Surface Chemistry at High Ionic Strength

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Vincent S. J. Craig
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Executive Summary

The aim of this study is to understand the behavior of mineral particles in concentrated electrolyte solutions using surface force techniques. To this end there are two significant challenges.

1) The first challenge relates to the type of surface forces that dominate at high electrolyte concentration. They are very short in range and poorly understood theoretically, but it is known that they are related to the solvation of the surface layer of a material or ions adsorbed to that layer, hence they are called salvation forces, or in aqueous solutions, hydration forces.

2) The second challenge is to prepare surfaces that are suitable for investigation by surface force measurement techniques and is intimately related to the first challenge as the very short range over which hydration forces operate requires that surface roughness is controlled at a level comparable to or less than the range of the hydration forces.

At this point in the project we have successfully produced titania and alumina surfaces that are ideally smooth. The alumina surfaces have proved to be unstable in electrolyte solutions, though it was possible to measure the forces between alumina surfaces bearing adsorbed molecules that passivate the surface to dissolution. We have some evidence that stable Alumina surfaces can be formed by annealing the Ald surfaces and this is related to an increase in crystallinity. Investigations using Titania surfaces are continuing both in salt solutions and in the presence of both cationic and anionic surfactants and in the presence of a range of organic acids.