Electrokinetics of Diffuse Soft Interfaces. III. Interpretation of Data on the Polyacrylamide/Water Interface

Abstract

Streaming potential measurements were carried out on a family of polyacrylamide-co-sodium acrylate gels cross-linked with N,N'-methylenebisacrylamide in a homemade electrokinetic cell. Measurements of the ionic conductivity within thin films of these gels allowed the equilibrium Donnan potential difference between the bulk gel and the bulk electrolyte environments to be estimated at various ionic strengths. The resulting Donnan potential data were combined with the directly measurable streaming potential data and used to evaluate the diffuse soft interface model of electrokinetics (Langmuir 2004, 20, 10324). The model introduces the concept of a gradual decay of polymer density and fixed charge density within a narrow inter-phase at the gel/solution interface. The nature of the decay at the interface has a dramatic effect on the magnitude of the streaming potential as predicted by the diffuse soft interface model. In this investigation, the gradual decay of polymer density within the inter-phase is described with a hyperbolic tangent function. For the gels mentioned, the characteristic length scale of the decay, alpha, as calculated from the fit to the model, increases significantly with decreasing ionic strength, suggesting an osmotically driven swelling of the loosely cross-linked polymer chains at the interface. The experimental data and the results of the fitting are discussed in terms of the physical picture of the interface and compared to fitting results for a model which assumes a simple step function at the gel-solution interface.

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

“…(18). The theory was recently successfully applied for the characterization of Na-acrylate gels [33].…”

“…Recently there was much interest in both the experimental investigation and the modeling of the electrokinetics at soft surfaces [17][18][19].…”

Faradaic double layer depolarization in electrokinetics: Onsager relations and substrate limitations

“…As such, L 23 again stands for an integral operator over the space, that is the position y under the thin-double layer assumption [23]. In cases where the latter assumption does not hold, the ion distribution within and outside the gel layer is significantly depending on the lateral field distribution.…”

“…Such cases constitute the direct pendant of that of electrokinetics of metallic surfaces depolarized by bipolar (positiondependent) faradaic reactions. In view of recent developments on the electrokinetics of soft systems such as permeable gel layers [22][23][24][25][26][27], precision should be added to the physical relevancy of the electrokinetic potential ζ in Eq. (14).…”

“…Not surprisingly, this analogy is also found in the methodology for detecting the impact of EDL depolarization on the electrokinetic properties of the gel layers. The key experiment to be performed is the measuring of streaming potential values as a function of ionic strength for gel layers with various space charge densities (charge variations may be obtained by appropriate gel synthesis following controlled polymerization routes, as e.g., described in [23]). For a given flow profile within the gel layer, the ionic conduction within the bulk gel phase is reflected in a decrease (in magnitude) of the streaming potential at a given ionic strength with increasing space charge density and concomitant increase in counterion concentration.…”