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

Partitioning equilibrium between bulk and slit-like pores in dilute solutio n was studied by Monte Carlo (MC) simulations on a cubic lattice in the pre sence of attractive, polymer-pore interaction. Athermal chains with exclude d volume of variable lengths were generated in a direct simulation of the e quilibrium partition coefficient K. The results show that by the variation of the polymer-pore adsorption energy, epsilon, three modes of liquid chrom atography of polymers in good solvents can be reproduced. In contrast to id eal chains, the compensation point where K = 1, relevant to critical chroma tography, was found to be a function of the chain length. The attraction en ergy 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 wit h the adsorption theta point in infinite chains. The counterbalance of ster ic exclusion (the depletion layer) and wall attraction (the enrichment laye r) at the compensation point was confirmed by a flat concentration profile across the pore. The distribution functions of the chain end-to-end distanc es perpendicular and parallel to slit walls were calculated. It was inferre d that in wide pores corresponding to size exclusion chromatography the par titioning proceeds by the coil orientation, and, additionally, that the cri tical chromatography operates in the regime of weakly adsorbed chains chara cterized by a diffuse adsorption layer. (C) 1999 Elsevier Science Ltd. All rights reserved.