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UBC Theses and Dissertations

Studies relating to the structures of ionic surfaces Siddiqui, Rafiq Ahmad

Abstract

The experimental work described in this thesis is designed to give information on the thermodynamic properties and chemical reactivity of the surface of an ionic crystal. The principal studies concern halogen exchange between hydrogen chloride gas and the surface of sodium bromide. For this reaction, like any ordinary chemical reaction, both kinetics and equilibrium may be investigated. In the present study, the most Interesting feature has been the measurement of the equilibrium constant and the calculation therefrom of the changes in thermodynamic properties attending the replacement of a bromide ion by a chloride ion in the surface. This appears to be the first reported measurement of an equilibrium constant involving concentrations in a solid surface. The work differs sharply from previous studies of isotopic exchange, which have been exclusively kinetic. The equilibrium is best represented by the equation [formula omitted] where X and (1-X) are the fractions of the surface anion sites occupied by Cl⁻ and Br⁻ respectively, ∆°S refers to the entropy changes of the exchanged ions at X = 0, and Si(X) describes the effect of any anion on the entropy of its neighbours in the surface. The equation indicates the formation of a two-dimensional solid solution of NaCl and NaBr, deviating from ideality through the term Si(X), so that the equilibrium constant K is a function of X. Since K is independent of temperature (20°-95°C.), ∆H = 0 and the equilibrium equation contains entropy terms only. From the value K = 6X10⁻³ at X = 0 it is estimated that [formula omitted] where S°Cl⁻ and S°Br⁻ refer to surface ions. The variation of K with X is used to arrive at a tentative estimate, based on some simplifying assumptions, of [formulas omitted]. Of this, about -2 cal/mole deg. K may be a simple mass effect. These large decreases in entropy are discussed in relation to the probable structure of the surface of sodium bromide; they are consistent with the "Verwey distortion" in which the anions are displaced outwards from the ideal surface, the distortion increasing, ceteris paribus, with the size and polarizability of the anions. The kinetics of the exchange reaction have been studied. It has not proved possible to analyze the rate curves in detail. The most striking feature, in comparison with previous studies, is that all reactions involving bromine (initially in either the gaseous or the solid phase) are faster than those involving chlorine isotopes only. The reason for this may be either the availability of d-orbitals of bromine for bonding in a transition complex, or the polarizability of the bromine atom. The exchange reaction has been used to study diffusion of chloride ions into sodium bromide at room temperature. It is possible to measure diffusion coefficients of about 10⁻²²cm²sec⁻¹, smaller by a factor of about 10⁵ than can be measured by most conventional methods. The results suggest that diffusion takes place principally along dislocations. The dehydration of high surface area sodium bromide has been studied, and is found to take place at a vapour pressure different from the equilibrium value for the system, NaBr.2H₂0; NaBr; H₂O. This is attributed to the formation of a "surface hydrate" overlying the lattice of anhydrous NaBr. This process may be regarded as adsorption with a discontinuous isotherm, and implies that the NaBr surface is energetically homogeneous. It has been found that color centers may readily be produced in the alkali halides by a Tesla Coil discharge. Centers thus produced in sodium chloride have been examined spetroscopically. The technique may be useful in experiments to establish the mechanism of exchange reactions such as that described in this thesis.

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