Excess ultrasonic attenuation due to solid-solid and solid-liquid transitions in emulsified octadecane

The crystallization and melting of emulsified n-octadecane (C₁₈) is monitored by microcalorimetry and by ultrasonic (2.25 MHz) velocity and attenuation measurements. During cooling, the lipid showed two thermal events (a large exothermic DSC peak at 12.8°C and a smaller one at 7.8°C) and during heating showed a similarly spaced pattern in reverse (a minor endothermic DSC peak at 15.9°C and a major DSC peak at 27.3°C). The minor DSC peaks during heating and cooling were attributed to a transition between a rotator phase stable at higher temperatures and a crystal phase stable at lower temperatures. The major DSC peaks were attributed to a transition between the liquid and rotator phases. The major DSC peaks corresponded to small but measurable changes in the speed of sound in the emulsions, and the major and minor endothermic DSC peaks during heating corresponded to peaks in ultrasonic attenuation. The peaks in attenuation were not seen during the cooling cycle. Scattering theory provided a good model for the temperature-dependent ultrasonic properties of the emulsion with the exception of the peaks in attenuation observed during melting transitions. However, by calculating an effective specific heat, density, and cubical expansion coefficient over the melting transition from the microcalorimetry data, it was possible to extend scattering theory to include the excess attenuation on melting.