Abstract
Calcite, the most stable crystalline form of CaCO3, is both an important biomineral and a major constituent of host rocks in carbonate reservoirs, which host drinking water and natural oil and gas. When biological organisms grow their shells, they control the crystal morphology, size, orientation and even the crystal phase of the precipitated calcium carbonate, using a range of organic molecules and inorganic ions. This results in materials with physical and chemical properties that differ significantly from those of inorganically precipitated calcite. Gaining more insight into the surface reactivity of calcite and the effects of surface impurities will bring us one step closer to being able to synthesize biomimetic materials, which mimic the properties of biogenic calcite. In addition, increased knowledge about the surface reactivity of calcite in the presence of impurities will help us predict processes occurring in carbonate rich aquifers and oil reservoirs, with implications for drinking water supply, oil recovery and sequestration of CO2. In this thesis, I had two main focus points: The first and most extensive was the development of an experimental protocol for analysing the chemical composition and speciation in the calcite-water interface region and the use of this technique to determine the effects of inorganic species, CO2, Mg2+, Sr2+ and Ba2+ on the calcite-water interface. The second was to investigate the effects of two carboxylates, acetate and hexanoate, on calcite crystal growth, to determine the effects of the carboxylate functional group and the length of the alkyl chain.
The chemical speciation of the calcite-water interface controls surface processes such as sorption of impurities and crystal growth and dissolution but spectroscopic resolution of a wet calcite surface is difficult when the analytical technique requires ultrahigh vacuum so relatively few spectroscopic studies exist of the pure interface and very few studies of the effects of impurities are available. In my research, I developed a method to analyse calcite powders in equilibrium with solutions. My results suggest that CO2 adsorbs weakly from the gas phase and not at all from solution and that bicarbonate adsorbs at high (above 10 %) partial pressures of CO2. The alkaline earth metal ions could control the hydrophilicity of the surface and this was concentration dependent. This supports existing theories that cations present during biomineralisation can be used by organisms to control the affinity of the surface for the organic molecules used to regulate crystal morphology. My study of the effects of carboxylate anions on calcite crystal growth had two main conclusions: First, that the length of the alkyl chain significantly affects the interaction between the carboxylate ion and the growing calcite surface. Acetate had no effect on calcite surface growth in the studied concentration range, i.e. 2 mM to 25 mM, whereas hexanoate did. My results also suggest that hexanoate affects the two growth mechanisms for calcite, spiral growth and 2D nucleation, differently. I studied growth with two methods. I observed spiral growth on single crystals with atomic force microscopy and average growth, under the relevant conditions where supersaturation was high mainly 2D nucleation, of calcite powders with the constant composition setup. I observed an apparent promotion of spiral growth, while 2D nucleation was inhibited.
The ability of a small organic molecule to cause a switch between growth mechanisms has consequences for our understanding of biomineralisation and for the design of biomimetic materials, because it provides another mechanism for biological control on material properties.
The chemical speciation of the calcite-water interface controls surface processes such as sorption of impurities and crystal growth and dissolution but spectroscopic resolution of a wet calcite surface is difficult when the analytical technique requires ultrahigh vacuum so relatively few spectroscopic studies exist of the pure interface and very few studies of the effects of impurities are available. In my research, I developed a method to analyse calcite powders in equilibrium with solutions. My results suggest that CO2 adsorbs weakly from the gas phase and not at all from solution and that bicarbonate adsorbs at high (above 10 %) partial pressures of CO2. The alkaline earth metal ions could control the hydrophilicity of the surface and this was concentration dependent. This supports existing theories that cations present during biomineralisation can be used by organisms to control the affinity of the surface for the organic molecules used to regulate crystal morphology. My study of the effects of carboxylate anions on calcite crystal growth had two main conclusions: First, that the length of the alkyl chain significantly affects the interaction between the carboxylate ion and the growing calcite surface. Acetate had no effect on calcite surface growth in the studied concentration range, i.e. 2 mM to 25 mM, whereas hexanoate did. My results also suggest that hexanoate affects the two growth mechanisms for calcite, spiral growth and 2D nucleation, differently. I studied growth with two methods. I observed spiral growth on single crystals with atomic force microscopy and average growth, under the relevant conditions where supersaturation was high mainly 2D nucleation, of calcite powders with the constant composition setup. I observed an apparent promotion of spiral growth, while 2D nucleation was inhibited.
The ability of a small organic molecule to cause a switch between growth mechanisms has consequences for our understanding of biomineralisation and for the design of biomimetic materials, because it provides another mechanism for biological control on material properties.
| Original language | English |
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| Publisher | Department of Chemistry, Faculty of Science, University of Copenhagen |
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| Publication status | Published - 2018 |