CRYSTALLIZATION IN OIL-IN-WATER EMULSIONS CONTAINING LIQUID AND SOLID DROPLETS

The kinetics of crystallization of supercooled oil droplets in emulsions containing a mixture of solid and liquid n-hexadecane droplets has been monitored at constant temperature from time-dependent measurements of the velocity of ultrasound. In emulsions stabilized by non-ionic surfactant (Tween 20 or C12E8) or anionic surfactant (sodium dodecyl sulphate, SDS), crystallization is induced in liquid droplets when solid droplets are present; in the absence of solid droplets crystallization proceeds at a negligible rate. Increasing the aqueous phase surfactant concentration leads to a substantial increase in the crystallization rate in Tween 20 emulsions. Ionic strength has no significant effect on the rate in SDS emulsions. The mechanism of this crystallization probably involves crystals on solid droplets penetrating into supercooled liquid droplets during collisions and thereby acting as nucleation sites for crystal growth. The process is enhanced by aggregated surfactant molecules bridging between the colliding droplets.