FREEZING PHENOMENA IN ADSORBED WATER AS STUDIED BY NMR

The freezing of water adsorbed on high-surface-area materials such as silica gel, controlled-pore glass, and activated charcoal is investigated with NMR methods. Part of the pore water freezes at the temperature predicted by the Kelvin equation and the rest of the water does not freeze in the sense that it does not assume the structure of ice on cooling below this freezing point. This bound water exhibits a distribution of correlation times, and information about the width of this distribution is obtained. Values for the activation enthalpy of the bound water are also deduced. An alternative method for the determination of the pore volume of an adsorbent is proposed and it is shown that. for adsorbents with small pores (r < 100 Å), this method results in a much better estimate for the total pore volume than the more common mercury-intrusion method. © 1993 by Academic Press, Inc.