Surface Chemistry at High Ionic Strength

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Professor Vincent S. J. Craig
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Executive Summary

The aim of this research program was to develop the means to accurately measure the surface forces between mineral like surfaces in concentrated solutions of aqueous electrolytes.

At high salt concentrations,the electrostatic forces are highly screened and become veryshort in range. Therefore,the forces in concentrated salt solutions are governed by dispersion and hydration forces, the latter have a decay length of approximately 0.3 nm.This demands that the surfaces between which the measurements are performed have exceptionally low levels of roughness. We pursued Atomic Layer Deposition as a means to produce nearly ideal surfaces of the mineral oxides Alumina, Titania, Hafnia and Zirconia. These surfaces were characterised for roughness, composition, thickness, surface charge and stability and employed in surface force measurements for the first time. Also extensive theoretical calculations have been performed to evaluate the van der Waals forces in these layered systems. The adsorption of the surfactant CTAB and the forces between ALD surfaces in CTAB solutions was also studied.

Numerous surface force studies have been completed using Alumina and Titania surfaces covering the influence of pH, electrolyte concentration and a range of additives. A particularly surprising result is found for Titania surfaces at high pH, where the influence of the van der Waals force is not seen. The surface forces between Titania surfaces at very high salt concentrations has also been investigated extensively. For concentrated solutions we find that the forces are always attractive regardless of the type of salt or pH of the solution. The results obtained for the adhesion measurements between Titania surfaces in different electrolytes are not reproducible. This is attributed to extreme sensitivity to contaminationas well as small differences in local geometry in the contact region.We conclude that the strength of the adhesion in systems of practicalinterest is likely to be governed by the nature and concentration of contaminants.

Our investigations in this area are ongoing.Hafnia and Zirconia surfaces have been produced and characterised and will shortly be used for surface force measurements. Additionally, we are developing a theoretical approach for including the effect of surface roughness in theoretical models of surface forces. This will allow experiments performed on non ideal surfaces to be compared to theory with the influence ofroughness rigorously accounted.