<i>In Situ</i> Monitoring the Nucleation and Growth of Nanoscale CaCO₃ at the Oil–Water Interface

Interfaces can actively control the nucleation kinetics, orientations, and polymorphs of calcium carbonate (CaCO₃). Prior studies have revealed that CaCO₃ formation can be affected by the interplay between chemical functional moieties on solid-liquid or air-liquid interfaces as well as CaCO₃'s precursors and facets. Yet little is known about the roles of a liquid-liquid interface, specifically an oil-liquid interface, in directing CaCO₃ mineralization which are common in natural and engineered systems. Here, by using <i>in situ</i> X-ray scattering techniques to locate a meniscus formed between water and a representative oil, isooctane, we successfully monitored CaCO₃ formation at the pliable isooctane-water interface and systematically investigated the pivotal roles of the interface in the formation of CaCO₃ (i.e., particle size, its spatial distribution with respect to the interface, and its mineral phase). Different from bulk solution, ∼5 nm CaCO₃ 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 CaCO₃. There is a clear tendency for enhanced amounts and faster crystallization of CaCO₃ 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 CaCO₃ nucleation at an oil-water interface, which can deepen our understanding of pliable interfaces interacting with CaCO₃ and benefit mineral scaling control during energy-related subsurface operation.