پایان نامه بررسی دینامیکی پراکندگی مولکولهای آلی سبک و نمکهای یونی در حین تبلور خنک شونده

This project was concerned with observing the process of nucleation of crystalline species during the cooling crystallisation of supersaturated solutions prepared from both ionic salt and small organic solutes. Cooling crystallisation induces a steady temperature-dependant destabilisation of the supersaturated liquid phase where the chemical potential of the liquid state increases and so becomes less thermodynamically stable. This decrease in stability triggers the onset formation of the corresponding solid state, which has a lower chemical potential. This driving force permits the spontaneous formation of critical nuclei and their subsequent growth into larger crystalline structures. Desire to understand the processes occurring at the earliest stages of crystal nucleation have attracted much interest over the years. Dynamic light scattering experiments were employed to closely observe the precursors to crystal nucleation. More specifically the phenomenon of molecular clustering and their consequent dispersion to form solute-rich regions within solution, where the generation of viable critical nuclei would be most likely occur, according to current theory. The pre-nucleation mechanism for cooling crystallisation was observed to involve large nano-droplet sized molecules of non-specific composition, for solutions of urea and glycine and almost micron-sized droplets for solutions of sodium chloride and sodium nitrate. Interestingly, there was evidence that clustering of larger aggregates was a phenomenon not wholly restricted by supersaturation as they were found in undersaturated solutions of all prepared samples. The relative size of particle radii formed in pre-crystalline solution tended to depend on the extent of the supersaturation i.e., the concentration of the solute present within the solution. This is not true in all cases though as supersaturated glucose indicated. This may suggest an alternate pathway in the route of nucleation of crystals than was previously assumed. The theory of a metastable phase forming within the bulk solution, where critical nuclei are thermodynamically more stable and hence are able to grow once formed might not hold true for all crystallising solutions or another factor may need to be better understood and manipulated in order to claim more control over the desired process. Further understanding of this could give an increased degree of control over yield and quality of products in pharmaceuticals and other material producing industries