Concentration effects in partitioning of macromolecules into pores with attractive walls

The influence of polymer concentration on the partitioning of flexible macromolecules into adsorptive pores was examined by simulations in an open system under good solvent conditions. In dilute solutions, the partition coefficient K(epsilon,phi) is sensitive to small variations of both polymer-pore attraction strength epsilon and concentration phi, whereas in semidilute solutions both influences level off. At weak attraction below critical adsorption energy epsilon(c), the coefficient K increases with phi in a fashion that resembles the weak-to-strong penetration transition found for purely steric exclusion (epsilon = 0) but modified as if the width of pores effectively increased. At moderate attraction above the critical condition the increasing concentration brings about a dramatic drop in the value of K. Additionally, the K(phi) dependences in attractive pores were calculated by an approximate method based on expressions for chemical potentials, and the results were in good agreement with the simulation data. The critical adsorption energy epsilon(c) for excluded volume chains was found to depend on concentration; the compensation point K = 1 was located at about phi = 0.012 irrespective of the pore width. The energy and entropy contributions to the free energy of confinement were calculated and the compensation of their values at the critical condition was demonstrated. Furthermore, it was shown that the effect of concentration in polymer partitioning with adsorptive pores can alternatively be regarded as a confined adsorption and two alternatives of the adsorbed amount Gamma were calculated. The adsorption and depletion concentration profiles phi(I)(x) in the pore in equilibrium with the bulk solution were presented and their variation with concentration phi and attraction epsilon was analyzed.