Role of Electrolyte in Liesegang Pattern Formation.

Pattern formation based on the Liesegang phenomenon is considered one of the useful models for gaining a mechanistic understanding of spontaneous spatiotemporal pattern formations in nature. However, for more than a century, the Liesegang phenomenon in chemical systems has been investigated by using electrolytes as both the reaction substrate and aggregation promoter, which has obfuscated the role of the electrolyte. Here, we distinguish the electrolyte (Na2SO4) from the reaction substrates (Ag+ ion and citrate), where Na2SO4 does not participate in the reaction step and acts as an aggregation promoter. The addition of Na2SO4 in Ag+-citrate-type Liesegang rings gave well-resolvable clear bands with a larger spacing coefficient. The observed changes were discussed by using the classical DLVO (Derjaguin-Landau-Verwey-Overbeek) theory, where the role of the electrolyte is to shield the electrostatic repulsive interaction among the reaction products. Furthermore, the numerical simulation of the reaction-diffusion equation with different aggregation thresholds reproduced the salt-dependent change in the spacing coefficient. We expect that an understanding of the exact role of the electrolyte as the aggregation promoter reported here will offer novel insight into how nature spontaneously forms beautiful spatiotemporal patterns.