Partition Coefficients and the Free Energy of Confinement from Simulations of Nonideal Polymer Systems

Abstract

The investigation of the partitioning equilibrium in real polymer systems by Monte Carlo (MC) simulations is reported. In contrast to ideal chain treatments the factors such as flexibility and excluded volume in real chains, the finite concentration of macromolecules φ, and polymer adsorption on pore walls of the attractive strength ε w are considered in computations. The free energy change Δ A / kT due to chain confinement in slitlike structures under nonideal conditions is estimated from simulations. The dependencies of the partition coefficient K on λ, the solute to pore size ratio, relevant to the static partitioning and to size exclusion chromatography (SEC), were computed. In all cases the factors considered enhance the partition coefficient K for a given λ and lead to more gradual changes of K with λ; i.e., the resolving power of the nonideal partitioning decreases relative to a case of ideal chain partitioning. The concentration effect on the steric partitioning coefficient K was found to depend markedly on the solvent quality in dilute solutions. In mixed-mode partitioning by a combination of steric exclusion and adsorption, the function K (λ,ε w ) is computed by a fit of the simulation data. This function properly reproduces for nonideal chains all liquid chromatography regimes, including the critical chromatography. An interpretation of the free energy changes Δ A / kT in nonideal partitioning, based on the loss of conformational and orientational entropies of coils on their confinement, is discussed.