Molecular Origins of Phase Stability in Phase-Change Nano-Emulsions for Thermal Energy Storage by NMR Spectroscopy

Author

Jungeun Park, CUNY City CollegeFollow

Date of Award

2024

Document Type

Dissertation

Department

Chemical Engineering

First Advisor

Robert J. Messinger

Keywords

Thermal Energy Storage, PCM nano-emulsion, Phase stability of PCM nano-emulsion, Supercooling, NMR, Rheo-NMR, MRI Velocimetry

Abstract

Phase change materials (PCMs) are latent heat storage materials that can store or release thermal energy during phase transitions. Organic PCM nano-emulsions are formed by emulsifying oil in water in the presence of surfactants, enhancing thermal conductivity and pumpability. However, PCMs nano-emulsions can become unstable due to repeated thermal cycling and shear in heat transfer systems.

To better understand the molecular origins of phase instability in PCMs nano-emulsions, liquid-state nuclear magnetic resonance (NMR) measurements were applied to a model PCM nano-emulsion. Quantitative ¹H single-pulse NMR measurements established the liquid fraction of oil within nano-emulsions as a function of temperature, providing insight into octadecane supercooling. Quantitative ¹³C single-pulse measurements revealed that the surfactant head groups existed in multiple environments while their signal intensities decreased upon repeated thermal cycling, leading to a loss of molecular mobility. The results explain, in part, the origin of phase instability upon thermal cycling.

To investigate the simultaneous effects of shear and thermal cycling, ¹H rheo-NMR and MRI velocimetry methods were applied on the model PCM nano-emulsion in a concentric double cylinder at different rotating frequencies during thermal cycling. Double-slice selection was applied to measure the velocity profile and concentration distribution of the oil within the gap, revealing non-linear velocity profiles and the shear-induced mass transport of nano-emulsion droplets. ¹H NMR Transverse relaxation times increased at higher shear rates, indicating increased molecular mobility due to shear-induced structural changes.

Overall, this dissertation presents the effective and non-invasive applications of NMR spectroscopy, rheo-NMR, and MRI velocimetry to understand and differentiate the effects of thermal cycling and shear on the phase stability of PCM nano-emulsions up from the molecular level.

Recommended Citation

Park, Jungeun, "Molecular Origins of Phase Stability in Phase-Change Nano-Emulsions for Thermal Energy Storage by NMR Spectroscopy" (2024). CUNY Academic Works.
https://academicworks.cuny.edu/cc_etds_theses/1131