Nanodroplets of Metal Carbonate Conform to Critical Nuclei of Classical Nucleation Theory

Mineral nucleation is a fundamental process of paramount importance to various fields, including geology, biomineralization, and industrial manufacturing. Recently, studies on the biominerals of calcium carbonate and phosphate have revealed the presence of a dense liquid phase prior to the formation of an amorphous solid phase. However, there is vigorous debate on whether the nucleation of calcium carbonate clusters can be adequately described by the classical nucleation theory (CNT) or if one must turn to the non-classical nucleation theory. Here, we show that liquid-liquid phase separation (LLPS) occurs in a non-aqueous solution where magnesium and calcium carbonates are dissolved in ethanol with excess triethylamine. The nanodroplets can be kinetically trapped by triethylamine. The size and number of the nanodroplets, in the vicinity of critical nuclei, can be adequately described within the framework of CNT. We found that bicarbonate-like species formed by triethylamine, CO2, and ethanol molecules are in constant exchange with sizable carbonate clusters. Our results demonstrate that the concepts of critical nuclei and the energy barrier of nucleation are indeed physically relevant. We argue that nanodroplets comprising highly solvated carbonate clusters are the key entities in the nucleation process, which falls within the realm of CNT.