Impurity effects on crystallization rates of n-hexadecane in oil-in-water emulsions

Crystallization rates of n-hexadecane dispersed in oil-in-water (O/W) emulsion toil 20 wt% and water 80 wt%) were studied by an ultrasonic velocity measurement technique. Tween 20 was employed for emulsification to form the O/W emulsion. A highly hydrophobic food emulsifier, a sucrose polyester involving palmitic acid moiety (P-170), was added to n-hexadecane, as an impurity, in an attempt to modify the rate of the crystallization of n-hexadecane. The rates of crystallization of n-hexadecane were monitored by variations in the ultrasonic velocity values, which increase with increasing amount of crystal fractions in the oil phase of the emulsion. In comparison with the results of the O/W emulsion systems with those of the bulk systems, the following results were obtained; (a) the addition of P-170 accelerated the nucleation in the emulsion system, yet no acceleration was revealed in the bulk system, (b) the rates of crystal growth were retarded by the addition of P-170, (c) the acceleration of nucleation occurred through two stages with increasing concentrations of P-170: e.g., a sharp increase occurred in a low concentration range of P-170 up to 0.3% with respect to n-hexadecane, then a moderate acceleration occurred at P-170 concentrations above 0.6%. These results were discussed in terms of heterogeneous nucleation occurring at interfaces of the O/W emulsion droplets. (C) 1999 Elsevier Science B.V. All rights reserved.