Calcium carbonate crystallisation at the microscopic level

The primary concern of this thesis is the investigation of crystal nucleation and growth processes, and the effect of foreign substrates on the rate, extent and mechanism of crystallisation, with particular emphasis on the calcium carbonate system. A methodology, based on the in-line mixing of two stable solutions, which permits the continuous delivery of a solution with a constant, known supersaturation, has been developed and characterised. This has been used to induce CaCO_3 crystallisation in experimental systems involving the channel flow and wall jet techniques. The channel flow method has been adapted to facilitate the study of crystal growth at a single calcite crystal. Ca"2"+ ion selective electrodes have been employed as a means of monitoring depletion of the supersaturated solution, downstream of the crystal substrate. The data obtained suggested a growth rate constant of 3x10"-"1"2 mol cm"-"2 s"-"1 (and a reaction order of 1.52 on supersaturation). The ex-situ techniques of optical microscopy and atomic force microscopy (AFM) were employed to visualise changes in the calcite surface topography resulting from exposure to the growth solution. A technique based on an impinging jet of supersaturated solution was developed and characterised as a method for inducing crystal growth on foreign substrates under defined hydrodynamic control. When used in conjunction with the ex-situ techniques of scanning electron microscopy (SEM), optical microscopy and micro-Raman spectroscopy, the role of substrate and supersaturation on the morphology and polymorphology of the CaCOs microcrystals was determined. The technique also proved to be a powerful tool for the evaluation of scale inhibiting surface coatings. The combination of the impinging jet method with thin transparent substrates allowed in-situ observation, through optical microscopy, of the induction and growth of CaCO_3 microcrystals on foreign substrates. A number of substrates, displaying various surface energies and chemistries, allowed the study of the role of the substrate on crystal induction. Crystal growth and induction time data were analysed in terms of transport versus surface-controlled mechanisms. Crystal induction was catalysed to a greater extent on higher energy surfaces. The effect of protonation of acidic and basic surfaces on crystal induction has also been investigated by electrochemical control. AFM was used to study cleaved calcite crystals before and after growth in a Kitano solution, followed by in-situ observation of crystal growth under a Kitano solution. This revealed the mechanism to be through the growth of islands. The combination of an AFM with an inverted optical microscope allowed the same measurements to be made on freshly grown, calcite microcrystals, with observation of a single growth spiral on each. The effect of a known growth inhibitor (maleic acid) was also investigated in-situ. This was found to change the morphology of the growth spiral on microcrystals in a manner consistent with preferential adsorption. (author)Available from British Library Document Supply Centre- DSC:DXN059424 / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo