The Crystallization and Nucleation of Stearic Acid Containing Molecules Under Non-Isothermal Cooling Conditions
Crystallization is commonly used in the production of many products such as ice cream, butter and chocolates. Due to the practical limits of the equipment, crystallization in the industry occurs non-isothermally. Currently, there are a limited number of models which can characterize the crystallization behaviour under such conditions. Crystallization under non-isothermal cooling conditions was studied by using molecules with a stearic acid moiety. These stearic acid containing molecules were selected for their different dimensional crystal growths. 12-hydroxystearic acid (12HSA) was selected to represent one-dimensional crystal growth, stearic acid for two-dimensional crystal growth and trihydroxystearin for three-dimensional crystal growth. In study 1, the modified Avrami model was experimentally validated to model crystallization using non-isothermal cooling conditions. Four techniques were tested which included: small deformation rheology, differential scanning calorimetry, polarized light microscopy and Fourier transform infrared spectroscopy (FT-IR). The experimental validation of the model was done by accurately fitting the parameters of the modified Avrami model; such as induction time, maximal phase change and the Avrami exponent; to the data. FT-IR was the most accurate method because the data collected fitted well to the modified Avrami model. The Avrami exponent obtained from FT-IR was the only technique to be sensitive to both the mode of nucleation as well as the dimensionality of crystal growth. By using the modified Avrami model to characterize crystallization under non-isothermal cooling conditions, the apparent rate constant obtained from the model gave further insights to the kinetics of crystallization under these conditions. Study 2 investigated the nature of crystallographic mismatches in 12HSA fibres which causes branching due to the imperfect incorporation of 12HSA molecules into the crystal lattice. FT-IR was used to monitor the changes during crystallization in the 1700 cm-1 and 3200 cm-1 peaks which corresponded to the dimerization of carboxylic acid monomers and the formation of non-specific hydrogen bonding, respectively. When FT-IR data was fitted to the modified Avrami model, the rate constants obtained increased linearly with the cooling rate for hydrogen bonding while the dimerization of carboxylic acid monomers plateaued at cooling rates above 5 °C/min. Therefore at cooling rates above 5 °C/min, 12HSA does not effectively dimerize when incorporating into the crystal lattice which causes crystal imperfections leading to branching in 12HSA fibres. In study 3, the activation energy for nucleation under non-isothermal cooling conditions was determined using a statistical method. The activation energies for stearic acid, 12HSA, trihydroxystearin and triglycerides were 1.52 kJ/mol, 5.40 kJ/mol (Rogers & Marangoni, 2009), 7.87 kJ/mol and 24.8 kJ/mol (Marangoni, Tang, & Singh, 2006); respectively. The activation energy for nucleation for a molecule is partially affected by its polarity relative to the solvent such that an increase in polarity would result in a decrease in activation energy. However, this was not always observed as the activation energy for stearic acid was less than that for 12HSA. Since the polarity of the molecule does not fully explain the activation energy, a specific interaction was used to account for the larger activation energy observed in 12HSA. The specific interaction describes how molecules are arranged in a nucleus and its ability to hide the polar groups away from the crystal-solvent interface. When the polar groups were not effectively hidden, an increase in the activation energy for nucleation was observed.
12-hydroxystearic acid, trihydroxystearin, stearic acid, non-isothermal, nucleation, crystallization, kinetics, Avrami model, activation energy, crystallographic mismatches
Master of Science (M.Sc.)
Food and Bioproduct Sciences