Most confinement studies have focused on the effects of freezing and melting pure liquids like water, hydrogen and helium; inert gases and organic liquids in nanoscale pores. Thus, many studies have shown that melting points and enthalpies of fusion in nanoscale crystals can differ significantly from their bulk scale counterparts. It has been shown (Christenson H.K., 2001) that melting points decrease in crystalline solids embedded within nanoporous matrices and melting point depression becomes more significant with decreasing size of pore. However, the confinement has other less known effects that will be discussed in this report.
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The study of biominerals shows that many biological reactions take place in specific spaces which can provide a structural and chemical environment that enables the controlled formation of crystals. The formation of biominerals in these specific volumes assigns them certain characteristics which would change if the growth environment changed. This kind of organization suggests that there should be similar systems which are available and exploitable in synthetic materials chemistry. Some studies have focused on these changes and have attempted to confine compounds such as calcium carbonate or calcium phosphate. These studies have been conducted in synthetics systems to observe the differences on their crystallization in confinement and in bulk solutions. The results obtained in these studies are discussed in this report.
There have been numerous investigations into the precipitation of calcium carbonate in different confined environments. For example, it has been grown within regular arrays of picoliter droplets created on patterned self-assembled monolayers (SAMs) (Stephens, C. J., et al. 2011).
Figure 1. SAM deposited on a gold-mica substrate with a droplet of CaCO3 solution by passing a Na2CO3/CaCl2 solution across the SAM. (copied from reference 6)
It is also possible to confine CaCO3 within the confines of a annular wedge, formed around the contact point of two crossed half cylinders. The cylinders are functionalized with SAMs. (Stephens, C. J., et al. 2010).
Formerly, CaCO3 biomineralization was considered to be based in a combination of ions pumped into the mineral deposition site. However, CaCo3 confinement has helped to increase our understanding of biomineralization processes due to a higher study of its formation. Currently, it is well known that calcite and aragonite formation in biological systems frequently proceeds via ACC (amorphous CaCO3) precursor. It was shown (Stephens, C. J., et al. (2011) how the crystallisation of CaCO3 was slower in droplets than in a bulk solution. This allows us to observe the mechanism of crystallisation more easily and then study the different stages of the process or even discover new polymorphs. The research shows that the crystals that had precipitated within the droplets were quite different in morphology and were smaller than those precipitated in bulk. While the crystals precipitated in bulk solution were rhombohedral calcite crystals, most crystals formed in the droplets were tetrahedral ACC crystals. The explanation for these results is that the rate of crystallization of the ACC phase decreases in confinement. In bulk solution, the tetrahedral growth form converted into the more common rhombohedral morphology faster than in the droplets because the lifetime of the ACC precursor phase increased in small volumes. In particular, precipitation reaction of CaCO3 terminates at an earlier stage of crystallization in SAMs than in the bulk solution. Thus, under the limited reagent conditions, we might be able to observe the frozen intermediate and then, better understand its mechanism of crystallization.
Other work has confined the CaCO3 in phospholipid bilayer vesicles (Tester C., Ryan E. Brock, et al. 2011). The results obtained are similar to those obtained in droplets: confinement allow the stabilization and control transformation of metastable amorphous precursor phases of calcite.
Figure 2. Phospholipid bilayer vesicle (liposome) (copied from reference 18)
Liposomal encapsulation offers excellent control over calcite nucleation and growth.
CaSO4 was also confined in a annular wedge formed around the contact point of two crossed half cylinders (Wang, Y. W., H. K. Christenson, et al. 2013). A remarkable stabilization of the metastable phases amorphous calcium sulfate and calcium sulfate hemihydrates was observed. Confinement caused changes in the crystal shapes, sizes and observed polymorphs.
The effect of confinement on Calcium Phosphate has also been studied. The importance of Calcium Phosphate confinement is highlighted by the fact that it allows understanding of bone and teeth structure and its formation mechanism (Cantaert B., Elia Beniash et al. 2013). By precipitating calcium phosphate within the pores of track-etched membranes, it is demonstrated that the formation of polycrystalline hydroxapatite and octacalcium phosphate (results of the crystallization of calcium phosphate) occurs by means of an intermediate amorphous calcium phosphate which is significantly stabilised in confinement. In addition, it is shown that in confinement the spatial constraints can provide a more organised structure of apatitic crystals because preferred orientations of the calcium phosphate nanocystals might appear. This research shows that orientation of the polycrystalline hydroxapatite in confinement is comparable or even superior to that seen in bone.
Figure 3. Precipitation of CaPO3 within membrane pores (copied from reference 1)
Some other studies have also reported that solid crystallization within nanometer-scale often exhibit preferred orientations with respect to the direction of the pores. Organic crystals were grown in the porous polymer monoliths and it was shown that pore direction affects the orientation of the nanocrystal. (Hamilton et al. 2012). Moreover, it is demonstrated that the sizes of crystals embedded in pores and the polymorph stability and selectivity during crystallization are affected by the pore size.
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The importance of confinement is explained through the crystallization processes since most of biological and chemical reactions can be affected by confinement, due to the changes in stability, morphology, polymorph, orientation, polycrisystalline character of crystals and also in the rate of crystallization. Indeed there are many crystallization phenomena that we cannot describe correctly from bulk solution. In conclusion, the confinement effects are important in general material synthesis since many optical, ferroelectric, electronic and magnetic proprieties of crystalline material depend on the control of the polymorphic forms of materials. Moreover, the study of early stages crystal growth is hard because they are transient and form randomly. Hence, to study the different polymorphic forms and orientation of crystals, it will be useful for solid crystallization to be conducted in localized volumes.
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