Abstract:

The NMR relaxation studies give information about both the dynamic behavior of adsorbed molecules and the physical properties of the adsorbent, such as specific surface and pore size distribution. The aim of this work is to study the adsorption state of benzene and cyclohexane on charcoals with various pore structures. The physical properties of charcoals studied were given elsewhere. Spin lattice and spin spin relaxation times(T_1 and T_2)were measured on a Varian XL-200 spectrometer by inversion recovery and CPMG spin echo method respectively.
The ln(M_0-M(t))/M_0)-t relation for a single exponential decay is a straight line but for biexponential decay it is bent (Fig.1). Experimental results show that the magnetization of liquid benzene decays exponentially while molecules adsorbed on charcoals obey the biexponential decay rule (Fig.2). Simulation of the magnetization decay curves gives two groups of relaxation times and their relative amounts (Tab.1 and 2). The rapid relaxing molecules (T_(1s)~1-2 ms) are solid-like adsorbed on the solid surface, while the slow relaxing molecules (T_(1v)~0.1-1.0s) are liquid-like condensed in the capillaries. The volume of the solid-like adsorbed layer can be obtained by mutiplying the relative amount of these molecules (M_(0s)/M_0) by the adsorption capacity of the charcoal, and the thickness of this adsorption layer by dividing this volume by the specific surface ares (Tab.2). This thickness ranges from 0.10 to 0.16 nm for all of the charcoals studied.
The calculated thickness of the adsorbed layer from ~(13)C T_1 data agrees well with those obtained by ~1H T_1 measurements (Tab.3).
~1H T_2 are also biexponential (Tab.4) and depend on charcoal species.