than those cor responding to the monolayer value, only. The isotherm was tested, using pub lished experimental equilibrium data of various biopolymers.