Abstract
In this work, the melting-point depression and molecular dynamics of hexamethyldisilane confined within five controlled pore glasses, with mean diameters ranging from 7.9 to 23.9 nm, are studied by high-field (9.4 T) nuclear magnetic resonance (NMR), and the results are discussed with reference to the bulk substance. The melting-point depression in pores with radiusR follows the simplified Gibbs-Thompson equation ΔT=k p/(R−s) with ak p value of 74 K · nm and ans value of 1 nm. To our knowledge, this is the first time thek p value of hexamethyldisilane is reported. Proton spin-lattice relaxation times (T 1), spin-spin relaxation times (T 2), and diffusivities (D) are reported as a function of temperature. The confinement in the pores gives rise to substantial changes in the molecular dynamics and the phase behavior. The line-shape measurements reveal a two-phase system assigned to a relatively mobile component at the pore walls and a crystalline solid at the center of the pores. However, theT 2 measurements show that the mobile phase also embraces a minor component attributed to nonfrozen liquid in pockets or micropores. The diffusivity of the major narrow-line component is approximately three orders of magnitude larger than that in the plastic bulk phase, reflecting fast diffusion of mobile molecules. Below the melting region,T 1 of the narrow line is significantly shorter thanT 1 of the broad line, suggesting that the molecular reorientation is more hindered close to the surface than at the center of the pore.
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Aksnes, D.W., Førland, K. NMR investigations of hexamethyldisilane confined in controlled pore glasses: Pore size distribution and molecular dynamics studies. Appl. Magn. Reson. 25, 297–311 (2003). https://doi.org/10.1007/BF03166691
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DOI: https://doi.org/10.1007/BF03166691