Contrasting mechanisms of dodecyltrimethylammonium chloride versus tetramethylammonium chloride in the crystallization of calcium sulfate hemihydrate whiskers from titanium gypsum

Abstract

The substantial accumulation of industrial by-product gypsum, primarily from China's power generation, phosphate fertilizer, and fluorine chemistry sectors, poses a significant environmental challenge. To address this issue and valorize the waste, this study provides, to the best of our knowledge, the first systematic mechanistic comparison of successfully synthesized high-aspect-ratio calcium sulfate hemihydrate whiskers (CSHW) from titanium gypsum (TMG) via a hydrothermal method, using dodecyltrimethylammonium chloride (DTAC) or tetramethylammonium chloride (TMAC) as crystal modifiers. The adsorption behaviors and underlying growth mechanisms were systematically investigated through integrated experimental and theoretical approaches. Optimal aspect ratios were achieved at 0.50 wt% DTAC and 0.75 wt% TMAC, corresponding to significant enhancements of 107.2 % (reaching 71.24) and 48.9 % (reaching 51.19), respectively. Mechanistic analyses revealed that the (CH₃)₃N⁺ groups of both surfactants chemically adsorb onto the side facets, inhibiting radial growth and promoting axial elongation. Crucially, DTAC, with its dodecyl chain, forms hemi-micellar or bilayer adsorption structures—a capability lacking in TMAC—which endows the CSHW with markedly enhanced hydrophobicity and a higher surface potential, both of which are crucial for improving interfacial compatibility and reinforcement efficiency in composite materials.