Institute of Biochemistry & Biophysics, Tehran university, P.O. Box 13145-1384, Tehran, I. R. IRAN
This articles is comprised of two parts: a) an experimental investigation on the behavior of an acrylic gel under DC electric field and b) a physico - mathematical description.a) Gel rods made of poly [acrylamide-co-bisacrylamide] were partially hydrolyzed to different extents at pH 12 by teteramethylethylene diamine. Equlibrium properties of the resulting gels rods (water content, number of carboxyl groups and pore size) were determined. Gel rods were then placed in water parallel to planar platinum electrodes. Under the field strengths geater than 2 V/cm the gels gradually bend towards cathode and after reaching a maximum, they traverse a smooth reverse deformation, finally bending towards anode. The speed and extent of these deformations depend on the electric field strergth; length, diameter, charge density (extent of hydrolysis) of the gel rods; temperature, and the pH of the bathing medium. In all cases the bending behavior follows the relation for the tree – point mechanical bending of solid rods. Anodic swelling and bending towards cathode is attributed to the difference in the osmotic pressure between the anodic and the cathodic sides of the gel, while the reverse deformation and bending towards anode is assigned to the migration of H+ ions from the anolyte into the gel and neutralization of COO¯ groups.b) Theoretical analysis: These attributes are quantitatively represented by a proper theoretical formulation based on Donnan and Flory-Huggins theories. The relation obtained for the osmotic pressure within the gels, in the absence of an electric field, is modified to include the ionic flux in response to concentration and electric field gradients. Considering the ionization of water and the network carboxyl groups, together with the principle of charge neutrality, and assuming Donnan equilibrium at the gel boundaries under the applied electric fields, the equations for ionic fluxes are derived and solved by Laplace transform. It is found that the concentration of cations decreases in the anodic side of the gel while it increases in the cathodic side, leading to an osmotic swelling gradient in the gel causing it to bend.