Chemical reactions and electric field induced ion migration in agarose gels

Gel reactors have been used to investigate numerous phenomena involving spatio-temporal localization of reactions because gels provide a convection-free medium where reactions are controlled by the diffusion process. In this dissertation, I studied diffusion-limited precipitation reactions in a gel. Effects of varying reactant fluxes and different geometries were examined. I also examined the question of what happens to the agarose gel structure when it is subjected to DC electric fields. For the experiments reported in this dissertation video photography and small angle light scattering methods were used.; The profile and kinetics of the precipitation reaction of Na2HPO 4 and CaCl2 diffusing from opposite sides of an agarose gel showed a crossover between a spatially confined precipitate and spatially separate bands (known as Liesegang bands) depending on the reactant flux J. For the case of equal flux, the spatially confined precipitate grows linearly in intensity. The width of the precipitate (o) scales as o ∼ (J/D)-alpha with alpha = 0.40 +/- 0.02, where D is the diffusion coefficient. For the case of unequal flux, the exponential intensity increase of the Liesegang bands leads to a saturation of the intensity of the broad central band which widens as a function of time. The time evolution of the first Liesegang band's profile provides a direct confirmation of the supersaturation model. Two-dimensional precipitation patterns were examined by varying the geometry of the gel reactor.; In order to address the question of how the structure of agarose gels used in electrophoresis is influenced by the applied electric field, I have performed a light scattering study of agarose gels in an electric field. I designed an apparatus to simultaneously photograph the entire gel sample as well as measure 2-dimensional light scattering patterns from the gel. The results showed that at low fields, 3--10 V/cm, the gel shrinks from the cathode side. This shrinking begins when the H+ and OH - ion-fronts migrating in opposite directions meet and neutralize each other. The resulting pH changes on either side of the neutral zone create differences in osmotic pressure causing the gel to shrink. The increased density of gel fiber bundles in the direction of the electric field leads to a highly anisotropic light scattering pattern. The characteristic shape of the 2-dimensional pattern suggests that fiber bundles are predominantly oriented perpendicular to the electric field direction.