Self-regulation of iron ion concentration in hydrated porous glasses

Abstract

The water proton spin-lattice relaxation rate was measured in Fe contaminated water filled porous glasses with pore diameters of 548 and 2917 Å at 0.65T at room temperature as a function of pore hydration. Two distinct regimes could be defined. A ‘‘wet’’ pores regime (0.6<V0/V<1) and a regime with water only within the pores (1<V0/V<4), where V0 is the pore volume and V the water volume. In both regimes, the proton relaxation rate varies linearly with V0/V. A small Fe ion concentration in porous silica glass water remains constant while the hydration of pores is changed by a factor of ≊4, which is a necessary assumption to model the observed proton spin-lattice relaxation rate. Such a constant small concentration premise agrees with the Langmuir isotherm, which for systems with large surface to volume ratios (S/V0), predicts a constant small volume concentration. In the two studied porous glasses with S/V0 of 9.2×106 and 42×106 m−1, the condition for large S/V0 was satisfied, while the Fe volume concentration which was ∼1 mM was small, as required. An analysis of the paramagnetically enhanced proton relaxation is presented.