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

<p>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.</p> <p>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.</p> <p>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.</p> <p>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.</p>

Name thesis:degree_name

Doctor of Philosophy (Ph.D.)

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Chemical Engineering

Year dc:date.available

2024

Author dc:creator

• Park, Jungeun

Contributors dc:contributor

• Robert J. Messinger

• Thermal Energy Storage
• PCM nano-emulsion
• Phase stability of PCM nano-emulsion
• Supercooling
• NMR
• Rheo-NMR
• MRI Velocimetry
• Chemical Engineering
• Complex Fluids
• Engineering

Repository record dc:identifier

https://academicworks.cuny.edu/cc_etds_theses/1131

OAI identifier oai:identifier

oai:academicworks.cuny.edu:cc_etds_theses-2208