(698b) Molecular-Level Understanding of Phase Behavior in Phase-Change Nano-Emulsions for Thermal Energy Storage

Phase Change Materials (PCMs) are latent heat storage materials that can store or release large quantity of energy while undergoing thermodynamic phase transitions. Organic PCMs are low cost, melt congruently, and exhibit good nucleating properties. To enable control over mixture transport properties, enhance thermal conductivity, and reduce cost, organic PCMs can be emulsified in water in the presence of surfactants to form PCM nano-emulsions. However, PCMs nano-emulsions can lose phase stability due to repeated melting and freezing processes during thermal cycling, which can further be deteriorated by shear flow during their applications in heat transfer systems. The distribution and dynamics of surfactants, and how they respond upon thermal cycling and shear, are poorly understood, but are expected to correlate with the observed phase, momentum, and heat transfer instabilities during their use.

To better understand the molecular-level origins of phase stability in PCMs nano-emulsions, multi-dimensional ¹H and ¹³C solution-state nuclear magnetic resonance (NMR) methods have been applied to a molecularly simple octadecane-water-stearic acid system under thermal cycling. Octadecane freezing and melting were readily observed, yielding information on octadecane supercooling effects. ¹H pulsed-field-gradient (PFG) NMR methods were applied to measure the diffusion coefficients of the octadecane nano-emulsions, enabling estimates of their average sizes by the Stokes-Einstein equation. Chemical shift, relaxation, and diffusion of the component species were examined as indicators of phase stability as a function of temperature and number of thermal cycles. The results lay the groundwork towards molecular understanding of phase stability under thermal cycling in oil-water-surfactant phase-change nano-emulsions.