Fractionating polydisperse polyelectrolytes in packed beds of cellulose fibers

Low molecular weight fractions of polydisperse polyelectrolytes can penetrate the porous structure of cellulose fibers, whereas the high molecular weight fraction cannot. The effects of polydispersity on polyelectrolyte adsorption, especially for polyelectrolytes with a very broad size distribution, often encountered in industry, can be very pronounced. Previously we showed that the adsorption rate constant for polyethylene imine (PEI) adsorbing on pulp fibers, as well as the maximum adsorption capacity, roughly increased by a factor of 4, when the pH was increased from 6 to 10. The increase in adsorption capacity was explained by the decrease in the size of a PEI molecule, by almost a factor of two. The increase in rate was explained by the increase in the low molecular weight fraction of PEI which could penetrate the fibers. To explain the results, this low molecular weight fraction should be four times as large at pH 10, compared to pH 6. Here, we discuss the results of experiments on PEI adsorption in packed beds of cellulose fibers, which confirm this hypothesis.