UBC Theses and Dissertations

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UBC Theses and Dissertations

A molecular dynamics investigation of the dissolution of molecular solids Anand, Abhinav


The dissolution of molecular solids is an important process, which has been studied for over a century. However, a lot of work is still needed for a detailed understanding of the molecular mechanism of dissolution, because of the complex nature of many molecular solids, and the large time scales required for simulation studies. In this thesis we study the dissolution of molecular solids, to examine if classical models (which assume that the rate is proportional to an active surface area) can be used to describe the dissolution profile of these solids. Urea and aspirin molecules are used as models, to study the dissolution process in water under sink conditions, because of their contrasting solubility in water. The dissolution rate in different water models was examined and it was found that they differ considerably. However, the overall mechanism for the dissolution process remains the same. Dissolution was found to be an activated process with the detachment of molecules from the crystal being the rate limiting step. Crystals with different shapes (cubic and cylindrical) were used to study the effect of shape on the dissolution process. The dissolution process for urea was found to occur in three steps, an initial rapid stage, where the molecules at the edges and corners go into the solution, a long intermediate stage with a nearly constant dissolution rate, and a final stage where the crystals lose their crystalline structure and dissolve completely. The fixed rate law stage was found to be described by a simple rate law derived from classical models. It was found that there is an additional step in the dissolution process for aspirin, occurring between the initial rapid stage and the fixed rate law stage, during which the crystal attains a solution annealed shape. The fixed rate law stage was again found to be described by a simple rate law. The results obtained are in agreement with an earlier dissolution study of NaCl crystals, thus it appears that the classical rate laws can be used to describe the dissolution of a variety of complex molecular and ionic crystals.

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