Association of water molecules in a nanoporous carbon material

Differential scanning calorimetry was used to study thermal effects during structural transformations of aqueous associates (phase transitions involvi ng evaporation and melting) occurring in a carbon material containing nanop ores (d = 0.82 nm) with a narrow size distribution and macropores (d = 20-3 0 mu m). The properties of the water molecules in the material and the mech anism of their association are governed by the size of nanopores and their morphology. Three coexisting states of water molecules were identified. It was shown that water molecules enclosed in nanopores are unable to crystall ize at temperatures from 0 to -70 degrees C. These pores are filled with wa ter molecules due to the presence of graphite-like conjugated structures wh ose pi-systems are capable of forming hydrogen bonds with water molecules. Once the nanopores are occupied by nonfreezing water, the formation of an a dsorption layer of water molecules on the walls of macropores begins, after which the bulk of the macropores is filled. The specific enthalpy of melti ng of water Delta H-fus in the nanoporous carbon sample (recalculated to th e weight of the water enclosed in the macropores) strongly depends on the w ater content in the sample; the value of enthalpy tends to diminish drastic ally as the concentration of water in the matrix decreases. Based on the ch aracter of water association in the carbon material, it was concluded that it is composed of a nanoporous fibrillar network, with typical distances be tween the fibrils being 20-30 mu m. The aqueous associates encapsulated bet ween the fibrils form highly defective ice-like structure upon freezing.