(474a) Molecular-Level Understanding of Phase Stability in Model Phase-Change Nano-Emulsions for Thermal Energy Storage Investigated By NMR Spectroscopy

Phase Change Materials (PCMs) are latent heat storage materials that can store or release large quantities of energy while undergoing thermodynamic phase transitions. Organic PCMs have been great interest of large-scale applications since they are low cost, melt congruently, and exhibit good nucleating properties. Organic PCMs can be emulsified in water in the presence of surfactant to enhance thermal conductivity significantly as well as the transport property. However, PCMs nano-emulsions often become unstable during thermal cycling due to repeated melting and freezing cycles, which can further be affected by shear flow during their applications in heat transfer system. However, the distribution and dynamics of surfactant upon thermal cycling and shear are poorly understood, but are expected to correlate with observed phase, rheological and heat transfer instabilities.

To better understand the molecular origins of stability in PCMs nano-emulsions, a model PCM nano-emulsion system was synthesized using octadecane as the oil phase, stearic acid as the surfactant, and dilute aqueous NaOH as the medium. Solution-state nuclear magnetic resonance (NMR) methods have been applied to a PCM nano-emulsion in the presence of ¹³C-enriched (50% uniformly ¹³C-labeled) stearic acid under thermal cycling. Freezing and melting of ¹H octadecane nano-emulsion were investigated quantitatively, yielding information on octadecane supercooling and the content of liquid and oil within octadecane. Quantitative single-pulse ¹³C NMR methods applied to show that the ¹³C carbonyl head groups of the surfactants were present in multiple local environments and their signal intensities decreased upon thermal cycling thus losing molecular mobility (>100 cycles). Rheo-NMR and velocimetry methods were applied to measure the temperature effects on the flow of PCM nano-emulsion compared to the flow of water at room temperature, at different shear rates. As the octadecane changes phase from solid to liquid, the shear banding disappears and the velocity profile returns to near-Newtonian behavior. We analyze the shear banding effects in terms of a mechanical instability due to emulsion aggregation, which changes the local viscosity. NMR chemical shift, relaxation, and flow behavior of the component species were investigated as indicators of phase stability. Overall, the results lay the groundwork towards molecular-level understanding of phase stability under thermal cycling and shear in PCM nano-emulsions for thermal energy storage.