Investigation of the Nucleation and Initial Growth of Nanosilica Using In Situ Small-Angle X-ray Scattering and Reactive Molecular Dynamics Simulation

Abstract

A method based on the combination of in situ small-angle X-ray scattering (SAXS) and reactive molecular dynamics (MD) simulation is proposed to investigate the nucleation and initial growth of nanosilica. The formation and significant growth of silica nuclei within 1 s can be observed using in situ SAXS in a custom-designed sample cell. Meanwhile, reactive MD simulation confirms that the nucleation and initial growth take place over a very short time period and quantitatively describes the morphology of the formed particles. Both the experimental and simulation methods reveal changes in the particle size and number. A sudden increase in particle size is observed in the experiment, which corresponds to a decrease in particle number, indicating the transformation of the growth mode from ortho-silicic acid attachment to particle coalescence. The simulation snapshots also captures the process of particle coalescence, confirming the existence of this growth mode. The effects of the synthesis conditions, including temperature, sodium silicate concentration, and pH value, are studied and discussed in detail. The results show that the combination of in situ SAXS and reactive MD simulation is a useful method that provides detailed information on the preparation of precipitated silica and its regulatory mechanism.

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“…Importantly, the pH factor strongly enhanced the speed of the nucleation step in the precipitation process. When the pH value decreases in the aqueous medium, the precipitation process gradually decreases, and this mainly affected the particle size growth in the precipitation process . Hence, the precipitating agent’s eminent factors such as atomic radii, solubility, and pH factors extremely affected the nucleation and also the self-assembly step of the precipitation process.…”

“…When the pH value decreases in the aqueous medium, the precipitation process gradually decreases, and this mainly affected the particle size growth in the precipitation process. 43 Hence, the precipitating agent's eminent factors such as atomic radii, solubility, and pH factors extremely affected the nucleation and also the self-assembly step of the precipitation process. Additionally, in the precipitation process, reaction kinetics such as temperature, time, pressure, and annealed temperatures play a very significant role in obtaining the specific morphology.…”