Nucleation monitoring in cooling crystallization with a quartz crystal oscillator Original Research Article

A crystallization monitoring system using a quartz crystal oscillator was implemented in the cooling crystallization of dilute lauric acid solutions for the investigation of the nucleation process of the solute. In addition, the microscopic observation of the oscillator surface was conducted to examine the number and size of yielded nuclei, and the observed results and the resonant frequency variation of the oscillator were analyzed to explain the nucleation process. The change of the resonant frequency indicated the beginning moments of emulsification and nucleation of lauric acid near the oscillator surface. The experimental results show that the average crystal size was raised from 139 to 238 nm and the nucleation rate was reduced as the solution concentration increased from 0.05 to 0.25 g/L. When the solution temperature lowers, the crystal grows more rather than new nuclei form in the high concentration solution. The estimated crystal mass from microscopic observation ranges between 18% and 28% of that from the frequency measurement. It was also demonstrated that the proposed monitoring device is useful to investigate the crystallization process in dilute solutions, which often requires large analytical instruments.