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

Area of Science:

• Surface science
• Atomic force microscopy
• Physical chemistry

Background:

• Atomic force microscopy (AFM) involves tip-surface interactions including electrostatic, van der Waals, and hydration forces in electrolyte solutions.
• Understanding these forces is crucial for accurate imaging of charged surfaces.

Purpose of the Study:

• To investigate electrostatic interaction forces versus distance curves in electrolyte solutions.
• To characterize repulsive forces and their dependence on salt concentration, pH, and tip surface charge.
• To identify and attribute observed repulsive forces to electrostatic and hydration origins.

Main Methods:

• AFM force-distance curves were recorded using various charged tips (silicon nitride, Al(2)O(3), glass, diamond) on negatively charged surfaces (mica, glass).
• Experiments were conducted across a range of salt concentrations and pH values.
• High salt concentrations (>3 M divalent cations) were used to isolate hydration forces.

Main Results:

• Neutral and negatively charged tips exhibited repulsive forces dependent on salt concentration, decaying exponentially with a length similar to the Debye length.
• Positively charged tips showed only attractive forces.
• A distinct repulsive force, attributed to hydration, was observed with silicon nitride tips on mica at high salt concentrations, decaying exponentially with a 3 nm length.

Conclusions:

• The observed repulsive forces are primarily electrostatic in origin, influenced by salt concentration and tip charge.
• Hydration forces become significant and observable in the absence of electrostatic interactions at high divalent cation concentrations.
• AFM is a valuable tool for quantifying and differentiating surface forces in electrolyte environments.