Kinetics of Crystallization in n-Hexadecane and Cocoa Butter O/W Emulsions Accounting for Droplet Collision-Mediated Nucleation

Hindle S., Povey M.J.W., Smith K.W., Kinetics of Crystallization in n-Hexadecane and Cocoa Butter Oil-in-Water Emulsions Accounting for Droplet Collision-Mediated Nucleation, Journal of Colloid & Interface Science, 2000, 232, 370-380

Droplet collision is shown to be important in the propagation of nucleation through supercooled oil-in-water emulsions by the use of a novel membrane technique. On the other hand, nucleation mediated by Tween 20 micelles is shown to be of relatively much less importance in both n-hexadecane and cocoa butter oil-in-water emulsions. The droplet collision phenomenon probably occurs via a surfactant bridge between the colliding droplets. When this process is taken into account we show that the Turnbull model for crystal nucleation kinetics explains very well nucleation in cocoa butter oil-in-water emulsions by seed crystals. On the basis of this model we characterized the seed crystals in cocoa butter via isothermal crystallization studies at 14.2, 15.0, 15.5, and 15.8°C. We suggest that there are few seed crystals whose size exceeds 0.28 μm at 80°C. In our cocoa butter samples there were between 10¹⁶ and 10¹⁷ seed crystals m⁻³ whose average size we inferred to be less than 0.09 μm. A value of 0.133 mJ m⁻² is obtained for the Gibbs free energy of the nucleating surface in our West African cocoa butter samples. There is evidence that the α-polymorph of POS comprises the nucleating layer in the seed crystal. There is no evidence that surfactant influences the primary nucleation of oil crystals. However, the surfactant has a big effect on the kinetics of the secondary nucleation process, mediated by droplet collision.