Composite sorption of pure vapors on polyolefins, carbon, and minerals

A general composite sorption isotherm is developed to predict experimental isotherms of the types I, II, and III. The isotherm, based on heterogeneous chemical thermodynamic equilibrium considerations, is used to represent single vapor sorption isotherm data at high concentrations. Sorbents investigated include polybutylene, low-density polyethylene, isopolypropylene, polytetrafluo- roethylene, polyvinyl chloride, granular activated carbon, graphon, cabosil, Iceland spar, and quartz. Sorbates include ethane, n-butane, n-pentane, n-hexane, n-octane, trichloroethylene, tetrachloroethylene, methyl bromide, dichloromethane, vinyl chloride, benzene, ethanol, and ethyl ethanoate. The isotherm has two terms with two parameters each. One term represents the absorption of individual vapor molecules, or monolayer adsorption. The second term represents the absorption of vapor molecule clusters, or multilayer adsorption. Contributions of terms 1 and 2 to overall sorption are generally predominant at relatively low and high sorbate concentrations, respectively. In contrast with the classic concept of infinite sorption for the BET isotherm, composite isotherm term 2 sorption capacity is finite.