An electrokinetic characterization of low charge density cross-linked polyacrylamide gels

Hydrogels of polyacrylamide have found wide application in separation science and, more recently, in speciation techniques. These applications ideally require an uncharged, inert polymer matrix to act as a conduit for either diffusion or electrically driven migration. However, the electrical effects due to the structural charge of the gel itself are typically neglected. In order to address this issue, streaming potential and conductivity measurements were carried out on thin films of porous polymer gels of varying fixed charge density. The films consisted of gels of polyacrylamide-co-sodium acrylate, cross-linked with N,N'-methylenebisacrylamide. Conductivity data allowed the estimation of the Donnan potential difference between the bulk gel and electrolyte solution as a function of ionic strength. The data illustrated that even a gel with very modest fixed charge density can give rise to Donnan potentials of tens of millivolts in the submillimolar regime of electrolyte concentration. Streaming potential data were used to calculate zeta potentials using a form of the Smoluchowski equation expanded to account for conductance in the gel layers. The resulting zeta potentials are not consistent with the Donnan potentials obtained from the analysis of the conductivity data and demonstrate the need for a more complete electrokinetic model for soft interfaces.