Exploiting Confinement to Study the Crystallization Pathway of Calcium Sulfate

Clara Anduix‐Canto,Jose R A Godinho,Alexander N Kulak,Jonathan P Wright,Carlos González Niño,Nikil Kapur,Philip J Withers,Fiona C Meldrum,Yi‐Yeoun Kim,Hugo K Christenson,Mark A Levenstein

doi:10.1002/adfm.202107312

Abstract

AbstractCharacterizing the pathways by which crystals form remains a significant challenge, particularly when multiple pathways operate simultaneously. Here, an imaging‐based strategy is introduced that exploits confinement effects to track the evolution of a population of crystals in 3D and to characterize crystallization pathways. Focusing on calcium sulfate formation in aqueous solution at room temperature, precipitation is carried out within nanoporous media, which ensures that the crystals are fixed in position and develop slowly. The evolution of their size, shape, and polymorph can then be tracked in situ using synchrotron X‐ray computed tomography and diffraction computed tomography without isolating and potentially altering the crystals. The study shows that bassanite (CaSO4 0.5H2O) forms via an amorphous precursor phase and that it exhibits long‐term stability in these nanoscale pores. Further, the thermodynamically stable phase gypsum (CaSO4 2H2O) can precipitate by different pathways according to the local physical environment. Insight into crystallization in nanoconfinement is also gained, and the crystals are seen to grow throughout the nanoporous network without causing structural damage. This work therefore offers a novel strategy for studying crystallization pathways and demonstrates the significant impact of confinement on calcium sulfate precipitation, which is relevant to its formation in many real‐world environments.

Highlights

Understanding the mechanisms by which crystals form promises the ability to control these processes in a vast array of applications
Supersaturations were calculated using Visual Minteq, taking into account all ions present. These analyses indicate that the system is supersaturated ([Ca2+] = [SO42−] ≈ 60 mM, S > 4) with respect to gypsum[23] at the time at which precipitation is first observed in the Controlled Pore Glass (CPG)
We have described an imaging-based strategy that exploits confinement effects to track the evolution of a population of individual particles in situ using synchrotron X-ray computed tomography and diffraction computed tomography, and explore the possible pathways by which crystals form

Summary

Introduction

Understanding the mechanisms by which crystals form promises the ability to control these processes in a vast array of applications. Significant progress has been made thanks to advances in analytical techniques, and it is recognized that crystallization can occur via processes based on atom-by-atom addition, the assembly of clusters and nanoparticles, and the transformation of liquid-like phases.[1,2,3,4,5] This potentially opens up a huge reaction space, where crystalline materials can form via multiple pathways, which can potentially operate simultaneously.[6] Characterizing crystallization processes becomes highly challenging, where techniques that average over an ensemble of particles,[7] or take a snapshot,[8,9] cannot unambiguously resolve individual pathways. Imagingbased methods that operate in situ and can follow the development of individual crystals to give both morphological and structural data are required.[10,11]

Results

Discussion

Conclusion