Relaxation studies of adsorbed water on porous glass: I. Varying coverages and pore size at constant temperature

Abstract

Pulse nuclear magnetic resonance spectroscopy (pnmr) was used to measure the proton relaxation times of water adsorbed on powdered porous glass of varying pore size as a function of percentage relative humidity at room temperature. The results appear to fit a three-state model, each characterized by a different correlation time τ c. The model incorporates both the adsorption and the pnmr T 1 relaxation data. From the model, it is concluded that (i) τ c for the first adsorbed monolayer is about 20 times longer than that for bulk water, (ii) τ c for the second adsorbed monolayer is about 6 to 10 times longer than that for bulk water, and (iii) the rest of the porous water is similar to bulk water. Also, the fraction of motionally restricted states increases with decreasing pore size such that at an average pore diameter of less than 74 Å their fraction sharply increases from about 0.26 to 0.80 at an average pore diameter of 20 Å.