Adsorption isotherm and conformations of polymer chains at a liquid-solid interface

A thermodynamic approach to the derivation of isotherms of adsorption of polymer chains at liquid solid interfaces is proposed. It is based on the models of a loose and a dense adsorption layer and takes into account the Helmholtz energy and its change during adsorption. For a loose layer formed by virtually undistorted Flory coils, the adsorption isotherm has a typical Langmuir-type shape. For a dense layer composed of compressed and entangled polymer chains, the adsorption isotherm is described by a modified Langmuir equation that includes the macromolecule concentration in the solution in a fractional power. The transformation from one type of isotherm to another can be considered as a first-order phase transition. The critical parameters of this transition are evaluated from experimental isotherms of adsorption of styrene-maleic anhydride statistical copolymers.