The presence of alcohol in binary alcohol− water mixtures can affect the precipitation pathways of anhydrous crystalline CaCO 3 polymorphs and their morphology. We explored the formation pathways and the effects of several parameters on calcite, vaterite, and aragonite: concentration of simple alcohols, time, and shaking speed, and we derived a multiparameter model for predicting what phase is preferred. We found that shaking speed and alcohol concentration are the most important parameters for affecting the stability of vaterite and aragonite and for changing vaterite morphology, from cauliflower-shaped, spherical aggregates, to dendritic, flatter structures. In all our experiments, the precipitated aragonite was twinned, and both the vaterite and aragonite can be interpreted to form through spherulitic growth. Classical growth theory fully describes their formation; there is no need to invoke the popular hypothesis for nonclassical growth by self-assembly of nanocrystals. These studies, and future work with solutions of low water activity, are paving the way to a better understanding of how organisms select their preferred polymorph and engineer CaCO 3 morphology during biomineralization.

understanding of how additives modify CaCO 3 growth kinetics and the mechanisms that control precipitation is therefore a topic of considerable interest. [ 1-3 ] The initial steps of CaCO 3 crystallization can occur via the formation of a poorly ordered amorphous calcium carbonate (ACC) phase. Synthetically formed ACC is often short lived and quickly transforms to more stable crystalline CaCO 3 polymorphs such as vaterite, calcite, and aragonite. [ 4,5 ] In contrast, biogenic ACC can be stable for much longer, even over the entire lifetime of an organism. [ 6,7 ] The enhanced stabilization of ACC has been explained by the incorporation of considerable amounts of magnesium, phosphate, and silicate, as well as the occlusion of associated proteins and other macromolecules. [ 8,9 ] For example, ACC aggregates in the intestinal tract of the seabream, Sparus aurata , are stabilized by the incorporation of up to 54 mol% Mg. [ 10 ] Similarly, ACC formed in the exoskeleton and gastroliths of the crayfi sh, P. clarkii , contain phosphoenolpyruvate and 3-phosphoglycerate (intermediates of the glycolytic pathway), which were shown to be responsible for ACC stabilization. [ 11 ] While numerous laboratory studies have quantifi ed the effects of inorganic additives on ACC formation, stability, and crystallization, [ 9,12,13 ] the role of biomolecules is less well constrained, mainly because of the vast diversity of organic compounds, their variety of composition, chain length, and structure. Highly carboxylated species have been shown to extend ACC lifetime, [ 3,14-16 ] whereas molecules with a lower number of carboxyl groups, such as single unit amino acids, do not exert much control on ACC stability. [ 3,17 ] However, a mechanistic understanding of how these organic molecules affect ACC composition, structure, and lifetime is still lacking. This study reports on investigations of the role of citrate (CIT) in ACC formation and crystallization. CIT is an intermediate in the tricarboxylic acid cycle and can form during glycolysis. It is used in various industrial processes, for example, as a fl avoring additive in food, as a cleaning and chelating agent, and as a scale inhibitor in pipes, boreholes, and subsurface reservoirs. [ 18 ] More recently, CIT has been shown to be an ideal coating agent to protect and stabilize metallic nanoparticles and to control their size. [ 19 ] The effect of CIT on CaCO 3 polymorph selection and crystal growth rates has been examined

The interaction of OH-containing compounds with calcite, CaCO(3), such as is required for the processes that control biomineralization, has been investigated in a low-water solution. We used ethanol (EtOH) as a simple, model, OH-containing organic compound, and observed the strength of its adsorption on calcite relative to OH from water and the consequences of the differences in interaction on crystal growth and dissolution. A combination of atomic force microscopy (AFM) and molecular dynamics (MD) simulations showed that EtOH attachment on calcite is stronger than HOH binding and that the first adsorbed layer of ethanol is highly ordered. The strong ordering of the ethanol molecules has important implications for mineral growth and dissolution because it produces a hydrophobic layer. Ethanol ordering is disturbed along steps and at defect sites, providing a bridge from the bulk solution to the surface. The strong influence of calcite in structuring ethanol extends further into the liquid than expected from electrical double-layer theory. This suggests that in fluids where water activity is low, such as in biological systems optimized for biomineralization, organic molecules can control ion transport to and from the mineral surface, confining it to specific locations, thus providing the organism with control for biomineral morphology.

Molecular dynamics simulations have been used to model the interaction between ethanol, water, and the {1014} surface of calcite. Our results demonstrate that a single ethanol molecule is able to form two interactions with the mineral surface (both Ca-O and O-H), resulting in a highly ordered, stable adsorption layer. In contrast, a single water molecule can only form one or other of these interactions and is thus less well bound, resulting in a more unstable adsorption layer. Consequently, when competitive adsorption is considered, ethanol dominates the adsorption layer that forms even when the starting configuration consists of a complete monolayer of water at the surface. The computational results are in good agreement with the results from atomic force microscopy experiments where it is observed that a layer of ethanol remains attached to the calcite surface, decreasing its ability to interact with water and for growth at the {1014} surface to occur. This observation, and its corresponding molecular explanation, may give some insight into the ability to control crystal form using mixtures of different organic solvents.