Intermittent molecular motion and first passage statistics for the NMR relaxation of confined water

The structure and dynamics of fluids confined in nanoporous media differ from those in bulk, which can be probed using NMR relaxation measurements. We here show, using atomistic molecular dynamics simulations of water in a slit nanopore, that the behavior of the NMR relaxation rate, R_{1}, with varying surface interaction and confinement strength can be estimated from the exchange statistics of fluid molecules between the adsorbed surface layer and the bulk region. In detail, we test the applicability of intermittent molecular water dynamics, a theoretical framework that relates NMR relaxation to the statistics of molecular exchange between these two states. We confront this framework with the direct evaluation of dipolar NMR relaxation rates from atomistic molecular dynamics simulations of water confined in a slit nanopore, allowing to assess its accuracy across varying surface interactions. We employ first return passage time calculations to quantify the molecular exchange statistics, thereby linking microscopic parameters of the confined fluid-such as adsorption time, pore size, and diffusion coefficient-to the NMR relaxation rate. Our results show that the theoretical framework proposed by Levitz et al. captures the qualitative trends of NMR relaxation rates observed in our molecular simulations. This approach allows to predict, interpret, and analyze the molecular relaxation of fluids at interfaces using merely concepts of statistical mechanics that can be generalized to closed and open geometries and thus yields direct insights into the experimental study of NMR relaxation with varying surface chemistry or confinement strength.