Yun-Wei Wang

Additives stabilize calcium sulfate hemihydrate (bassanite) in solution

Recent work on the precipitation of the mineral gypsum (calcium sulfate dihydrate) has shown that contrary to long-standing opinion, this phase can precipitate from solution at room temperature via a bassanite (calcium sulfate hemihydrate, or plaster of Paris) intermediate phase. An amorphous calcium sulfate (ACS) phase can also precede precipitation of hemihydrate. Here, we profit from these observations to generate ACS and hemihydrate particles that have considerable stability in solution. By precipitating calcium sulfate in the presence of poly(acrylic acid), poly(styrene-4-sulfonate), sodium triphosphate and magnesium ions, we show that it is possible to use these additives to retard the transformation of these metastable mineral phases, and thereby readily isolate hemihydrate. These additives are also active in controlling the morphologies of the hemihydrate crystals, which can play a key role in defining properties such as porosity and mechanical strength. The results confirm the stepwise-precipitation of gypsum via amorphous and hemihydrate intermediates and suggest an alternative to the energy-intensive calcination processes which are currently widely used to prepare hemihydrate.

Occlusion of Sulfate-Based Diblock Copolymer Nanoparticles within Calcite: Effect of Varying the Surface Density of Anionic Stabilizer Chains

Polymerization-induced self-assembly (PISA) offers a highly versatile and efficient route to a wide range of organic nanoparticles. In this article, we demonstrate for the first time that poly(ammonium 2-sulfatoethyl methacrylate)-poly(benzyl methacrylate) [PSEM–PBzMA] diblock copolymer nanoparticles can be prepared with either a high or low PSEM stabilizer surface density using either RAFT dispersion polymerization in a 2:1 v/v ethanol/water mixture or RAFT aqueous emulsion polymerization, respectively. We then use these model nanoparticles to gain new insight into a key topic in materials chemistry: the occlusion of organic additives into inorganic crystals. Substantial differences are observed for the extent of occlusion of these two types of anionic nanoparticles into calcite (CaCO3), which serves as a suitable model host crystal. A low PSEM stabilizer surface density leads to uniform nanoparticle occlusion within calcite at up to 7.5% w/w (16% v/v), while minimal occlusion occurs when using nanoparticles with a high PSEM stabilizer surface density. This counter-intuitive observation suggests that an optimum anionic surface density is required for efficient occlusion, which provides a hitherto unexpected design rule for the incorporation of nanoparticles within crystals.

The Effect of Additives on the Early Stages of Growth of Calcite Single Crystals

Abstract As crystallization processes are often rapid, it can be difficult to monitor their growth mechanisms. In this study, we made use of the fact that crystallization proceeds more slowly in small volumes than in bulk solution to investigate the effects of the soluble additives Mg2+ and poly(styrene sulfonate) (PSS) on the early stages of growth of calcite crystals. Using a “Crystal Hotel” microfluidic device to provide well‐defined, nanoliter volumes, we observed that calcite crystals form via an amorphous precursor phase. Surprisingly, the first calcite crystals formed are perfect rhombohedra, and the soluble additives have no influence on the morphology until the crystals reach sizes of 0.1–0.5 μm for Mg2+ and 1–2 μm for PSS. The crystals then continue to grow to develop morphologies characteristic of these additives. These results can be rationalized by considering additive binding to kink sites, which is consistent with crystal growth by a classical mechanism.