Early Homogenous Amorphous Precursor Stages of Calcium Carbonate and Subsequent Crystal Growth in Levitated Droplets

Abstract

An in situ study of the contact-free crystallization of calcium carbonate in acoustic levitated droplets is reported. The levitated droplet technique allows an in situ monitoring of the crystallization while avoiding any foreign phase boundaries that may influence the precipitation process by heterogeneous nucleation. The diffusion-controlled precipitation of CaCO3 at neutral pH starts in the initial step with the homogeneous formation of a stable, nanosized liquid-like amorphous calcium carbonate phase that undergoes in a subsequent step a solution-assisted transformation to calcite. Cryogenic scanning electron microscopy studies indicate that precipitation is not induced at the solution/air interface. Our findings demonstrate that a liquid-liquid phase separation occurs at the outset of the precipitation under diffusion-controlled conditions (typical for biomineral formation) with a slow increase of the supersaturation at neutral pH.

A classical view on nonclassical nucleation

“…Faatz et al (11) presented the basis for a phase stability diagram including liquid-liquid phase separation. Wolf et al (12) performed experiments in acoustically levitated droplets and observed the formation of emulsion-like structures in transmission electron microscopy (TEM), which were proposed to be a dense liquid phase (DLP). Bewernitz et al (13) performed titration experiments at moderate pH levels, in which they supported the proposed emergence of a DLP by 13 C nuclear magnetic resonance (NMR) T 2 relaxation and 13 C pulsed field gradient stimulated-echo self-diffusion NMR measurements.…”

Now You See Them

“…Calcium carbonate is highly polymorphic in that it can exist in six different crystal structures. The first polymorph formed after nucleation is often amorphous calcium carbonate (ACC), which subsequently crystallizes (6,7). ACC has no long-range order, but it often has short-range structural order that appears to determine the lattice structure after crystallization (8).…”

Control of organic polymorph formation: crystallization pathways in acoustically levitated droplets

“…To access these levels of supersaturation any container walls which might act as heterogeneous nucleation sites have to be circumvented. In the past few years, acoustic levitators generating an ultrasonic standing wave for the containerless levitation of objects, such as droplets or solid samples, have gained considerable interest to study crystallization processes [15][16][17][18][19][20][21][22][23][24][25][26]. We briefly describe the experimental development up to the introduction of a climate unit for the control of the chemical potential of the surrounding vapor phase [27].…”

“…Our group pioneered a few years ago with the first time resolved in situ XRD experiments providing a continuous set of XRD patterns during a crystallization process [17,18,30]. In one of the early experiments, the non-classical crystallization pathway of calcium carbonate system involving a liquid/liquid phase separation and an emulsified of a highly hydrated liquid amorphous calcium carbonate (LACC) was investigated [20,31]. The LACC develops from pure, neutral, and saturated calcium bicarbonate solution crystallizing homogeneously in acoustic levitation [20].…”

“…In one of the early experiments, the non-classical crystallization pathway of calcium carbonate system involving a liquid/liquid phase separation and an emulsified of a highly hydrated liquid amorphous calcium carbonate (LACC) was investigated [20,31]. The LACC develops from pure, neutral, and saturated calcium bicarbonate solution crystallizing homogeneously in acoustic levitation [20]. These characteristic intermediates behave like a classical emulsion and are stabilized electrostatically [31].…”