Journal of Crystal Growth 197 (1999) 263—270 Impurity effects on crystallization rates of n-hexadecane in oil-in-water emulsions N. Kaneko, T. Horie, S. Ueno, J. Yano, T. Katsuragi, K. Sato*  Faculty of Applied Biological Science, Hiroshima University, Higashi, Hiroshima 739-8528, Japan  Mitsubishi Chemical Co., Yokohama Research Center, Kamoshida, Aoba-ku, Yokohama 227-8502, Japan Received 23 July 1998; accepted 17 August 1998 Abstract Crystallization rates of n-hexadecane dispersed in oil-in-water (O/W) emulsion (oil 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.  1999 Elsevier Science B.V. All rights reserved. Keywords: Emulsion crystallization; Heterogeneous nucleation; n-Hexadecane; Ultrasonic velocity measurement; Impu- rity effect 1. Introduction There is an increasing interest in exploring crys- tallization phenomena occurring in encapsulated systems like emulsions, vesicles, forms, etc., with an * Corresponding author. Fax: #81 824 22 7062; e-mail: kyosato@ipc.hiroshima-u.ac.jp. attempt to develop novel methods of crystallization for the production of functional materials. In case of the emulsion systems where an oil phase is emul- sified in a water phase (O/W emulsion), it has been known that a transformation from liquid oil to fat crystals remarkably influences the physical proper- ties of the emulsions, such as emulsion stability, rheology, appearance, etc. [1—4]. Therefore, it is important to clarify the mechanisms of nucleation 0022-0248/99/$ — see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 0 9 3 3 - 6