Self-regulation of iron ion concentration in hydrated porous glasses

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.