Chloride ion substitution in alkali bromide surfaces: cooperative interactions, including a surface transition

Abstract

Thermodynamic information on surfaces of vacuum -sublimed NaBr and KBr has been obtained by stud y in g equilibrium in: (a) halogen exchange between HCl(g) and the surfaces; (b) adsorption of HCl an d HBr. Both types of study disclose a transition in the pure KBr surface at 293 K, induced by HCl adsorption. From exchange results, ΔHt = + 18.4 kJ mol^-1 and ΔS_t = + 62.8 J mol^-1 K^-1. Adsorption indicates a similar transition in the pure NaBr surface at 268 K. It is proposed that, in the transition, anions acquire free two-dimensional translation. Comparison with exchange studies of Harrison & Siddiqui (1962) on solution-precipitated NaBr suggests that in those surfaces both anions and cations were mobile. Partly exchanged surfaces show non-ideal behaviour which is interpreted in term s of repulsion between like anions up to a threshold distance beyond which they attract each other. In KBr, the effects are large, and lead to formation of a surface compound K₄Br₃Cl. Beyond this composition further Cl^- does not substitute into the KBr surface below 10 °C at ordinary HCl pressures. The transition temperature of KBr surfaces 5 to 15 % exchanged with Cl^- is depressed. This is attributed to a tendency to two-phase separation of the surface as KBr and K₄Br₃Cl. In NaBr, corresponding interactions are weaker, and give only slight non-ideality. This has been matched by computations using the 'quasi-chemical’ aproximation, with first-neighbour repulsion of like anions about 2 kJ mol^-1 and similar second-neighbour attraction. The effects are believed to be associated with the Verwey distortion. They are smaller in NaBr than in KBr because the former is being studied with anions in the mobile state in which such effects are only residual. Data for KBr suggest that, above its transition, it may lose these enthalpy effects more completely than NaBr and acquire an entropy-dominated non-ideality like that found by Harrison & Siddiqui (1962) in solution-precipitated NaBr.