Monolayers calcite

Several crystals, such as vaterite and calcite forms of CaC03, or a-glycine, have been nucleated (induced oriented crystallization) at the water surface covered with a monolayer film of carboxylic acids or aliphatic alcohols (compressed to "suitable" distances of the hydrophilic groups with a Langmuir balance) (Mann et al., 1988). 

An interesting analogous in vitro experiment was performed by Xu el al. [92] in which the presence of a polyelectrolyte in solution resulted in a layer of amorphous calcium carbonate forming under a structured monolayer. This subsequently transformed into a thin layer of polycrystalline calcite. The crystal growth in this in vitro system occurs by phase transition of amorphous calcium carbonate into calcite and not by dissolution of amorphous phase and reprecipitation of calcite crystals. 

Extensive seeded calclte growth experiments in the presence of phosphate ion indicate that the phosphate ion adsorbs onto the crystal surface as a monolayer. At a concentration of 10" M, phosphate ion can strongly inhibit calcite formation however, short term experiments show that this monolayer adsorption removes insignificant amounts of phosphorus from solution. In experiments lasting several days a further decrease in solution phosphate concentration occurs, presumably caused by nucleation of a surface calcium phosphate phase on the calcite seed. 

The presence of both oriented vaterite and calcite crystals on charged monolayers indicates that the electrostatic interactions between nuclei and the organic surface are influenced by structural relationships at the interface. The nucleation of vaterite on films of positive and negative charge indicates that Ca binding is not a prerequisite for stabilization of this metastable phase. Oriented calcite, on the other hand, requires Ca binding at the carboxylate head-groups. ... 

Nucleation of calcite on the (110) face under stearate monolayers can be rationalized in terms of charge, stereochemical and geometric com-... 

Fig. 27. Optical micrographs of (A) oriented calcite nucleated under stearate mono-layers at [Ca] = 10 mM bar = 100 p,m. (B) Oriented vaterite crystals nucleated under octadecylamine monolayers. Arrows indicate crystals of different crystallographic orientation bar =100 /im.

Geometric correspondence cannot, however, be solely responsible for (llO)-oriented calcite nucleation. For example, the (001) face of calcite comprises a hexagonal lattice of coplanar Ca atoms of 4.96-A periodicity and such an arrangement matches the monolayer-binding sites almost exactly. A significant difference between the (110) and (001) faces is the orientation of the carbonate anions they lie perpendicular to the (110) surface but parallel to (001). Thus the stereochemistry of the carboxylate headgroups mimics that of the anions in the (110) crystal face but not in the (001) face. 

The change from calcite to vaterite nucleation on stearate films at low [Ca] suggests that the extent of Ca binding is important for polymorph selection. The nucleation of calcite is favored by the formation of a well-defined Ca-carboxylate layer that mimics the first layer of the (110) face of the unit cell. By contrast, the structural requirements for vaterite formation are less precise. This is consistent with vaterite being the dominant phase on amine monolayers where no Ca binding is present, and suggests that kinetic factors of charge accumulation... 

X-RAY REFLECTIVITY X-ray reflectivity measurements can provide important information about mineral-water interfaces in situ by accurately determining die position of an adsorbed monolayer relative to the substrate surface. By measuring x-ray reflectivity of calcite, with and without lead, Sturchio et al. (1997) established that the lead ions were located in the surface atomic layer. X-ray reflectivity measurements found rubidium to be specifically adsorbed to the rutile surface at the tetradentate site (Zhang et al., 2004). These authors were able to include this information in the CD-MUSIC model to obtain an accurate description of rubidium adsorption. 

Probing large molecule adsorption stearate monolayers on calcite... 

Figure 22. (A) The specular reflectivity of the calcite-water interface (squares), with similar data for a calcite surface in contact with a 5 mM solution of stearic acid in methanol (The calcite-water data are offset vertically by a factor of 10, for clarity). The measurements were performed in a transmission cell. The solid lines are best-fit structure factor calculations for selected models. The thick solid line through the stearate data is optimized for an extended stearate molecule adsoibed on top of the calcite surface, as shown in (B). The thin solid line through the stearate data is optimized for the carboxylic head group substituting for surface carbonate ions of the calcite lattice. (B) Best-fit model for the stearate monolayer adsorbed on calcite.

There is little published information concerning the adsorption of stearic acid on the carbonates, although in several cases commercial products exist. Suess studied the adsorption of stearic acid onto dolomite from organic and aqueous solutions at room temperature [16]. In both cases, monolayer coverage was observed at almost exactly half the quantity of stearic acid required by calcite. It was concluded that the magnesium sites were inactive for adsorption under the conditions of the experiment. They may however react at higher temperatures. 

All peptides formed stable monolayers, with 189A molecule" for (b) and 61molecule" for (c). With monolayers of (b), calcium carbonate crystallized as calcite with two different habits. Apart from a small amount of pyramidal [01./]-oriented crystals (/ = 1,2), a new type of indented crystals nucleated from a [10.0] face. With monolayers of (c), similar crystals formed, but less efficiently. 

From the experimental data it can be concluded that the films have a diameter of about 5-10 xm and consisted of stearic acid, calcium stearate, and calcium carbonate (mainly calcite, no experimental information could be given concerning amorphous calcium carbonate at this point). The viscoelastic data pointed to a glassy state with a crystalline hardness. It is important to note that the films did not consist of a stearate monolayer, cross-linked by calcium ions (chalk soaps) as control experiments with CaCl2 observed. Derived from other control experiments it became clear that the growth of the films was due to a specific interaction of crystalline (or precrystalline) calcium carbonate and stearic acid. 

---