Variation in Properties of Pre-Nucleation Calcium Carbonate Clusters Induced by Aggregation: A Molecular Dynamics Study

Bing-Bing Wang,Yi Xiao,Zhi-Ming Xu

doi:10.3390/cryst11020102

Abstract

Numerous studies have speculated calcium carbonate (CaCO3) nucleation induced by pre-nucleation clusters (PNCs) aggregation. However, it is challenging for experiments to directly obtain the relationship between PNCs aggregation and nucleation. Herein, we employ molecular dynamics simulations to explore the variation during PNCs aggregation, which can describe the beginning stage of CaCO3 nucleation induced by PNCs aggregation in supersaturated solutions. The results reveal that the formation of CaCO3 nucleus consists of PNCs spontaneous growth, PNCs solubility equilibrium, and aggregation of PNCs inducing nucleation. The PNCs aggregation, accompanied by the variation in the configuration and stability of CaCO3 aggregate, breaks the solubility equilibrium of PNCs and creates conditions for the formation of the more stable nucleus. Besides, the CaCO3 nucleus with the higher coordination number and the lower hydration number form when decreasing the CaCO3 concentration or increasing the temperature. This work not only sheds light on the formation of the CaCO3 nucleus but also contributes to the explanation for CaCO3 polymorphism.

Highlights

pre-nucleation clusters (PNCs) with a hydrodynamic diameter of ~2 nm were widely detected by analytical ultracentrifugation (AUC) in the supersaturated stage near CaCO3 nucleation, while larger species with hydrodynamic diameter of ~4 nm were detected in the early postnucleation stage [11]
We present the results of an molecular dynamics (MD) study of the spontaneous growth and aggregation of PNCs in supersaturated CaCO3 solutions to explore the beginning stage of CaCO3 nucleation
It is worth noting that three PNCs are near each other at t = 4.6 ns and the aggregation of PNCs occurs

Summary

Introduction

Calcium carbonate (CaCO3 ) is ubiquitous in marine sediments and biominerals [1,2]. it is a common scale component from hard water resulting in practical issues [3]. PNCs with a hydrodynamic diameter of ~2 nm were widely detected by analytical ultracentrifugation (AUC) in the supersaturated stage near CaCO3 nucleation, while larger species with hydrodynamic diameter of ~4 nm were detected in the early postnucleation stage [11] This mutation in cluster size during nucleation supports the hypothesis of non-classical nucleation. The Ca-C coordination number of PNCs, a precursor of nucleation, generated in supersaturated CaCO3 solutions at ambient temperature is close to two, which is similar to the chain-like model [25]. While biogenic species of ACC, an intermediate product of the crystallization, have coordination numbers commensurate with crystalline species [17] These PNCs are stable and remain in solubility equilibrium in the pre-nucleation stage instead of dissolving spontaneously. The influences of CaCO3 concentration and temperature on PNCs aggregation are discussed to gain a better understanding of CaCO3 polymorphism

Simulation Section

Evolution of Nucleation

Effect

Effect of Temperature

Conclusions