Understanding mineral precipitation and dissolution, and ion transport in porous materials is critical to improved management of natural subsurface water sources and to improving the way in which water is treated for public consumption. Additionally, brine water is a by-product of water desalination plants and of enhanced oil recovery operations, and the precipitation of gypsum, CaSO4, is common. As these industries consider the re-injection of high mineral content wastewater into subsurface reservoirs, the importance of understanding various precipitation effects, has prompted the investigation of factors that determine precipitation in pores of various dimensions. In this study, the sorption of water and transport of ions into 25 nm outer diameter hollow spherical silica nanoparticles (HS) consisting of five nm-thick porous spherical shells are examined. The shell micropores (1-2 nm wide) allow transport between the HS primary inner volume and its surroundings. A core-shell sphere model with hard sphere interaction potential describes the HS structure and spatial distribution, while a Guinier model describes the micropore structure in small-angle x-ray scattering (SAXS) results. Water diffusion and its structural properties within the hollow spheres, as well as ion transport of CaCl2 and Na2SO4 solutions in contact with the HS material were studied using a combination of SAXS, calorimetry, gas sorption analysis, x-ray diffraction, and pH measurements. In situ SAXS water sorption measurements show distinct time domains for sorption tied to the HS surfaces, inner volume, and the inter-HS external void spaces. HS-confined water phase transitions near -29 °C and -43 °C will be discussed. In the case of ion transport, the impact of ion solvation and possible ion pairing, on the estimated HS inner volume ion densities will be discussed. These investigations lay critical groundwork for understanding how to control mineral precipitation in nanoconfined environments and enable further studies of nucleation-, growth-, and dissolution-dependent phenomena.

Water properties and ion transport in nanometer-scale hollow silica spheres with porous shells

ACS Fall 2021

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