Stability of n-hexadecane phase change material emulsions using sugar-based surfactants and the HLD platform

Abstract

Storing thermal energy and using renewable energy are some of the most important challenges humanity faces today. Using phase change materials (PCM) is a suitable solution to prevent heat loss. PCM emulsions (PCMEs) have several advantages and are specifically applied in heat exchangers. Here, the formulation of these emulsions, the type of surfactants, and their composition based on freeze–thaw cycle stability are investigated. The Hydrophilic–Lipophilic-Deviation (HLD) platform is used for this purpose. Considering the importance of stability during temperature changes in PCMEs, three types of surfactants that have different behaviors with respect to temperature are used: ethoxylates, ionics, and sugar-based surfactants. By performing freeze–thaw cycles on emulsions made by individual surfactants, after 10 freeze–thaw cycles it was observed that the oil separation in each of the samples containing polysorbate80, AOT, lauryl-myristyl-alcohol-polyethylene-glycol-ether, SDS, sorbitan-monooleate, and decylglucoside occurred at the 2nd, 4th, 6th, 7th, 8th, and 10th-cycle, respectively. Therefore, the sorbitan-monooleate and decylglucoside had better freeze–thaw cycle stability compared to the others because of their non-sensitivity to temperature. We also applied the emulsion cycle stability with two combinations of surfactants: (1) polysorbate80 and sorbitan-monooleate, (2) decylglucoside and sorbitan-monooleate. The former, despite the effect of temperature, showed no oil phase separation in the nine freeze–thaw cycles, but in the latter, the oil phase separation occurred in the 5th-cycle. This study shows that synergism of surfactants is the most important factor in the stability of emulsions. To better explore these emulsion systems, the storage stability, freeze–thaw stability, droplet morphology, and viscosity tests were performed on different samples at various HLD values.