Charge density distribution at interfaces between polyelectrolyte layers and aqueous solutions—Experimental access and limitations of traditional electrokinetics

Charge formation within surface-confined polyelectrolyte layers (PL)—including biopolymer films—is of highest importance in the application of biomedical materials in demanding products. However, due to the lack of adequate analytical tools the impact of electrical charging on the intra- and intermolecular structure of surface-confined PL so far remained poorly understood. The traditional characterization of hard surfaces by electrokinetic (zeta potential) measurements cannot be applied for the characterization of the internal structure of thick PL, although the traditional electrokinetics remains important for characterizing PL/electrolyte interfaces. Systematic investigations revealed that surface conductivity (SC) measurements provide a unique opportunity for the characterization of PL, including the determination of Donnan and surface potentials, maximal PL charge at complete dissociation, fractional PL charge, counterion condensation, and even PL thickness. This was achieved through advanced electrokinetic measurements in microchannels and an extension of the related theoretical modeling. A serious restriction in modeling as well as in the quantitative interpretation of experimental data is the assumption of a uniform segment distribution within the PL while the gradual decay of the segment concentration with the distance to the solid surface is rather abundant. Recently, we showed that the concept of local Donnan potentials holds true for cases of a nonuniform segment concentration if the characteristic length h of the segment concentration decay exceeds the Debye length κ m −1 of the PL. We demonstrate that the incorporation of the concept of local Donnan potentials into the SC theory permits us to derive an analytical equation for the fractional charge of PL and for the SC at nonuniform segment distribution. In addition, the measurement of the fractional PL charge can provide information about layer thickness, the length of the segment concentration decay, and concentration values near the solid surface and near the PL/electrolyte interface. A concept for the characterization of charging and structural features of nonuniform polyelectrolyte layers is presented and discussed.