The interaction of adsorbed liquids on solid surfaces has been studied by NMR spectroscopy and relaxometry for decades. Despite models existing for well-defined systems [1], quantities such as the T1/T2 ratio are often treated merely empirically. One promising approach to improve our understanding is the addition of paramagnetic species; the unpaired electrons dominate dipolar nuclear relaxation, while at the same time providing opportunities to exploit the DNP mechanism for signal enhancement and for highlighting those molecules that come close to the radical site. In liquids, Overhauser Effect (OE) is expected but the Solid Effect (SE) has been found to be equally important for viscous materials [2]. The quantification of these two effects, and determining relaxation properties with and without radicals, much improves the ability to discern different relaxation mechanisms.
In this study, we investigate protic and aprotic liquids on the surface of silica nanoparticles, a model material with a large specific surface; its hydrophilicity is modified either by reducing OH group density or by silanation with APDMES. In a second step, the surface is treated by nitroxide radicals or vanadyl octaethylporphyrine (VOOEP) radicals; the latter compound is similar to the vanadyl radicals found in many crude oils [3]. Adsorbed liquids include water and alkanes and their deuterated analogues; employing 1H and 2H relaxometry, we have investigated the relaxation dispersion for all steps of sample treatment from the native state to the final stage with grafted radicals [4]. Considering pH-dependent measurements we are able to separate intra- from intermolecular processes and the role of hydrogen exchange. As the observed correlation times cover five orders of magnitude, both OE and SE are found for all liquids. The obtained DNP enhancement factors are discussed in terms of a suitable approach to selectively probe wetting and non-wetting liquids in single- and multiphase adsorption.