Adsorption of Organic Vapors on Water-Preadsorbed Alkali Halide

Abstract

The adsorbed amounts of organic vapors, i.e. CH₃OH, (CH₃)₂CO and (C₂H₅)₂O, on NaBr particles preadsorbed by water Vapor have been determined at -78, -63.5, -45 and 0°C. Three different amounts of the preadsorbed water, V_a, V_b and V_c, are used in this study, where V_b=2V_a and V_c=3V_a. The value V_a was approximately equal to the value necessary for covering the unit surface area of NaBr particles with localized monolayer of water.
The monolayer capacity of methyl alcohol estimated from its adsorption isotherm for NaBr surface V_a-preadsorbed was found to agree closely with the amount of preadsorbed water.
This result is explained by the following two assumptions that water molecules are adsorbed on (100) planes of NaBr crystals in the ratio of one watar molecule to two adjacent anions at V_a-adsorption and that methyl alcohol molecules are adsorbed on such localized water molecules in a ratio of 1:1. Moreover, the adsorptive activity of V_b-preadsorbed NaBr surface for ethyl ether was slightly larger than that of the V_a-preadsorbed surface. Therefore, in the V_b-preadsorbed surface, water molecules in the upper layer, i.e. second layer, are expected to adsorb on the localized monolayer so as to interact strongly with ethyl ether molecules. By using the three organic vapors in such“double adsorption”system, some significant informations were obtained about the adsorptive structure of water molecules attached on NaBr surface, and the interactions between preadsorbed water molecules and the organic molecules. From these results, the double adsorption method is considered to be an effective tool for studying the mechanism of a solid-gas adsorption.