Ultrasonic-Assisted green synthesis of nano-anhydrite from calcium hydroxy glycolate in an ethylene glycol-sulfuric acid system

Within the framework of green chemistry, additive-free and morphology-controllable synthesis of inorganic nanomaterials has become an emerging research frontier. In this study, we developed an efficient liquid-phase synthesis route using calcium hydroxy glycolate (CHG) as the calcium source, sulfuric acid as the sulfur source, and ethylene glycol as the solvent. Under ultrasound-assisted conditions, without the use of soluble salts or surfactants, high-purity nano-anhydrite calcium sulfate (CaSO₄) was successfully synthesized. By systematically varying the precursor concentration and ultrasonic parameters, the resulting products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy (FT-IR), which confirmed the formation of spherical, plate-like, and rod-shaped nano-anhydrite. Specifically, by adjusting the Ca²⁺ concentration, rod-like crystals (length: 300-450 nm, width: 50 nm, aspect ratio: 7), spherical particles (mean diameter: 23.19 nm), and flake-like structures (diameter: 115.40 nm, thickness: 10-30 nm) were obtained at 0.3, 0.5, and 0.7 mol·L^-1, respectively. The smallest particle sizes across these morphologies were achieved under optimized ultrasonic conditions of 750 W for 30 min. Molecular dynamics simulations revealed that ethylene glycol concentration modulates its selective adsorption on specific crystal planes of anhydrite, thereby differentially inhibiting growth rates along certain directions and enabling morphology-controlled synthesis. The simulated adsorption energies for the (200), (020), (011), and (002) faces were -15.04, -7.96, -2.08, and -0.45 kJ·mol^-1, respectively. These results indicate that preferential adsorption occurs particularly on the (200) and (020) planes. This integrated experimental and simulation study elucidates the coupled mechanism of "precursor concentration - crystal plane adsorption - ultrasonic dynamics," offering theoretical insights and technical support for the environmentally sustainable and controllable synthesis of nano-anhydrite and other sulfate-based nanomaterials.