Formulation of highly stable PCM nano-emulsions with reduced supercooling for thermal energy storage using surfactant mixtures

This study was to develop stable phase change material (PCM)-water nano-emulsions with low supercooling and low viscosity using two-surfactant mixtures by the phase inversion temperature (PIT) method. The PCM agent n-hexadecane was mixed with Brij L4 as the primary surfactant and polyethylene-block-poly(ethylene glycol) (PE-b-PEG), Tween 60 or Tween 80 as a co-surfactant to form stable nano-emulsions. The results showed that the droplet size, viscosity and PIT point as well as the stability of PCM nano-emulsions were strongly dependent on the combination, mass ratio and total concentration of the two surfactants. In 25% (w/w) PCM nano-emulsions, an optimum formulation was the surfactant combination of Brij L4 and Tween 60 at 6:4 mass ratio, mixed with PCM at 11:20 mass ratio. The corresponding nano-emulsion had a high stability, a small droplet size of 60 nm and desirable Newtonian fluid behaviour with a relatively low viscosity of 50 mPa s. The addition of n-octacosane as a nucleating agent was effective to reduce the emulsion supercooling, though the supercooling degree was notably increased after multiple thermal cycles. The optimized 25% PCM nano-emulsion with 2% (w/w) n-octadecane showed an excellent stability over 120 days and 300 thermal cycles with a droplet size below 80 nm, a supercooling degree of ~5 °C and a latent heat of ~50 J/g, which was very promising for further development and application as a cooling storage medium.