New approaches for the study of dissolution kinetics at the microscopic level
This thesis is concerned with the development, application and theoretical treatment of the scanning electrochemical microscope (SEeM), with the aim of obtaining new insights into the kinetics and mechanisms of ionic crystal dissolution processes. The ultramicroelectrode (UME) probe of the SEeM, placed at close distances to the surface of an ionic single crystal face in contact with a saturated solution, was used to induce and monitor the dissolution processes of interest. This was achieved by stepping the potential at the UME from a value at which no electrode reaction occurred to one where a component of the saturated solution was electrolysed at a diffusion-controlled rate. The resulting undersaturation induced the dissolution process and dissolving material, after traversing the tip/substrate gap, was subsequently collected at the UME probe. The current-time behaviour provided quantitative information on the local dissolution rate. The SEeM was successfully used to determine the dissolution characteristics of the (010) face of monoclinic potassium ferrocyanide trlhydrate. A second-order dependence on the interfacial undersaturation was found, consistent with the classical Burton, Cabrera and Frank dissolution model. This investigation proved that the SEeM was capable of delivering high mass transport rates under well-defined conditions and demonstrated that the dissolution of an un symmetric salt could be described by classical theories. In addition, through the development of SEeM dissolution rate imaging, it was shown that it was possible to map the dissolution activity across single pits in the crystal surface with micrometre resolution. The kinetics and mechanism controlling the dissolution of silver chloride is a classical system which, despite a number of studies, remains unresolved. SECM studies of the dissolution of pellets and electrochemically grown films of Agel in aqueous solutions, both in the absence and presence of supporting electrolyte (where the supporting electrolyte does not contain a ion common to Agel), were carried out and the corresponding mass transfer theories developed. In the latter case dissolution was found to be diffusion-controlled, due to the build up of electroinactive ions in the tip/substrate gap, suppressing the attainment of high interfacial undersaturations. In contrast, in the absence of supporting electrolyte, where the principle of electroneutrality prevented this process, the dissolution kinetics were determined unequivocally. In order to significantly increase the spatial resolution of electrochemically induced SECM imaging, a new integrated electrochemical-atomic force microscopy (IEAFM) probe was developed, which simultaneously measured the topography of the surface while electrochemically inducing dissolution under conditions which closely mimicked those of SEeM experiments. Using this technique, it was demonstrated, for the first time, that dissolution of an ionic crystal surface (the (100) face of potassium bromide), under conditions of very low interfacial undersaturation, occurred by the dynamic unwinding of steps at the sites of screw dislocations. Through use of the high spatial resolution and well-defined mass transport characteristics of the SEeM, it was possible to determine the dissolution characteristics, in an area of a crystal surface devoid of dislocations and defects, i.e. a 'perfect' surface. Studies on the (100) face of copper sulfate pentahydrate demonstrated that dissolution, in low dislocation density areas, occurred via an oscillatory mechanism. A new hydrodynamic technique, the microjet electrode, was developed and found to be capable of achieving mass transfer coefficients up to 0.82 cm s-l. The ability of the technique to characterise fast surface processes was demonstrated through kinetic studies of the oxidation of ferrocyanide ions at a Pt electrode. Possible modifications to the technique in order to facilitate the characterisation of dissolution processes were considered.