Dissolution kinetics of synthetic amorphous silica in biological-like media and its theoretical description

Different types of synthetic amorphous silica have been dissolved under biological-like conditions in a buffer system. The buffer solution named TRIS buffer provides a system with comparable pH value, buffer capacity, and osmotic pressure as it can be found in extracellular fluid. Additionally, the phospholipid L-alpha-dipalmitoylphosphatidylcholine (DPPC) has been used in several dissolution experiments to simulate conditions which are found in extracellular lung fluid. The molybdic acid method (used as standard procedure) and partly ICP-OES have been applied to determine the total amount of silica dissolved. The dissolution of nanodisperse silica shows a strong size effect, which leads to a significant higher silica concentration when compared with the bulk phase. At longer dissolution time a slight decrease of solubility can be observed. The Gibbs free energy of cluster formation is calculated numerically for the dissolution process to predict the temporal development of silica concentration. Theoretical curves are adapted by nonlinear regression of a free parameter. This parameter can be understood as the rate constant of the dissolution process.