Correlation between Electrostatic and Hydration Forces on Silica and Gibbsite Surfaces: An Atomic Force Microscopy Study

Abstract

The balance between hydration and Derjaguin−Landau−Verwey− Overbeek (DLVO) forces at solid−liquid interfaces controls many processes, such as colloidal stability, wetting, electrochemistry, biomolecular self-assembly, and ion adsorption. Yet, the origin of molecular scale hydration forces and their relation to the surface charge density that controls the continuum scale electrostatic forces is poorly understood. We argue that these two types of forces are largely independent of each other. To support this hypothesis, we performed atomic force microscopy experiments using intermediate-sized tips that enable the simultaneous detection of DLVO and molecular scale oscillatory hydration forces at the interface between composite gibbsite:silica−aqueous electrolyte interfaces. We extract surface charge densities from forces measured at tip−sample separations of 1.5 nm and beyond using DLVO theory in combination with charge regulation boundary conditions for various pH values and salt concentrations. We simultaneously observe both colloidal scale DLVO forces and oscillatory hydration forces for an individual crystalline gibbsite particle and the underlying amorphous silica substrate for all fluid compositions investigated. While the diffuse layer charge varies with pH as expected, the oscillatory hydration forces are found to be largely independent of pH and salt concentration, supporting our hypothesis that both forces indeed have a very different origin. Oscillatory hydration forces are found to be distinctly more pronounced on gibbsite than on silica. We rationalize this observation based on the distribution of hydroxyl groups available for H bonding on the two distinct surfaces.

Field theory of structured liquid dielectrics

“…We believe that, beyond the introduction of our approach and first results in the linear regime, our theory is capable of addressing a wide class of continuum models for polarization phenomena in confined solvents, in particular phenomena pertinent to the nano-confined aqueous systems [25,26]. Recent experimental work has shown that the observed oscillatory behavior of electrostatic and hydration forces is largely determined by the bulk ion and polarization charges, while the solution pH essentially determines the boundary conditions [44]. Our work is straightforwardly amenable to further extensions, allowing for nonlinear polarization functionals in the bulk as well as the incorporation of surface polarization functionals.…”

Interaction between Hydrated Smectite Clay Particles as a Function of Salinity (0–1 M) and Counterion Type (Na, K, Ca)

“…84 The third is expressed as oscillations with a period of ≈3 Å likely associated with molecular layering of water on the clay surfaces. 94,95 Parametric Fit to the Non-DLVO Interactions. To facilitate comparison with future studies, we fitted the non-DLVO interaction curves presented in Figure 4 using the following empirical relation…”

“…84 The third is expressed as oscillations with a period of ≈3 Å likely associated with molecular layering of water on the clay surfaces. 94,95 Parametric Fit to the Non-DLVO Interactions. To facilitate comparison with future studies, we fitted the non-DLVO interaction curves presented in Figure 4 using the following empirical relation (3) In eq 3, A s , A l , and A o are the amplitudes of the short-range, long-range, and oscillatory components, and w = 0.32 nm is the period of the oscillations.…”

A comprehensive continuum theory of structured liquids

“…The experimentally observed oscillations in the force-distance curves are clearly due the ordering of bulk water molecules in the vicinity of the surface. The details of these curves are nevertheless dependent on pH and hence on charge regulation effects [6]. We believe that our approach will be flexible enough to disentangle and identify these different relevant effects on electrostatic and hydration forces at complex substrate surfaces.…”