Molecular-Level Understanding of Phase Stability in Phase-Change Nanoemulsions for Thermal Energy Storage by NMR Spectroscopy.

Phase change materials (PCMs) are latent heat storage materials that can store or release thermal energy while undergoing thermodynamic phase transitions. Organic PCMs can be emulsified in water in the presence of surfactants to enhance thermal conductivity and enable applications as heat transfer fluids. However, PCM nanoemulsions often become unstable during thermal cycling. To better understand the molecular origins of phase stability in PCM nanoemulsions, we designed a model PCM nanoemulsion system and studied how the molecular-level environments and dynamics of the surfactants and oil phase changed upon thermal cycling using liquid-state nuclear magnetic resonance (NMR) spectroscopy. The model system used octadecane as the oil phase, stearic acid as the surfactant, and aqueous NaOH as the continuous phase. The liquid fraction of octadecane within the nanoemulsions was quantified noninvasively during thermal cycling by liquid-state