TEMPERATURE-DEPENDENCE OF SOLID-LIQUID SCALAR INTERACTIONS IN AQUEOUSCHAR SUSPENSIONS BY NONSTATIONARY DNP AT LOW MAGNETIC-FIELD

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 proto n DNP enhancement changed from negative to positive. indicating a chan ge in the relative contributions from exchange (scalar) and dipole-dip ole nuclear-electron coupling. A cross-over temperature was found at w hich the contributions of the dipole-dipole and exchange interactions canceled each other and the DNP effect disappeared. A model of molecul ar migration at the solid-liquid interface is constructed to account f or these observations. Correlation of experimental and theoretical res ults has been achieved through consideration of the proton-electron sp in dipole-dipole interaction and chemical exchange of solvent molecule s. The increase with temperature in the contribution of the contact in teraction to proton relaxation is a result mainly of a decrease in the lifetime tau(h) of water in the short-lived hydrated complex at the h ardwood char interface. The enthalpy of activation E-h=13.5 kcal/mol f or surface exchange diffusion as well as the lifetime of water molecul es on the surface tau(h)(300 K)=1.9 x 10(-4) s and hyperfine scalar co nstant a=5.3 x 10(-3) MHz are estimated. The results are characteristi c of weak short-lived chemisorption bonds at the solid-liquid interfac e. (C) 1997 Elsevier Science B.V.