Nonuniform Surfaces and the Inflection Point in Polylayer Adsorption Isotherms

Abstract

The position of the inflection point in polylayer adsorption isotherms, which has been traditionally used to determine the specific surface of many disperse systems, is analyzed. The inflection point positions versus the molecular parameters of the adsorption system and the degree of surface nonuniformity are considered. The surface heterogeneity is exemplified by the droplet roughness formed on rapid cooling. The vapor–liquid-droplet interface creates the thermal heterogeneity of the surface on which gas-phase molecules are adsorbed after cooling. Conditions for using the inflection point to estimate the specific surface of silica samples during argon and nitrogen adsorption are discussed, along with distinctions between the monolayer capacity found from the isotherm and complete filling of the monolayer itself determined by the degree of filling surface centers.