Proton spin-lattice relaxation of colloidal silica suspensions in H2O/D2O mixtures

The proton spin-lattice relaxation rates of amorphous colloidal suspensions are reported over a wide Larmor frequency range for varying degrees of iso topic dilution. At low magnetic fields, the amplitude of the relaxation dis persion reveals a non-linear dependence upon deuteration. This behaviour is primarily attributed to isotopic fractionation effects of slow exchanging hydrogens situated at solvent-shielded sites on the silica surface, but whi ch are involved in dipolar interactions with readily solvent accessible pro ton species. A theoretical spin relaxation model is proposed which accounts for the isotope dilution effects in a multiple-phase system composed of th e abundant slow-relaxing bulk spins and spin environments in mutual dipolar interaction at the oxide interface. The broadness of the observed relaxati on dispersions is treated as a superposition of a discrete number of Lorent zian-like power spectra. At least two relaxation components are resolved fr om the present dispersion data which obviously reflect the heterogeneity of the proton sites at the silica surface. Each component seems to consist of a water and a pH dependent silanol contribution. The exchange of the obser ved proton classes has been estimated to occur on the microsecond time scal e. An overall quantitative analysis of our relaxation dispersion data at di fferent H/D ratios reveals considerable isotope fractionation between some slow exchanging surface hydrogens and the water solvent. Fractionation fact or values <0.5 were obtained. (C) 1998 Elsevier Science B.V. All rights res erved.