Particle size effect on transverse NMR relaxation in aqueous char suspensions

A strong particle size influence on NMR transverse proton relaxation in aqu eous suspensions of several newly synthesized carbon based chars is reporte d. The results are quantitatively interpreted in terms of a two-stage molec ular exchange model. A porous cage effect leads to slow exchange between mo lecules inside and outside pores, in contrast to fast molecular exchange at the solid-liquid interface, where a familiar two-site formalism can be app lied. The dependence of surface properties on the relaxation data was detec ted. The transverse NMR relaxation time of water protons on the paramagneti c surface of chars is estimated to be T-2s approximate to 0.5 x 10(-5) s, w hich is comparable with the EPR relaxation time of the free radical centers . Dynamic nuclear polarization (DNP) is used to estimate the paramagnetic s hift Delta v of water protons at the surface. The value Delta v so obtained is much less than the transverse NMR relaxation rate of water protons on t he paramagnetic surface \Delta v\ much less than (T-2s (-1)). This inequali ty can account for the independence of the NMR transverse relaxation times on the interval tau observed in CPMG measurements. (C) 1999 Elsevier Scienc e B.V. All rights reserved.