S T R U C T U R E O F M A T E R I A L S

S C I E N T I F I C H I G H L I G H T S

1 3 4 H I G H L I G H T S 2 0 2 1 I

An imaging-based strategy to study crystallisation pathways has been developed. Crystallisation is performed within millimetre-diameter controlled pore glass (CPG) rods that have sponge-like structures and pore diameters of just 7 nm. This extreme confinement significantly slows crystallisation [1], making it possible to study processes that proceed rapidly in bulk solution. The developing crystals are also fixed in position in these environments such that their spatial and structural development can be followed in 3D. By studying crystallisation in situ within the CPG rods using synchrotron X-ray micro-computed tomography (μ-CT) and X-ray diffraction computed tomography (XRD-CT) at beamline ID11, the evolution of the size, shape and structure of a population of particles can be monitored over time.

This study focused on calcium sulfate, which is highly important in industry and in the environment. Gypsum (CaSO4∙2H2O) is the thermodynamically stable phase of calcium sulfate at room temperature, while bassanite (CaSO4∙0.5H2O) becomes the dominant phase above ≈ 95oC. However, recent ex-situ studies have reported bassanite and an amorphous phase as intermediates to the aqueous phase precipitation of gypsum at room temperature, suggesting a more complex behaviour [2].

Calcium sulfate was precipitated within the CPG rods by inserting a rod between two tubes, one of which contained CaCl2 solution, and the other (NH4)2SO4 solution. Counter- diffusion then leads to the precipitation over time. The development of particles within the rods was observed in situ using μ-CT and XRD-CT. μ-CT generates high-quality

images of the particles based on absorption of the X-ray beam, while XRD-CT is able to 3D image and identify the polymorph of the individual crystals. Amorphous calcium sulfate formed at early times, and subsequently transformed into spheroidal bassanite crystals (Figure 114). The number and sizes of these crystals continued to increase with time until they ultimately blocked the rod. They then remained stable as bassanite for over three weeks. The stabilisation of bassanite in purely aqueous environments at room temperature for such long periods is unprecedented.

The influence of the CPG surface chemistry in directing crystal growth was also studied by functionalising the CPG rods with carboxyl-terminated monolayers. While the rate of crystallisation was comparable to unfunctionalised rods, both bassanite and gypsum formed within the rods after 1-2h. The µ-CT images were of such high quality

Fig. 114: CT imaging of calcium sulfate precipitation in an unfunctionalised CPG rod.

a-b) 3D rendering showing bassanite crystals after (a) 3h and (b) 24h. c-d) 2D spatial

diffraction maps of bassanite crystals after (c) 3h and (d) 24h.

Fig. 115: 2D µ-CT slice of calcium sulfate crystals in a carboxylate- functionalised CPG rod, where the brighter crystals are bassanite and the darker are gypsum.