Ian Ralph Collins

Electrical properties of the gold—aqueous solution interface

Abstract The electrokinetic properties of gold particles dispersed in aqueous inorganic electrolyte solutions have been studied using the technique of microelectrophoresis. Results obtained for dispersions formed from vacuum-evaporated metal indicate that electrical charge development at the “pristine” surface of gold in contact with aqueous solutions arises from the presence of a surface oxide layer. The influence of halide ions on the electrokinetic properties of these particles is consistent with metal—halide interactions involving both ligand exchange and specific anion adsorption. Evidence has been obtained which indicates that in the case of dispersions formed by citrate reduction of chloroauric acid, strongly adsorbed citrate ions modify the metal surface and inhibit interactions with halide species.

Adsorption of Aerosol-OT at the calcite/water interface--comparison of the sodium and calcium salts.

The adsorption of the surfactant Aerosol-OT (AOT) at the calcite-water interface has been investigated using batch adsorption isotherms and neutron reflection. The adsorption isotherms showed that NaAOT adsorption followed S-type adsorption behaviour with a maximum surface excess of 2.5 mg m(-2) but the method could not be used for the investigation of Ca(AOT)2 adsorption owing to the changes in the bulk phase behaviour of the solution. The surface excess, determined by neutron reflection at the critical micelle concentration (CMC), was 2.5 mg m(-2) for Ca(AOT)2 and 1.8 mg m(-2) for NaAOT. The time dependence of the NaAOT adsorption suggests a slow conversion from the sodium to the calcium salt of AOT at the calcite-water interface by binding calcium ions released from the slightly soluble calcite. The layer thickness in both cases was 35 Å which indicates adsorption as bilayers or distorted micelles. At higher concentrations of NaAOT (~10× CMC) adsorption of an AOT lamellar phase was evident from Bragg peaks in the specular reflection. To our knowledge, this is the first time that adsorption of a surfactant at the calcite-water interface has been investigated by neutron reflection. The technique provided significant new insight into the adsorption behaviour of AOT which would not have been accessible using traditional techniques.

Neutron Reflection at the Calcite-Liquid Interface

The calcite-liquid interface is of great importance in many industrial and academic situations. In this paper, this interface is investigated using the powerful method of neutron reflection. Experimental approaches to overcome the challenges of surface roughness, cleanliness and mineral dissolution have been developed. Based on this protocol, the interfaces of calcite with the liquids water, toluene and heptane have been characterised successfully. Hence this study allows the technique of neutron reflection to be expanded to the investigation of mineral surfaces.