Measuring electrostatic, van der Waals, and hydration forces in electrolyte solutions with an atomic force microscope

Abstract

In atomic force microscopy, the tip experiences electrostatic, van der Waals, and hydration forces when imaging in electrolyte solution above a charged surface. To study the electrostatic interaction force vs distance, curves were recorded at different salt concentrations and pH values. This was done with tips bearing surface charges of different sign and magnitude (silicon nitride, Al(2)O(3), glass, and diamond) on negatively charged surfaces (mica and glass). In addition to the van der Waals attraction, neutral and negatively charged tips experienced a repulsive force. This repulsive force depended on the salt concentration. It decayed exponentially with distance having a decay length similar to the Debye length. Typical forces were about 0.1 nN strong. With positively charged tips, purely attractive forces were observed. Comparing these results with calculations showed the electrostatic origin of this force.In the presence of high concentrations (> 3 M) of divalent cations, where the electrostatic force can be completely ignored, another repulsive force was observed with silicon nitride tips on mica. This force decayed roughly exponentially with a decay length of 3 nm and was approximately 0.07-nN strong. This repulsion is attributed to the hydration force.

Force measurements with the atomic force microscope: Technique, interpretation and applications

“…Ducker et al [15] glued silica spheres onto cantilevers while Butt used glass spheres [16]. With smooth, spherical particles of defined radius (1) the force can be analyzed more quantitatively; (2) the total force is higher and thus the measurement can be more sensitive; (3) it is possible to make a variety of probes by attaching particles of different chemical composition to the cantilevers; and (4) hydrodynamic force measurements become possible.…”

“…Due to the high melting point of silica (1723 8C) it is unpractical to sinter them to cantilevers. In this respect glass particles [16,163,164], which are usually made from a borosilicate glass, are easier to sinter since their melting point is below 800 8C. Their surface is as smooth as that of silica particles.…”

“…It is not only of fundamental interest but also of direct practical relevance when it comes to dispersing solid particles in a liquid. One major step to measure surface forces quantitatively was the introduction of the colloidal probe technique [15,16]. In the colloidal probe technique a spherical particle of typically 2-20 mm diameter is attached to the end of the cantilever.…”

Probing the validity of the Derjaguin approximation for heterogeneous colloidal particles

“…With the SFA, the interaction forces are measured between two curved mica plates in the crossed-cylinder geometry. 2,14,15 The colloidal probe technique, which is based on an atomic force microscope (AFM), uses a colloidal particle of a few micrometers, attached to the end of the cantilever to probe the interaction force with a planar substrate (sphere-plane geometry) 10,[15][16][17][18][19][20][21] or with a second colloidal particle mounted to the substrate (sphere-sphere geometry). [22][23][24][25][26] While any of these techniques could be used to assess the validity of the Derjaguin approximation, to the best of our knowledge there is only one study addressing this question.…”

Characterisation of dry powder inhaler formulations using atomic force microscopy

“…Colloidal probe AFM, developed by Butt (1991) and Ducker et al (1991) in the early 1990s, involves attaching a particle to a tipless cantilever and using the modified probe to investigate the interactions between the attached particle and a substrate (Butt et al, 2005). The method requires careful preparation of the probe.…”