Nonideality of polymer solutions in the pore and concentration-dependent partitioning.

This Note rationalizes recently measured partitioning of polyethylene glycol, PEG-3400, from bathing solutions of varying polymer concentrations to -hemolysin channels. To explain the concentration dependence of the partition coefficient, we recognize that nonideality of polymer solution in the pore is weak because the overlap volume fraction of the polymer in the pore is higher than that in the bulk. The reason is that polymer molecules in the pore form cigars with higher intramolecular monomer density. Because the radius of gyration of PEG-3400 2.5 nm in the bathing solution is significantly greater than the radius of the pore 1 nm , polymers do not enter the -hemolysin channel at low concentrations. At high concentrations repulsion nonideality pushes the polymer into the channel with a sharp transition between exclusion and entry Fig. 1 . The partition coefficient was determined from purely electrical measurements of the effect of PEG on channel ionic conductance. This procedure seems to be more straightforward than that of the “apparent diffusion coefficient” of polymers or tracers measured by dynamic light scattering from porous glass beads. Analysis showed that the observed concentration dependence of the partition coefficient cannot be explained using an approach that assumes nonideality of the polymer solution in the pore to be identical to nonideality in the bathing solution: measured partitioning is a much sharper function of polymer concentration than identical nonideality predicts. The partition coefficient p is defined as a ratio of the polymer volume fractions in the pore p and in the bulk b, p= p / b. To find p one needs to know the polymer chemical potentials in the bulk and in the pore. The chemical potentials can be written as