Surface Chemistry at High Ionic Strength

Background to Surface Forces

In the first part of this report we provide some background to surface forces in order to explain the main considerations that apply when high concentration of electrolytes are present. That is the electrostatic component of the surface force is greatly reduced and the surface forces and therefore the stability of a system is dependent upon the balance between the repulsive hydration force and the attractive van der Waals forces.

Investigation of Interaction Forces

An investigation of the interaction forces in NaCl solutions at both low, intermediate and high salt concentrations illustrates this point further. Additionally the frictional forces in these systems have been measured in order to test the proposed approach of using the frictional and adhesive forces to characterize the hydration force in systems where the hydration force cannot be easily measured directly. This approach is validated.

New Technique for Solvent Structure Evaluation

The development of a new technique for the sensitive evaluation of solvent structure is described, termed photon pressure AFM. This technique is able to resolve the solvent structure adjacent to a solid material and observe alterations to that structure arising from the presence of cations in solution. It promises to provide exquisite detail on the interaction of ions with mineral surfaces.

Challenges and Future Directions

A major challenge has been to produce mineral surfaces that are sufficiently idealized in terms of geometry and surface finish to employ in these studies. To this end the technique of ALD is very promising. Funds from IFPRI have been employed to apply for a research grant to acquire this instrumentation.