Rapid crystal growth without inherent supersaturation induced by nanoscale fluid flows?

Abstract

Crystal growth is a process that only takes place under non-equilibrium conditions and a necessary prerequisite is that the crystal is exposed to a phase that is supersaturated in the material the crystal is composed of, be it a solution, a vapour or a supercooled melt. In industrial mass crystallization the growth rate for a population of crystals (in suspension growth processes [1]) rarely exceeds mean linear velocities of 10 -7 ms -1 . Here we present a mass crystallization process which is accompanied by rapid crystal growth several orders of magnitude faster and into a region of solution that is without inherent supersaturation. The material investigated is a solid hydrate that exhibits a solution mediated phase transition to its anhydrous form in the presence of methanol [2]. The phase transition is initiated simply by placing an amount of hydrate crystals into the solvent and is characterized by the rapid emergence of needle-shaped crystals. The needles emanate from the crystal faces of the hydrate crystals and grow into the solution, which is nominally free of the substance to be crystallized. The high growth rate of the crystals, which of the order of up to 10 -4 ms -1 is surprising. Although rapid needle growth has been observed before [3][4][5][6][7][8][9], to date a satisfactory explanation for needles growing under the abovementioned conditions is still outstanding. Based upon the topology of the crystals we propose a tentative mechanism for this phenomenon capable of explaining the unusually rapid growth and highlight those questions that need addressing in order to verify this mechanism. X-ray powder diffraction is used to characterize the crystal phase of the needles; confocal fluorescence microscopy reveals that the needles are hollow. The width of these needles is between 0.5 and 5 µm, their length appears to be limited only by the amount of hydrate available for their formation.

Design of Crystalline Solids

“…The ability to fill the needles has been demonstrated indirectly by confocal fluorescence microscopy [34]. The experiments designed to prove that the needles are hollow along their entire length require a fluorescent probe molecule to be present within the tunnel in order to visualize the void space.…”

“…A reasonable model for the growth mechanism, involving transport of a saturated solution from within the crystal and subsequent generation of supersaturation through micro-mixing in the vicinity of the crystal surface of the material, has been discussed by Jones et al [34]. The initial saturated solution is assumed to consist predominantly of the solvent contained within the crystal lattice of the solvate and the solute.…”

Decreasing the Diameter of Crystalline Tubes and an Approach to Explain its Nucleation

“…It has been assumed that the growth of needles starts at defects in the crystal structure [2,3]. Though the single crystals were prepared carefully, the occurrence of some defects on the surface cannot be avoided.…”

“…By immersing hydrates in an antisolvent, such as methanol, hollow needles can be produced via phase transformation [1,2]. Furthermore, the outer diameter of these hollow needles can be tailored by controlling the water content in methanol.…”

“…This definition plays an important role to investigate the starting point of the needle growth in this paper since the mechanism of the growth of hollow needles was already published. Hereby, the growth mechanism is characterized by the processes of the decomposition of the hydrate crystal (dehydration), the diffusion of saturated water in the methanol, the nucleation, and, finally, the growth of the hollow anhydrate needles (solvent-mediated phase transformation) [2,3].…”