Formation of Structured Particles Based on Reaction-Diffusion Kinetics in Gel

Abstract

Oscillatory zoning of solid solution of (Ba,Sr)SO₄ was investigated in gelatin. Then, Ba²⁺ and Sr²⁺ diffused into gelatin, and SO₄^2- also diffused from opposite side. Periodic change of molar ratio Ba²⁺/Sr²⁺, which was firstly found out by Putnis et al. (1992) in silica gel, was well reproduced. However, we were able to obtain fine-structured particles which are quite different from those in silica gel. We investigated effects of gelation time and gelatin concentration on the structure and chemical inhomogeneity of particles. Experiments with 16% and 24%-gelatin gave results that core portion of solid solution was composed of BaSO₄, which was different from the results in silica gel. Furthermore, we were able to make various kinds of particles with novel structures by controlling gelation time and gelatin concentration. For example, different structures of particles were successfully formed in separated zone in gelatin. As another, white turbid zones containing particles were clearly separated by a transparent gel. These particles were formed spontaneously without special operations. This was achieved by a reaction-diffusion mechanism. Gelation time and gelatin concentration controlled the diffusion rate mainly, which induced dramatic change in particle growth process in gel. On the basis of these findings, we discussed the mechanism of particle growth in this system.