MAGNETIC CROSS-RELAXATION AND CHEMICAL-EXCHANGE BETWEEN MICROPOROUS SOLID AND MOBILE LIQUID-PHASES

Nuclear magnetic spin-lattice relaxation rates are reported as a funct ion of magnetic field strength, pH, and temperature for water protons in Sephadex gels, The water proton rates report the relaxation profile characteristic of the solid at high pH values, At lower pH values, th e relaxation rate is limited by a slow chemical exchange process that makes the relaxation rate independent of magnetic field at low field s trengths, In cases where the proton exchange rate does not limit the t otal relaxation rate, the magnetic field dependence of the relaxation rate is given by a power law of the type R = Av(-B), where B is 0.75 e xcept at pH 11 where the data are better represented with B = 0.5. The exchange events important for the water proton relaxation are represe nted as a two-stage process in which water within the pores of the gel exchanges protons and magnetization with the solid spin system rapidl y compared with the rate at which the water within the pores exchanges with the bulk water phase.