NMR study of phase transitions in pure water and binary H2O/HNO3 films adsorbed on surface of pyrogenic silica

Pyrogenic silica (aerosil) was employed as host within which the phase transitions in the adsorbed pure water and binary H2O/HNO3 films have been studied with NMR spectroscopy. The median freezing temperature and freezing temperature region were shown to be highly sensitive both to the average thickness of the adsorbed films and to the amount of adsorbed nitric acid. The molar concentration of nitric acid in the adsorbed films was found to be very small, on the order of 10(-3) - 10(-2) (M/liter). The concentration was found to be greater in the layers adjacent to the surface of silica and sharply decreases with distance from the surface. The difference between the median freezing temperatures for adsorbed pure water and for the binary system was found to be about 9 K for films of equal thickness. This is about 150 times greater than the difference between the freezing temperatures of bulk pure water and a solution with the same concentration of nitric acid. (C) 1996 Academic Press, Inc.