Partition coefficients and the free energy of confinement from simulationsof nonideal polymer systems

The investigation of the partitioning equilibrium in real polymer systems b y Monte Carlo (MC) simulations is reported. In contrast to ideal chain trea tments the factors such as flexibility and excluded volume in real chains, the finite concentration of macromolecules phi, and polymer adsorption on p ore walls of the attractive strength epsilon (w) are considered in computat ions. The free energy change DeltaA/kT due to chain confinement in slitlike structures under nonideal conditions is estimated from simulations. The de pendencies of the partition coefficient K on lambda, the solute to pore siz e ratio, relevant to the static partitioning and to size exclusion chromato graphy (SEC), were computed. In all cases the factors considered enhance th e partition coefficient K for a given lambda and lead to more gradual chang es of K with lambda; i.e., the resolving power of the nonideal partitioning decreases relative to a case of ideal chain partitioning. The concentratio n effect on the steric partitioning coefficient K was found to depend marke dly on the solvent quality in dilute solutions. In mixed-mode partitioning by a combination of steric exclusion and adsorption, the function K(lambda, epsilon (w)) is computed by a fit of the simulation data. This function pro perly reproduces for nonideal chains all liquid chromatography regimes, inc luding the critical chromatography. An interpreta tion of the free energy c hanges DeltaA/kT in nonideal partitioning, based on the loss of conformatio nal and orientational entropies of coils on their confinement, is discussed .