Interaction With Surface Spins as a Contribution to Nuclear Magnetic Relaxation of Liquids Adsorbed in Mesoporous Materials

ABSTRACT

NMR relaxation of liquids in the presence of solid interfaces, such as silica gel or porous glass, is affected by slowed‐down and anisotropic molecular reorientations compared to the bulk, but also by the presence of spins as potential relaxation partners on or close to the surface. The latter contribution is predominantly considered as unpaired electrons in metal centers or surface radicals, but dipolar interactions with spin‐bearing nuclei such as ²⁹ Si or ¹ H may also contribute to relaxation of adsorbed species. In this study, an attempt is made at identifying the most relevant relaxation contributions by either adding or removing relaxation sinks: While immobilized stable radicals (TEMPO) on the surface shorten relaxation times of adsorbed liquids, the replacement of ¹ H nuclei by ² H in hydroxyl groups is expected to remove the dominating part of nuclear dipolar spin–spin relaxation. In both cases, it is assumed that surface chemistry, which is the decisive parameter for spin relaxation mechanisms at interfaces, remains unchanged. Longitudinal and transverse relaxation, 2d T ₁ ‐T ₂ maps, and frequency‐dependent relaxation measurements of a number of homogeneous liquid phases as well as binary mixtures in the pore space of silica gel and porous glass are discussed in terms of the presence of radicals, whereas the replacement of OH by OD is observed to reduce relaxation rates in the systems under study.