Hexadecane-in-water emulsions as thermal-energy storage and heat transfer fluids: Connections between phase-transition temperature and period of hexadecane droplets dispersed in hexadecane-in-water emulsions and characteristics of surfactants

Abstract

Colloidal stability and fluidity of organic phase change material (OPCM)-in-water emulsions (OPCM/W) against temperature change and supercooling (solidifying-temperature; Ts) of OPCM droplets in OPCM/W emulsions were examined to develop the thermal-energy storage and heat transfer fluids of OPCM/W emulsions. Higher colloidal stability and fluidity of OPCM/W emulsions against temperature change in the range of −1–40°C were achieved by decrease in the droplet size below ∼2μm in diameter. Furthermore, the solidifying-temperature (Ts) of OPCM droplets in OPCM/W emulsions increased with longer hydrocarbon chain length of surfactants. Consequently, the solidifying-period (ts) of OPCM droplets in the OPCMW emulsion was shortened with longer hydrocarbon chain length of surfactants. On the other hand, Pluronic-type surfactants with poly(propylene oxide) as a hydrophobic part did not affect Ts and ts of OPCM droplets in the emulsions. These indicate that hydrocarbon chain of surfactants acts as nuclei in the droplets to promote the solidification of OPCM droplets in OPCM/W emulsions. We also revealed that the precipitation of surfactants in OPCM/W emulsions promotes the solidification of OPCM droplets in OPCM/W emulsions. So, the nucleation of OPCM in droplets by hydrocarbon chain and precipitation of surfactants in OPCM/W emulsions prevents the supercooling of OPCM droplets in OPCM/W emulsions.