Exchange and Cross-Relaxation in Adsorbed Water

Abstract

Previous investigations concerning the NMR relaxation in adsorbed water show that both the longitudinal and the transverse proton magnetization decay in a multiexponential way. Two or more components can always be distinguished in those decay curves. We propose a model for adsorbed water which can account for these different relaxation components and with the aid of this model we give a quantitative description of the longitudinal relaxation in water adsorbed on two silica gels and two controlled pore glasses. The model assumes three phases:the protons of the OH groups on the adsorbent surface, the protons of the water molecules close to the surface ("bound" water), and the protons in the rest of the water ("free" water). We prove the occurrence of cross relaxation between the OH protons and the protons in the bound water and of exchange between bound and free water. The OH protons have very slow longitudinal relaxation with T1 = 4-5 s and very fast transverse relaxation with a characteristic time of about 40 μs. The amount of OH protons corresponds to six OH groups per 100 Å2 for all adsorbents studied. We show that the influence of the surface on the longitudinal relaxation of the adsorbed water is limited to one adsorbed layer, that the longitudinal relaxation time for the bound water lies between 20 and 60 ms, and that the free water exhibits a longitudinal relaxation time comparable to the value for pure water (3.3 s). Values for the cross-relaxation times and exchange times are deduced.