Reaction and Recovery of Nucleation Particle Strontium Chloride Hexahydrate in Calcium Chloride Hexahydrate

Abstract

Isostructural solid phases with similar lattice parameters are known to promote nucleation in salt hydrate phase change material (PCM) systems, as they serve as templates for crystal growth. However, the phases with the closest structural and chemical similarities are generally also salt hydrates and thus, are susceptible to dissolution or reaction to form phases with different hydration states. Here, we investigate an isostructural system where the candidate nucleation particle, strontium chloride hexahydrate (SrCl₂⸱6H₂O), reacts in the liquid PCM, calcium chloride hexahydrate (CaCl₂⋅6H₂O). We demonstrate that when present, crystalline SrCl₂⸱6H₂O dramatically reduces undercooling of CaCl₂⋅6H₂O. However, when present in low concentrations, SrCl2⸱6H2O will react with CaCl₂⋅6H₂O to form a lower hydrate phase, SrCl₂⸱2H₂O, which does not promote nucleation in the system. Critically, we demonstrate that it is possible to regain the nucleation particle efficacy by supercooling the system, recovering the active SrCl₂⸱6H₂O phase. This result bounds the utility of isostructural compounds as nucleation particles for salt hydrate phases and illustrates that nucleation strategies relying on isostructural systems are complicated by the solubility relationships and reactivity between the phases present.