Hydration forces - A study of short-range interaction forces using Atomic Force Microscopy
Due to the COVID-19 crisis measurements the PhD defence of Simone van Lin will take place online without the presence of an audience.
The recording of this defence will be added to the video overview of recent defences.
Simone van Lin is a PhD student in the research group Physics of Complex Fluids. Her supervisor is prof.dr. F.G. Mugele from the Faculty of Science and Technology.
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.
In Chapter 2, I will give more information on AFM and on the analysis procedure of the experimental data. In Chapter 3 I show our study on the hydration force at mica surfaces in the presence of different types of monovalent cations (1+). I show that oscillatory hydration forces, as a result of the packing of water molecules, are stable in the absence of added ions. I show that both contributions to the force are affected by the type of cation present in solution. The monotonic contribution to the hydration force decreases in strength with decreasing the bulk hydration energy of the cation in solution, leading to a transition from an overall repulsive to an attractive force. The oscillatory part in contrast plays a binary character, being hardly affected by the presence of strongly hydrated cations but becomes completely suppressed in the presence of weakly hydrated cations, which agrees with complementary Molecular Dynamics simulations.
Chapter 4 focusses on the effect of different types of divalent cations (2+) on the hydration force at mica surfaces. Here I show that the oscillatory hydration force remains stable in the presence of different types of divalent cations, unlike what was found for the monovalent cations. While the oscillatory force is mostly unaffected, the monotonic contribution is attractive at intermediate salt concentrations, but becomes slightly repulsive at higher concentrations.
Not only ions in solution but also the surface itself influences the interaction forces. In Chapter 5 I show a study on the effect of the surface on the hydration force, where experimental data is presented that is acquired at mica and silica surfaces. Mica is atomically smooth after cleavage, while the thermally grown silica surfaces used in this study are amorphous. I show that the oscillatory hydration force is not only stable at atomically smooth surfaces such as mica, but that it is found at silica surfaces as well. The monotonic contribution to the force does show minor variations between the two surfaces but generally, the hydration forces at mica and silica surfaces show to be remarkably similar, that points to fundamentally similar hydration interactions at both surfaces.
In the last experimental chapter, Chapter 6, a study on the effect of temperature on the hydration force is presented where I show that the oscillatory hydration force is remarkably stable, even in ambient temperatures of 65°C. Additionaly, the monotonic contribution of the force measured in purified water becomes more and more attractive upon increasing the temperature, which is more pronounced in the presence of salts. Lastly, in Chapter 7 I summarize my conclusions and present recommendations for future work.
