Steric exclusion/adsorption compensation in partitioning of polymers into micropores in good solvents

Partitioning equilibrium between bulk and slit-like pores in dilute solution was studied by Monte Carlo (MC) simulations on a cubic lattice in the presence of attractive, polymer-pore interaction. Athermal chains with excluded volume of variable lengths were generated in a direct simulation of the equilibrium partition coefficient K. The results show that by the variation of the polymer-pore adsorption energy, epsilon, three modes of liquid chromatography of polymers in good solvents can be reproduced. In contrast to ideal chains, the compensation point where K = 1, relevant to critical chromatography, was found to be a function of the chain length. The attraction energy in the compensation point epsilon(c) is independent of the slit width and can be identified with the critical energy of adsorption as well as with the adsorption theta point in infinite chains. The counterbalance of steric exclusion (the depletion layer) and wall attraction (the enrichment layer) at the compensation point was confirmed by a flat concentration profile across the pore. The distribution functions of the chain end-to-end distances perpendicular and parallel to slit walls were calculated. It was inferred that in wide pores corresponding to size exclusion chromatography the partitioning proceeds by the coil orientation, and, additionally, that the critical chromatography operates in the regime of weakly adsorbed chains characterized by a diffuse adsorption layer. (C) 1999 Elsevier Science Ltd. All rights reserved.