SorptionIsothermen

Sorption isotherms are graphs and models that describe how much sorbate is taken up by a sorbent at equilibrium as a function of the driving force, such as concentration or partial pressure, at a fixed temperature. The term sorption covers both adsorption on a surface and absorption into a phase, and isotherms are used to characterize capacity, affinity, and energetics of the process. They are widely used to compare sorbents and to design separation, purification, and remediation systems.

Several mathematical models are used to fit sorption isotherm data. The Langmuir isotherm describes monolayer adsorption on a homogeneous surface with finite capacity, often written in a form involving maximum uptake and an affinity constant. The Freundlich isotherm is empirical and describes heterogeneous surfaces, with uptake increasing with concentration in a non-saturating manner. The BET model extends Langmuir to multilayer adsorption and is commonly applied to estimate the specific surface area of porous solids. Other models include Temkin, Dubinin–Radushkevich for microporous adsorption, and Redlich–Peterson, which blends Langmuir and Freundlich characteristics.

Measurement approaches include volumetric, gravimetric, and microbalance techniques. Data are plotted as isotherm curves at a given temperature, typically as amount sorbed versus equilibrium concentration or pressure. Isotherms can be classified by shape, and for gas adsorption, IUPAC recognizes types that relate to surface area, porosity, and energy heterogeneity. Desorption isotherms may exhibit hysteresis due to pore structure or condensation effects, reflecting differences between adsorption and desorption.

Applications span environmental engineering (removal of contaminants from water or air), catalysis (characterizing active sites), gas capture (such as CO2), chromatographic separations, controlled release in pharmaceuticals, and food packaging. Understanding sorption isotherms aids material selection and process design.