A theoretical model is proposed to account for the experimental results in part II. The basic assumption is that the total surface charge of the monolayer-formed DP fibers stems from (i) fixed ionized groups on the surface and (ii) adsorbed ions on the surface from surrounding solutions. The fixed ionized groups are the carboxyl groups of the fibers and quaternary ammonium groups of the polymer. The proposed model correlates the amount of adsorbed polymer with the zeta potential of the monolayer-formed DP fibers. Calculations made on the proposed model using the experimental data in part II suggest that a comparison of the charge calculated from zeta potential with that from the amount of adsorbed polymer yields ca. 0.04% of the fixed ionizable groups as effective charged sites, i.e., electrokinetically detectable. This finding is due to the binding of counterions to the fixed ionized sites on the surface. Zeta potentials of the DP with adsorbed monolayers largely stem from the fixed ionized groups with only a minor contribution from the adsorbed ions. The zeta potentials are nearly proportional to the difference between the number of cationic and anionic groups on the surface.