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This thesis documents the development and application of a computer model for gibbsite, an aluminium tri-hydroxide polymorph. In particular, the work has emphasized the idea of computer modelling techniques combining with ex- observations to provide greater insight than either could separately. Chapter One provides an overview and introduction to the fields of solid state chemistry, crystallization and computer modelling. These ideas are extended in Chapter Two to include a more detailed discussion of the theoretical principles behind the modelling in this project. The development of transferable oxalate and hydroxide potential models, intended primarily for sodium oxalate and gibbsite, is described in Chapter Three. Both ab initio hypersurface fitting and lattice fitting techniques were utilized, with an average structural fitting error of under two percent. In addition, the potentials were used to successfully reproduce several (related) crystal structures, thus establishing the quality of the model. In Chapter Four, the model for gibbsite was employed in generating equilibrium and growth morphologies. The equilibrium morphology was found to give excellent agreement with experiment, with all observed faces present. However, the importance of the prismatic planes is underestimated. Also discussed in the chapter is a method for predicting the phenomenon of crystalline twinning. This technique was successfully applied to a number of systems, including gibbsite and sodium oxalate. In Chapter Five, the equilibrium morphology calculations performed earlier were extended by probing the effects of cation incorporation on the habit of gibbsite. This study was conducted in order to provide a first step in estimating the role of the crystallizing solution. Calculations of the change in surface energy caused by the replacement of a surface proton with a cation from solution ++ / were made. Different crystal habits were constructed by applying a range of defect surface coverage values to each of the faces appearing in the morphology. The resulting defect morphologies were in excellent agreement with crystal habits commonly observed by experimentalists. Also, the work provided an explanation for the earlier underestimation of the prismatic faces. Chapter Six documents molecular simulations of solutions containing the major species known to be present in industrial and experimental Bayer liquors. The structuring in two solutions, one containing sodium hydroxide and the other potassium hydroxide, was probed by constructing graphs of the radial distribution functions. These plots indicated that a significant degree of ion pairing was occurring between the alkali metal cations (Na+ and K+) and the aluminate monomer ([Al(OH)4(subscript)]-). Furthermore, these cations were found to be acting as 'bridges' which stabilize multiple aluminate monomers, allowing them to form clusters. This data was used to assist in explaining vibrational spectra, and to postulate that clustering may be the origin of the fine particle suspensions noted during the induction period.

Reactivating quiescent cells to proliferate is critical to tissue repair and homoeostasis. Quiescence exit is highly noisy even for genetically identical cells under the same environmental conditions. Deregulation of quiescence exit is associated with many diseases, but cellular mechanisms underlying the noisy process of exiting quiescence are poorly understood. Here we show that the heterogeneity of quiescence exit reflects a memory of preceding cell growth at quiescence induction and immediate division history before quiescence entry, and that such a memory is reflected in cell size at a coarse scale. The deterministic memory effects of preceding cell cycle, coupled with the stochastic dynamics of an Rb-E2F bistable switch, jointly and quantitatively explain quiescence-exit heterogeneity. As such, quiescence can be defined as a distinct state outside of the cell cycle while displaying a sequential cell order reflecting preceding cell growth and division variations.

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Computer programs have been written which allow morphological calculations based on a knowledge of internal crystal structure to he carried out. Details of the programs are presented along with the guidelines developed for their use. The programs were used to compare and contrast the current methods employed for relating crystal shape to structure and to confront specific problems in that field. Calculations on a range of compounds show that the morphologies derived from the simple Donnay-Harker (DHI) model give almost as good a fit to the observed form as the more sophisticated attachment energy (AE) calculations except when strong bonding directions were present. In the first study of its type all the methods currently favoured in the literature including the Ising and PBC approaches as well as DH and AE models were applied to benzophenone. All the models gave the same theoretical morphology. One problem remaining in the field of relating crystal structure and morphology is that of polar morphology. None of the current methods can account for a polar morphology. Surface, bulk, isolated molecule charge distributions were used in a modification of the classical attachment energy model to account for urea exhibiting a polar morphology when growth from the vapour phase. For the disruptive type of tailor made additives an improvement in the current methodology is proposed with the calculation of an additional parameter. This additional parameter accounts for the morphology with an additive present and gives good agreement with the test case of benzamide crystals grown with benzoic acid as an additive. The additive approach also allowed the effects of toluene solvent on the crystal habit of benzophenone to be considered By treating toluene as a tailor made additive it was possible from calculations to identify the likely sites of toluene incorporation and the subsequent effect on crystal growth. The results from the calculation were consistent with experiment.