Temperature dependence of solid-liquid scalar interactions in aqueous char suspensions by nonstationary DNP at low magnetic field

Abstract

The temperature dependence of DNP enhancements in aqueous suspensions of hardwood chars by nonstationary DNP at low magnetic field has been investigated. With increasing temperature, the sign of the water proton DNP enhancement changed from negative to positive, indicating a change in the relative contributions from exchange (scalar) and dipole-dipole nuclear-electron coupling. A cross-over temperature was found at which the contributions of the dipole-dipole and exchange interactions canceled each other and the DNP effect disappeared. A model of molecular migration at the solid-liquid interface is constructed to account for these observations. Correlation of experimental and theoretical results has been achieved through consideration of the proton-electron spin dipole-dipole interaction and chemical exchange of solvent molecules. The increase with temperature in the contribution of the contact interaction to proton relaxation is a result mainly of a decrease in the lifetime τ h of water in the short-lived hydrated complex at the hardwood char interface. The enthalpy of activation E h = 13.5 kcal/mol for surface exchange diffusion as well as the lifetime of water molecules on the surface τ h(300K) = 1.9 × 10 −4s and hyperfine scalar constant a = 5.3 × 10 −3MHz are estimated. The results are characteristic of weak short-lived chemisorption bonds at the solid-liquid interface.