The Effect of Emulsifier Type and Droplet Size on Phase Transitions in Emulsified Even-Numbered n-Alkanes

Phase transitions in emulsified even-numbered n-alkanes (C₁₆, C₁₈, and C₂₀) are studied as a function of droplet size (0.15–3.45 μm) and surfactant type (polyoxyethylene sorbitan monolaurate or caseinate) using microcalorimetry (DSC) and ultrasonic attenuation measurements (2.25 MHz). Two DSC exothermic peaks were observed during the heating of C₁₈ and C₂₀ emulsions stabilized by Tween 20: a minor peak around 15 and 25 °C, respectively, and a major double peak about 10 °C higher. We tentatively attribute the minor peak to crystal-rotator phase transition, and the split major peak to melting of the surface and core of the droplets. The C₁₆ emulsions showed similar behavior for the major melting peak (15 °C), but the minor peak was absent-possibly as the sample was not cooled enough to cause the rotator phase to enter the low temperature crystalline state. For similar sodium caseinate stabilized emulsions of C₁₈ and C₂₀, the minor peak was much less pronounced (~25%), which we attribute to the lack of compatibility between the alkane and protein molecules. There were two ultrasonic attenuation peaks for the melting of C₁₈ and C₂₀ and one for C₁₆ corresponding to the DSC peaks. In all cases, the magnitude of the attenuation decreased with increasing particle size. Using an extended scattering theory approach we were able to relate the changes in ultrasonic attenuation to the changes in the effective physical properties of the alkane molecules during melting.