Adsorption of polar molecules on alkali-fluorides

Abstract

The isotherms and calorimetric heats of adsorption of the vapors of H 2O, CH 3OH, CH 3NH 2, and CH 3CN on micro single crystals of NaF and LiF have been measured. For H 2O on LiF, a type III isotherm and a low heat of adsorption (5–7 kcal/mole) for a major portion of the monolayer were found. The isotherm of H 2O on NaF, however, showed a vertical step typical of a phase transition with a concomitant high heat of adsorption of 13–15 kcal/mole. The adsorption of CH 3OH and that of CH 3NH 2 were similar on both surfaces with heats of adsorption in the range of 13–14 kcal/mole. The adsorption of CH 3CN was weaker on both surfaces and gave type III isothrems. The role of lattice dimensions and of hydrogen bonding with the surface F − ions in the determination of the adsorption behaviors is discussed. The non-(100) surface sites which were observed on the carbon replicas of the LiF surface are postulated as the active sites responsible for the initial higher heat of adsorption for all four adsorbates.