Surface area determination via NMR: Fluid and frequency effects

Abstract

Recently, the use of NMR spin-lattice relaxation measurements has been proposed as a tool for the determination of specific surface area. The use of this approach, which has several advantages and disadvantages as compared to conventional methods, was demonstrated using relaxation measurements of water at 20 MHz for two materials with similar surface chemistry. In principle, the surface-enhanced spin-lattice relaxation rate is a function of proton frequency, fluid, temperature and surface chemistry. In this paper, we describe relaxation measurements at 303 K using a range of fluids (water, methanol, ethanol, cyclohexane) at two proton frequencies (20 and 300 MHz) for a series of samples with different surface areas from three different porous solid groups (controlled pore glass, mixed ester cellulose membranes and carbon blacks). Surface area of samples studied were in the range of 4.4 to 179.0 m2/g. A three-fold increase in the sensitivity of the surface area determination is noted when the frequency is decreased from 300 to 20 MHz. In general, surface area sensitivity increases with fluid type as (cyclohexane ⪡ water < methanol < ethanol).