SELF-REGULATION OF IRON-ION CONCENTRATION IN HYDRATED POROUS GLASSES

The water proton spin-lattice relaxation rate was measured in Fe conta minated water filled porous glasses with pore diameters of 548 and 291 7 Angstrom at 0.65T at room temperature as a function of pore hydratio n. Two distinct regimes could be defined. A ''wet'' pores regime (0.6< V-0/V<1) and a regime with water only within the pores (1<V-0/V<4), wh ere V-0 is the pore volume and V the water volume. In both regimes, th e proton relaxation rate varies linearly with V-0/V. A small Fe ion co ncentration in porous silica glass water remains constant while the hy dration of pores is changed by a factor of approximate to 4, which is a necessary assumption to model the observed proton spin-lattice relax ation rate. Such a constant small concentration premise agrees with th e Langmuir isotherm, which for systems with large surface to volume ra tios (S/V-0), predicts a constant small volume concentration. In the t wo studied porous glasses with S/V-0 of 9.2X10(6) and 42X10(6) m(-1), the condition for large S/V-0 was satisfied, while the Fe volume conce ntration which was -1 mM was small, as required. An analysis of the pa ramagnetically enhanced proton relaxation is presented. (C) 1996 Ameri can Institute of Physics.