The Roles of Oil–Water Interfaces in Forming Ultrasmall CaSO₄ Nanoparticles

Abstract

In natural and engineered environmental systems, calcium sulfate (CaSO 4 ) nucleation commonly occurs at dynamic liquid−liquid interfaces. Although CaSO 4 is one of the most common minerals in oil spills and oil−water separation, the mechanisms driving its nucleation at these liquid−liquid interfaces remain poorly understood. In this study, using in situ small-angle Xray scattering (SAXS), we examined CaSO 4 nucleation at oil−water interfaces and found that within 60 minutes of reaction, short rodshaped nanoparticles (with a radius of gyration (R g ) of 17.2 ± 2.7 nm and a length of 38.2 ± 5.8 nm) had formed preferentially at the interfaces. Wide-angle X-ray scattering (WAXS) analysis identified these nanoparticles as gypsum (CaSO 4 •2H 2 O). In addition, spherial nanoparticles measuring 4.1 nm in diameter were observed at oil− water interfaces, where surface-enhanced Raman spectroscopy (SERS) revealed an elevated pH compared to the bulk solution. The negatively charged oil−water interfaces preferentially adsorb calcium ions, collectively promoting CaSO 4 formation there. CaSO 4 particle formation at the oil−water interface follows a nonclassical nucleation (N-CNT) pathway by forming ultrasmall amorphous spherical particles which then aggregate to form intermediate nanoparticles, subsequently growing into nanorod-shaped gypsum. These findings of this study provide insights into mineral scaling during membrane separation and can inform more efficient oil transport in energy recovery systems.