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

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 $\Delta$ 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 $\Delta$ 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.