Hydration forces: a study of short-range interactions using atomic force microscopy

The hydration force was introduced after the observation that clays and lipid bilayers swell spontaneously or repel each other in aqueous solutions, and silica dispersions and other colloidal particles remain stable in concentrated salt solutions, where one would expect that, due to the screened repulsive electric double layer, the attractive van der Waals will dominate resulting in aggregation of the particles. These ‘additional’ forces found in aqueous solutions have become known as ‘hydration forces’. The short-range interactions, that are not described by the DLVO theory, can be repulsive or attractive, and decay exponentially over 1-2 nm and can be oscillatory. The latter arise when water molecules are induced to order or ‘structure’ into quasi-discrete layers between two surfaces. Despite limited available theoretical models to describe the hydration force, its general form can be expressed by a monotonically- and an oscillatory decaying contribution.
Experimental studies in salt solutions have reported that the hydration force is both ion- and surface specific. However, it remains unknown how the oscillatory and the monotonic contribution to this force are affected by specific ions and how and if their contributions are correlated. In this work, I measured the hydration forces in a large variety of conditions and looked into both contributions of the hydration force. The general form of the experiments is as follows: forces were measured between a sharp AFM tip and a surface (either mica or silica) in an aqueous solution over 5 nm separation. Salt solutions of different cation types were studied and the ambient temperature was varied. The obtained forces were fitted to an empirical equation that consists of an oscillatory and a monotonic contribution, enabling us to study the effect of the environmental condition on both contributions to the force.