TheDonnan potential is successfully isolated from ion pair potentialon a ferrocene-labeledpolyelectrolyte (DNA) monolayer. The isolated Donnan potential shiftsnegatively upon the increase in NaClO4concentration witha slope of â58.8 mV/decade. With the salt concentration grownup to 1 M, the stretched DNA chains in low salt concentration arefound to experience a gradual conformation relaxing process. At saltconcentrations higher than 2 M, Donnan breakdown occurs where onlythe ion pair effect modulates the apparent potential. The apparentformal potential also shows strong dependence on solution pH, whichreveals that the charge density in the polyelectrolyte monolayer playsan important role in the establishment of Donnan equilibrium. [ABSTRACT FROM AUTHOR]