(450b) Molecular Origins of Phase Stability in Model Phase-Change Nano-Emulsions for Thermal Energy Storage Elucidated 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 are of particular interest of large-scale applications because they are low cost, melt congruently, and exhibit good nucleating properties. Their major disadvantage is low thermal conductivity. By forming PCM nano-emulsions in which the organic PCMs are emulsified in water in the presence of surfactant, thermal conductivity increases significantly as well as the transport property does. However, PCMs nano-emulsions often become unstable due to repeated melting and freezing cycles, which can further be affected by shear flow during heat transfer applications. The surfactant distribution and dynamics upon thermal cycling and shear are poorly understood, but correlate with observed phase and heat transfer instabilities during their application.

To better understand the molecular behavior of surfactants upon thermal cycling and how it affects the stability of PCMs nano-emulsions, we designed a molecularly simple PCM nano-emulsion as a model system using octadecane as an oil, stearic acid as a surfactant and pH-adjusted water as the continuous medium. Solution-state ¹H and ¹³C nuclear magnetic resonance (NMR) experiments were applied to a PCM nano-emulsion synthesized using stearic acid, half of which was uniformly ¹³C-labeled. The liquid content of octadecane within the nano-emulsions was measured quantitatively during melting and freezing, revealing a wide temperature range of solid-liquid co-existence that appears to be linked to phase instability. Interestingly, quantitative single-pulse ¹³C NMR experiments establish that the carbonyl surfactant head groups are in two different electromagnetic environments. ¹³C longitudinal (T₁) measurements also reveal two different T₁ relaxation times for the surfactant head groups. After repeated thermal cycling, ¹H signal intensities of surfactant head groups decrease (e.g., by 33% over 120 cycles), indicating that the surfactant head groups lose molecular mobility, a key finding whose origins are still under investigation. ¹H pulsed-filed-gradient (PFG) NMR methods were performed to measure the diffusion coefficients of the octadcane PCM nano-emulsions upon thermal cycling. ¹H chemical shift, relaxation, and diffusion data were measured over different temperatures and number of thermal cycles, which are analyzed as potential indicators of phase stability. To generalize our findings, we synthesized a more complicated PCM nano-emulsion system by replacing stearic acid with Tween and Span mixed surfactants. The above NMR experiments were performed and the results were compared with the stearic acid model system. Overall, the results lay the groundwork towards molecular-level understanding of phase stability under thermal cycling in oil-water-surfactant PCM nano-emulsions for thermal energy storage.