Hydration forces between silica surfaces: Experimental data and predictions from different theories

Abstract

Silica is a very interesting system that has been thoroughly studied in the last decades. One of the most outstanding characteristics of silica suspensions is their stability in solutions at high salt concentrations. In addition to that, measurements of direct-interaction forces between silica surfaces, obtained by different authors by means of surface force apparatus or atomic force microscope (AFM), reveal the existence of a strong repulsive interaction at short distances (below 2 nm) that decays exponentially. These results cannot be explained in terms of the classical Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory, which only considers two types of forces: the electrical double-layer repulsion and the London-van der Waals attraction. Although there is a controversy about the origin of the short-range repulsive force, the existence of a structured layer of water molecules at the silica surface is the most accepted explanation for it. The overlap of structured water layers of different surfaces leads to repulsive forces, which are known as hydration forces. This assumption is based on the very hydrophilic nature of silica. Different theories have been developed in order to reproduce the exponentially decaying behavior (as a function of the separation distance) of the hydration forces. Different mechanisms for the formation of the structured water layer around the silica surfaces are considered by each theory. By the aid of an AFM and the colloid probe technique, the interaction forces between silica surfaces have been measured directly at different pH values and salt concentrations. The results confirm the presence of the short-range repulsion at any experimental condition (even at high salt concentration). A comparison between the experimental data and theoretical fits obtained from different theories has been performed in order to elucidate the nature of this non-DLVO repulsive force.

Interactions between silica particles in the presence of multivalent coions

“…At low concentrations below about 1 mM we have fixed p = 1 in our fits in order to avoid the values above unity which are not consistent with the regulation model. Due to this modification the measured forces were slightly more repulsive than the calculated ones at short distances, and this behavior if probably connected to the short-range hydration repulsion which is known to be present for silica surfaces in aqueous solutions [23,[41][42][43][44].…”

“…The DLVO theory describes the force curves perfectly, except at very short separations below few nanometers, where the experimental curves are more repulsive then predicted by the theory. This short-range repulsion is probably due to the hydration forces [41][42][43][44] or overlapping hairy layers of polysilicilic acid [38] and it is not part of our theoretical description.…”

Dispersion forces acting between silica particles across water: influence of nanoscale roughness

“…This short-ranged repulsion has a range of about 0.3 nm and is probably due to the hydration force or the overlapping of hairy layers of polysilicilic acid. 9,20,[28][29][30] The different heat-treatment procedures influence this short-ranged component of the force somewhat, as can be inferred from different strengths of this component. However, changes in surface hydrophobicity were reported not to influence such short-ranged forces too strongly.…”

Two-Stage Dynamics ofIn VivoBacteriophage Genome Ejection

“…p 0 and c are, respectively, the amplitude and decay length of the hydration force [18] between adjacent DNA strands separated by a distance d s , ð1= ffiffi ffi 3 p Þp 0 d s e −ðd s =cÞ per unit length. The spatial organization of water molecules and ions near interfaces was suggested to be the origin of the hydration force [40]; however, the exact mechanism is still under debate [41][42][43]. We will treat p 0 and c as empirical parameters that change with buffer conditions.…”