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Title: Quantitative crystal investigations : surface dissolution and nucleation processes
Author: Parker, Alexander S.
ISNI:       0000 0004 6350 859X
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2016
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The aim of this thesis is twofold with both elements related to industrially relevant crystal systems and processes. The first element utilized a combined scanning electrochemical cell microscope (SECCM) and atomic force microscopy (AFM) method to study the dissolution of enamel surfaces with controlled proton flux to the surface. This was then extended to investigate the effect of both known surface treatments, fluoride and zinc ions, but also a novel treatment of calcium silicate and its methods of action. The second element investigated the use of a nanopipette to investigate the early nucleation and initial growth of calcium carbonate crystals. For the dissolution of enamel, an SECCM probe to selectively etch a surface for a defined period of time with a high spatial resolution was used. The extent of the etching in the resultant pits was then monitored through AFM to measure the volume of material removed along with other pit dimensions. The method allowed for multiple independent measurements on a single sample, which could be selectively treated to eliminate comparability issues associated with measurements on multiple samples. The system could be modelled via finite element method (FEM) to calculate an intrinsic rate of reaction for the proton induced dissolution of enamel. A proton induced rate constant of dissolution of k0= 0.099 ± 0.008 cm s-1 for bare untreated enamel was established, whereas treatment with 1000 ppm sodium fluoride (NaF) and/or zinc chloride (ZnCl2) decreased this rate constant. The work also characterised the use of calcium silicate as a novel additive in toothpaste and to determine its effect as both a remineralising agent and as a dissolution inhibitor. The release of Ca2+ ions into solution was measured which acts to promote the remineralisation of tooth enamel. The addition of phosphate buffer into this solution combined with micro-Raman spectroscopy was then used to confirm the formation of hydroxyapatite (HAP (Ca10(PO4)6(OH)2)) material. The extent of adhesion of calcium silicate onto rough and polished samples was also observed, showing the preference of particles to adhere to rough surfaces, and was quantified by investigating the effect of infilling of etch pits formed via the SECCM method above, which showed an average pit volume reduction of 77±12%. The second element of the thesis involved investigation into the initial phase of nucleation, nanoprecipitation and growth of calcium carbonate crystals using voltage driven ion migration within a nanopipette (~50 nm opening) geometry to control the mixing of constituent ions to selectively control and induce the nucleation and dissolution of crystals and monitor their growth. This was achieved using oppositely charged CO32- and Ca2+ ions, inside and outside the pipette respectively, which could be either driven together or apart depending on the applied polarity. This process was modelled using FEM to give quantitative information about the growth rate and nanocrystal size during growth as well as analysis of the saturation levels within the probe geometry. The nanocrystals formed were studied in situ using micro-Raman spectroscopy to give information about the polymorph of calcite produced. The effect of the driving bias was demonstrated and rationalised through simulation along with the effect of constituent ion concentration. This method was used to assess the effect of maleic acid as an inhibitor to the formation of calcium carbonate. Its potent effect was shown by the significantly larger time taken to block the pipette by crystal growth. This also provided evidence for the mechanism of crystal growth inhibition by comparison with ion concentrations expected as a result of a pure chelation effect.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QD Chemistry