Microencapsulated Alkane Wax Melting: Measured by 1H NMR Relaxometry and Diffusometry

Abstract

Phase change materials (PCMs) are useful for energy storage. Large latent heats make n-alkane waxes effective PCMs. Octadecane and eicosane were studied as both unencapsulated and microencapsulated with NMR relaxometry and diffusometry throughout melting. Each wax was stepped degree-wise through its melting point. At each temperature point, NMR T 2 relaxation, T 1 − T 2 correlated relaxation, and PGStE measurements as a function of molecular migration time Δ were acquired. These measurements detected molecular-level disordering due to microencapsulation. NMR relaxometry measurements on the solid waxes measured a major intermediate mobility domain in the microencapsulated waxes. This pseudo interface domain in the solid microencapsulated waxes is consistent with the disruption of long-range order, increasing heterogeneity and extending the melting point range. NMR diffusometry measured diffusion coefficients D in the unencapsulated wax melts, matching values established in the literature. D as a function of 1 / T was fit for Arrhenius activation energies E a as a function of Δ for each stage of melting. D values are reported for each solid wax sample, which are not measurements of only molecular diffusion but are consistent with magnetization transport via spin diffusion.