dynamics of non-dense sodium silicate - water system studied by molecular dynamics

Abstract

A wide range of applications of amorphous silica makes this mineral an object of various investigations conducted by the scientists and the industry. The procedure of its precipitation is well studied, and we dispose of a range of models describing the succeeding processes. However, the size restrictions of experimental methods limit the knowledge about the very first instants of the process. Therefore, to contribute to filling this gap, our study aimed at elucidating some aspects of silica oligomerization. To access the molecular details of the process, we applied the Molecular Dynamics method with the ReaxFF Reactive Forcefield. We studied sodium silicate solutions with high initial concentration. We took up the topic of the effect of temperature, the size of the simulated system, and the amount of water in the simulations. Our considerations led us to conclusions about the reaction mechanism. We applied this knowledge to propose later a simple model describing the evolution of small species. Finally, we estimated the corresponding kinetic constants, which explained well the succession of events and reasonably agreed with the theory.

Dynamics of oligomerization of silicate solution studied by Molecular Dynamics

“…To validate this assumption, we looked into the morphology of the system to compare it with previously discussed systems with low water content [30]. We noticed that the aggregates, similar to those observed for low water content systems, were present in the solution only at the early beginnings of simulations.…”

“…As expected, increasing the initial concentration resulted in a much more abrupt system evolution, accelerating the reactions by decreasing distances between the reacting molecules. Thus, in the current investigation, we focus on C0=115 g SiO2/L as we did for systems with 02 HS n  [30].…”

“…We discuss below the simulations ran at 1000-2000K in more detail as a means of comparison with the previously described systems with nHS<2 [30].…”

“…We compared a few simulations corresponding to different H2O/Si ratios: from silicate systems with low water content, i.e. nHS=0; 0.5; 1; 2, [30] to aqueous silicate system (nHS=29). To investigate the effect of the H2O/Si ratio, we compared the evolution of species for C0=115 g SiO2/L at 1000, 1500, and 2000K.…”

“…We focused on the systems with intermediate and high silicate concentrations but without undergoing gelation. In the previous paper [30] we described our MD study on the systems containing sodium silicate and silicic acid and low water content ( 02 HS n  ), i.e. the water concentration Cw<4M.…”