Monitoring the Nucleation and Growth of Nanoscale CaCO at the Oil-Water Interface.

Interfaces can actively control the nucleation kinetics, orientations, and polymorphs of calcium carbonate (CaCO). Prior studies have revealed that CaCO formation can be affected by the interplay between chemical functional moieties on solid-liquid or air-liquid interfaces as well as CaCO's precursors and facets. Yet little is known about the roles of a liquid-liquid interface, specifically an oil-liquid interface, in directing CaCO mineralization which are common in natural and engineered systems. Here, by using X-ray scattering techniques to locate a meniscus formed between water and a representative oil, isooctane, we successfully monitored CaCO formation at the pliable isooctane-water interface and systematically investigated the pivotal roles of the interface in the formation of CaCO (i.e., particle size, its spatial distribution with respect to the interface, and its mineral phase). Different from bulk solution, ∼5 nm CaCO nanoparticles form at the isooctane-water interface. They stably exist for a long time (36 h), which can result from interface-stabilized dehydrated prenucleation clusters of CaCO. There is a clear tendency for enhanced amounts and faster crystallization of CaCO at locations closer to isooctane, which is attributed to a higher pH and an easier dehydration environment created by the interface and oil. Our study provides insights into CaCO nucleation at an oil-water interface, which can deepen our understanding of pliable interfaces interacting with CaCO and benefit mineral scaling control during energy-related subsurface operation.