Cooperative interactions and two-dimensional ordering in the adsorption of HBr on KBr

Abstract

Adsorption isotherms of HBr gas on high surface area KBr have markedly non-ideal shapes. These are analysed using a two-layer model with repulsive interactions between molecules in the first layer. An earlier report from this laboratory of a transition in the surface layer at 20 °C, induced by adsorption, is confirmed by these data. Likewise, an apparent restriction of first-layer adsorption to no more than 25% coverage seems to correlate with the previous report from this laboratory that a ‘two-dimensional compound’ K₄Br₃Cl is formed in surfaces partly exchanged with HCl. Heat of adsorption is 48.5 kJ mol^–1(below transition) and 30.5 kJ mol^–1(above transition); mean-field repulsion constant C is ca. 40 kJ mol^–1; the transition has ΔH_t= 18.0 kJ mol^–1 and ΔS_t= 59.0 J mol^–1 K^–1. The remarkably large magnitudes of some of these quantities are discussed in terms of a model for adsorption involving normal weak hydrogen bonding plus additional electrostatic interactions between the Br of adsorbed HBr and adjacent K⁺ ions. Thereby, interactions between admolecules become intermediate-range, rather than nearest neighbour, averaging C/12 ≈ 3–4 kJ mol^–1.