Crystallization in miniemulsion droplets

Undercooling and crystallization in stable nanodroplets with a narrow size distribution were analyzed using differential scanning calorimetry (DSC) measurements. Hexadecane droplets in water and NaCl solution droplets in petrolether were prepared using the miniemulsion process. It was found that the undercooling required to obtain crystallization in such droplets is significantly increased, compared to the bulk material: for the hexadecane droplets, a shift from 12°C (bulk) to approximately -4°C (droplets) is observed, and for the NaCl solution, a shift from -22°C (bulk) to -46°C (droplets) is observed. This is explained by the fact that, in miniemulsions, each droplet must be nucleated separately and the nucleation mechanism is shifted from heterogeneous to homogeneous nucleation. The undercooling additionally increases as the temperature decreases, which is explained with finite size effects for spinodal decompositions. The interfacial tension does not have any influence on the crystallization process. It was found that the crystallization rate in miniemulsion droplets is higher than that of the bulk and is proportional to the droplet size.