An Ion-Exchange Approach to the Crystal Design of Barium Sulfate in the Presence of Ionic Surfactants

Chemical reaction occurring between an insoluble solid and a saltwater solution relates to an exchange of ionic components and is therefore referred to as ion-exchange reaction. In this paper, we used the ion-exchange reaction of solid-state barium carbonate (BaCO 3 ) and aqueous sodium sulfate (Na 2 SO 4 ) for the crystal design of barium sulfate (BaSO 4 ). This anionic ion-exchange, driven by the solubility discrepancy between BaCO 3 and BaSO 4, was rate-limited chemical transformation owing to the slow release of barium ions, resulting in the unusual BaSO 4 crystallite with the bounded (200), (002), and (210) faces. Therein, cationic surfactant of cetyltrimethylammonium bromide (CTAB) exerted almost no influence on the morphological control of crystalline products. In a striking contrast to this, the similar cationic surfactant of CTAOH (i.e., cetyltrimethylammonium hydroxide) promoted the crystal growth along the [020] direction. Surprisingly, the addition of anionic sodium dodecyl sulfate (SDS) enhanced the surface etching phenomenon of BaSO 4 crystals along the [002] direction at 120 °C. The Fajans rule of common-ion adsorption and the layer-like arrangement of pendent surfactant hydrocarbon chains accounted for the formation of BaSO 4 secondary architectures of trigonal bipyramids or nanorods at a relatively high concentration of SDS. In a word, through the rate-controlled chemical reaction assisted by ionic surfactants, we found a successful ion-exchange approach to the crystal design of BaSO 4, suggesting a potential application of solid-state BaCO 3 in the removal of aqueous sulfate ions from industrial wastewaters.