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 (BaCO3) and aqueous sodium sulfate (Na2SO4) for the crystal design of barium sulfate (BaSO4). This anionic ion-exchange, driven by the solubility discrepancy between BaCO3 and BaSO4, was rate-limited chemical transformation owing to the slow release of barium ions, resulting in the unusual BaSO4 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 BaSO4 crystals along the [002] direction at 120 degrees C. The Fajans rule of common ion adsorption and the layer-like arrangement of pendent surfactant hydrocarbon chains accounted for the formation of BaSO4 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 BaSO4, suggesting a potential application of solid-state BaCO3 in the removal of aqueous sulfate ions from industrial wastewaters.