Energy characteristics of adsorbed water in active carbons according to the NMR relaxation data

Nuclear magnetic relaxation measurements were used to determine activation energy Eact of the motion of water molecules adsorbed in active carbons. The Eact value was found to depend on the filling of active carbon pores due to changes in the state of water molecules under adsorption. It was established that the Eact = f(p/ps) plots, where p/ps is the relative pressure of water vapor observed for microporous active carbons (FAS-1, 2, N-15, SKT-6A), are similar in form to the corresponding plots of changes in water adsorption heats. In particular, we concluded that the plateau in the Eact = f(p/ps) dependences, as in the case of adsorption heats, reflects the volumetric filling of active carbon micropores with water. We show that a linear function describes the increase in Eact values for water upon the complete filling of micropores with an increase in the volume of adsorbed water clusters per one primary adsorption center (W0/am). We establish that, for water in the FAS-3 sample, the deviation of Eact values from this linear function was due to the contribution from the vapor phase in the mesopores (x0 = 0.7−1.2 nm) that make up a considerable part of the active carbon’s porous system.