Microphase assisted "autocatalysis"

Abstract

The mathematical model developed in this study considered reaction, crystallization, and microphase processes simultaneously to describe autocatalytic microphase action more effectively. Simulations were performed to determine the effect of crystal growth rates, nucleation rates and the initial crystal size distribution on the overall reaction rate enhancement produced by the microphase. The growth kinetics and the crystal size distribution were found to have a significant influence on the heterogeneous reaction rate. These results call into question previous models developed to describe autocatalytic microphases, as none of them include appropriate crystallization kinetic models. In the reaction system of solid calcium citrate and Hquid sulfuric acid, the solid reaction product, calcium sulfate, functions as an autocatals^tic microphase to enhance the rate of reaction and crystallization. Significant reaction rate enhancement was observed experimentally at the onset of calcium sulfate nucleation creating an autocatal5^c effect. Indeed, enhancements approaching a factor of 3 were observed under different initial conditions. The previously proposed microphase mechanism attributes the multiphase reaction rate enhancement to the increase in mass transfer of the sparingly soluble solute from the interface to the bulk reaction phase. This mechanism was e:q)erimentally verified, when increased mass transfer rates without reaction occurring were found to mirror the reaction rate enhancement. Reaction and mass transfer rate enhancement increased with increasing microphase loading until 1.87 g/L microphase (2.5 g/L CaS04 crystals), when the effect of new nuclei is negligible. These experimental results were used to validate the theoretical model.