The ability to tailor the micro- and nanoscale architecture of crystalline substances is a critical step toward application. There has been great success in controlling the morphology of many inorganic, water insoluble materials, such as metals and oxides. However, there has been less progress on the manipulation of water soluble salts, and only a few novel structures have been produced. New architectures of materials such as sodium chloride are important, as an understanding of how to manipulate these highly regular, fundamental structures could lead to great insights into the growth mechanisms of other crystals.
Researchers from the Institute of Intelligent Machines at the Chinese Academy of Sciences have recently advanced in this area, as they have managed to produce sodium and potassium chloride crystals with a hopper-like structure [Zhang et al., Angewandte Chemie (2011) doi:10.1002/anie.201101704]. These individual crystal blocks look just like normal, cubic salt crystals, apart from the large cavities which develop along one side.
The crystals grow on the surface of microdroplets of salt solution, resulting in larger structures, in the form of hollow microspheres. The exact growth mechanism is not fully understood at present, but the team has formed a hypothesis, as co-author of the paper Suhua Wang explained to Materials Today, “The nucleation should start at the organic-aqueous interface around the metastable water microdroplets when acetone diffuses into the water microdroplets. The continuous diffusion of acetone into [the] water droplets results in the saturation of NaCl in the droplet of the aqueous solution. The nutrient of ions from the water droplets sustainably fed the uniform growth of NaCl single crystals on the side of [the] aqueous solution. However, the cyclohexane molecules on the organic side prevent facet growth due to the limitation of [the] ion mobility. As a result, the cubic hopper-like single crystals and their uniform arrangement at the interface of the microdroplets [is] achieved.”
The team is now planning to test their hypothesis of the growth mechanism by modeling the system, as well as extend their method to other water soluble compounds.