Proton and deuteron field-cycling NMR relaxometry of liquids confined in porous glasses

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Polar and non-polar liquids in porous glasses have been studied by proton and deuteron field-cycling NMR relaxometry. The mean pore diameters were 4 nm, 30 nm and 208 nm. The frequency dependence of the spin-lattice relaxation time T 1 is strongly influenced by the polarity of the adsorbate. Non-polar liquids show a flat T 1 dispersion compared with polar species. We designate the two cases as “weak” and “strong” adsorption respectively. Molecules of liquids at surfaces are known to adopt a preferential orientation, whereas they retain a high diffusivity. Therefore, reorientations mediated by translational displacements (RMTD) must contribute to the correlation function which decays on a timescale of up to eight orders of magnitude greater than in the bulk. The high diffusivity at the surface is made plausible by a mechanism called bulk-mediated surface diffusion recently proposed by Bychuk and O'Shaughnessy [O.V. Bychuk and B. O'Shaughnessy, J. Chem. Phys., 101 (1994) 772]. These authors found that the displacements effective on the surface can be described as the result of Lévy walks. They therefore obey a Cauchy distribution. This work employs the Cauchy distribution for a numerical derivation of a surface correlation function, g( r c ) , which correctly reproduces the typical correlation lengths of the substrate and thus renders the notion of the liquid molecules performing Lévy walks reasonable. The difference in T 1 dispersion behaviours of polar and non-polar adsorbates disappears when the free liquid is frozen while the approximately two monolayers thick surface film remains liquid. The T 1 dispersions are then equally steep irrespective of the polarity. This indicates that a non-polar liquid confined to a thin, topologically two-dimensional layer on a polar surface undergoes the same relaxation mechanism as a strongly adsorbed polar liquid which is dominated by RMTD processes.