PerspectiveMaterials Science

Now You See Them

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Science  19 Dec 2008:
Vol. 322, Issue 5909, pp. 1802-1803
DOI: 10.1126/science.1167221

Crystallization lies at the heart of many natural and technological processes, from the production of pharmaceuticals and nanomaterials to the formation of bones and teeth, frost heave, and scale deposition. Crucial features of these crystals, such as lattice orientation, particle size, and size distribution, are defined by conditions during the earliest stages of precipitation—at nucleation. Yet, nucleation from solution is poorly understood, because experimental studies of nucleation are highly challenging (1). Recent studies have high-lighted the possible role of clusters in nucleus formation (2, 3). On page 1819 of this issue, Gebauer et al. provide support for this thesis (4) by demonstrating the presence of large, well-defined clusters before nucleation of one of the phases of calcium carbonate. Crystallization appears to proceed through aggregation of these clusters. The results challenge the conventional picture of crystal nucleation.

How do crystals nucleate? According to classical nucleation theory, calcium carbonate nucleation proceeds by addition of ions to a single cluster (top). Gebauer et al. now suggest a different mechanism, in which nucleation of ACC occurs by aggregation of stable, amorphous, precritical clusters (bottom). The nucleated ACC phase subsequently crystallizes to generate the final stable crystal product.

CREDIT: P. HUEY/SCIENCE

Classical nucleation theory provides a simple understanding of how crystals nucleate. Nucleation is often slow because of a free-energy barrier originating from the interface between the nucleus and its surroundings. The theory assumes that nuclei grow one molecule at a time (see the figure, top). As the nuclei grow, their Gibbs free energy increases, until a free-energy maximum is reached at the critical size. At least in simple systems such as argon, critical nuclei are expected to persist for microseconds or less, making them virtually impossible to observe. Beyond the critical size, the nuclei are stable and release energy during growth.

In their investigation of calcium carbonate nucleation, Gebauer et al. observe long-lived precritical clusters, about 2 nm in diameter, and suggest that they grow by colliding and coalescing. These results are clearly in contrast to the picture of nucleation presented by classical nucleation theory. The theory assumes that the structure of the nucleus is like a piece of the bulk phase and that its surface has the same interfacial tension as a bulk phase. However, if stable, precritical clusters are to exist, they must lie in a free-energy minimum. Such a minimum would only occur if the classical theory's assumptions are wrong, perhaps because the structure of the clusters is different from that of the bulk.

The possible structure of the precritical calcium carbonate clusters is open to speculation. If the ions are present in a bulk crystal lattice, then it is surprising that the clusters of about 70 ions neither shrink nor grow. Alternatively, the structures may be ordered but differ from that of the bulk phase, thus retarding growth. Modeling has shown that small ordered clusters of argon atoms are not just chunks cut from the bulk lattice but form different structures, such as icosahedra, which are incompatible with growth to fill space (5). In the case of argon, the true bulk phase nucleates on these ordered clusters, rendering them transient. However, calcium carbonate solutions are considerably more complex, so that nucleation of the true bulk phase from an ordered nanoparticle may be more difficult. 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). The most likely scenario is therefore that the precritical clusters are themselves amorphous or of low structural order. However, if they are amorphous, it is again unclear why they neither dissolve nor grow.

There is one system in which clusters and their contribution to the nucleation of the bulk phase have been extensively studied, and where we have at least a basic understanding of their behavior: water droplets in Earth's atmosphere (9). These droplets range in size from a few nanometers to tens of nanometers. They are stabilized by other species (such as sulfuric acid) that are both highly water-soluble and nonvolatile, so that they partition strongly into the nanometer-scale droplets. These ions provide an osmotic pressure inside the droplets that prevents their evaporating, even when the air is undersaturated with water vapor. Dusek et al. have shown that the super-saturation at which nucleation occurs is determined largely by the size of the nanodroplets present (10).

Could the precritical clusters observed by Gebauer et al. also be the result of stabilization of calcium carbonate clusters by another species present as an impurity? This mechanism would provide a basis for stabilizing precritical clusters in a free-energy minimum and does not contradict classical nucleation theory. Such impurities are ubiquitous and virtually impossible to eliminate from any solution. The results of Gebauer et al. may thus reflect the mechanism of nucleation of calcium carbonate in “real” systems. Nucleation could then occur by coalescence of the precritical clusters to give ACC, which will subsequently crystallize to a more stable crystalline polymorph. The latter mechanism is consistent with the observations of Gebauer et al., who show that ACC is the first phase precipitated after nucleation.

The idea that nucleation of calcium carbonate may proceed via an aggregation mechanism is highly topical. The past decade has seen great progress in understanding crystallization processes, and it is now well recognized that single-crystal growth (as distinct from nucleation) often occurs via the aggregation of small precursor units rather than by addition of ions or molecules to a nucleus (11). Cluster species have also been observed before nucleation in saturated solutions of compounds such as sodium chloride (2), urea (12), and glycine (3), and there have been suggestions that clustering can determine which polymorph is formed (13). However, none of these even remotely approach the size or stability of the clusters observed by Gebauer et al. Further investigation of precritical clusters and their role in the crystallization of calcium carbonate, and indeed other compounds, is eagerly anticipated.

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