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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow> <mml:msub><mml:mrow><mml:mi>T</mml:mi></mml:mrow> <mml:mrow><mml:mn>2</mml:mn></mml:mrow> </mml:msub> </mml:mrow> <mml:annotation>$$ {T}_2 $$</mml:annotation></mml:semantics> </mml:math> relaxation, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow> <mml:msub><mml:mrow><mml:mi>T</mml:mi></mml:mrow> <mml:mrow><mml:mn>1</mml:mn></mml:mrow> </mml:msub> <mml:mo>-</mml:mo> <mml:msub><mml:mrow><mml:mi>T</mml:mi></mml:mrow> <mml:mrow><mml:mn>2</mml:mn></mml:mrow> </mml:msub> </mml:mrow> <mml:annotation>$$ {T}_1-{T}_2 $$</mml:annotation></mml:semantics> </mml:math> correlated relaxation, and PGStE measurements as a function of molecular migration time <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mi>Δ</mml:mi></mml:mrow> <mml:annotation>$$ \Delta $$</mml:annotation></mml:semantics> </mml:math> 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mi>D</mml:mi></mml:mrow> <mml:annotation>$$ D $$</mml:annotation></mml:semantics> </mml:math> in the unencapsulated wax melts, matching values established in the literature. <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mi>D</mml:mi></mml:mrow> <mml:annotation>$$ D $$</mml:annotation></mml:semantics> </mml:math> as a function of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mn>1</mml:mn> <mml:mo>/</mml:mo> <mml:mi>T</mml:mi></mml:mrow> <mml:annotation>$$ 1/T $$</mml:annotation></mml:semantics> </mml:math> was fit for Arrhenius activation energies <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow> <mml:msub><mml:mrow><mml:mi>E</mml:mi></mml:mrow> <mml:mrow><mml:mi>a</mml:mi></mml:mrow> </mml:msub> </mml:mrow> <mml:annotation>$$ {E}_a $$</mml:annotation></mml:semantics> </mml:math> as a function of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mi>Δ</mml:mi></mml:mrow> <mml:annotation>$$ \Delta $$</mml:annotation></mml:semantics> </mml:math> for each stage of melting. <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mi>D</mml:mi></mml:mrow> <mml:annotation>$$ D $$</mml:annotation></mml:semantics> </mml:math> 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.