Adsorption of water on the fine fractions of Finnish tills

KELLOMÄKI, AARRE and NIEMINEN, PERTTI, 1986: Adsorption of water on the fine fractions of Finnish tills. Bull. Geol. Soc. Finland 58, Part 2, 13—19. This study deals with adsorption of water from the vapour phase on till samples at 25°C. The dry weights were determined after drying the samples in an oven at 105°C. Complete adsorption isotherms were measured for 41 tills with a "desiccator method" using saturated salt solutions as humidity standards. In addition, partial isotherms were determined for about 40 other samples. The isotherms were BET types I and II. The surface areas computed with the BET equation from water adsorption are considerably larger than the nitrogen adsorption areas of the same samples. In order to compare the isotherms with each other they have been presented as film thicknesses against relative humidity. All curves can then be satisfactorily represented by four model isotherms, which reflect the interactions of water molecules and the adsorbent surfaces. To a certain extent the model isotherms correlate with the mineral composition of the samples. Free energies of adsorption were calculated. Their negative values indicate that adsorption is highly exothermic at low humidities, or the samples are hygroscopic.