Nanometer-ranged attraction induced by multivalent ions between similar and dissimilar surfaces probed using an...

Area of Science:

• Colloid and Surface Science
• Physical Chemistry
• Materials Science

Background:

• Understanding inter-particle forces is crucial for controlling colloidal systems.
• Existing theories like DLVO explain long-range electrostatic interactions.
• Short-range forces often deviate from DLVO predictions and require further investigation.

Purpose of the Study:

• To directly measure forces between amidine latex (AL) and sulfate latex (SL) particles.
• To investigate the influence of multivalent cations (hexamines, N6) versus monovalent salts (KCl) on these forces.
• To analyze force profiles using DLVO theory and identify deviations at short distances.

Main Methods:

• Atomic Force Microscopy (AFM) was employed for direct force measurements.
• Experiments were conducted on three particle pairs: AL-AL, AL-SL, and SL-SL.
• Measurements were performed in solutions of KCl and multivalent hexamines (N6).

Main Results:

• DLVO theory successfully described forces at distances beyond several nanometers.
• A short-ranged, non-DLVO attractive force was observed at closer distances.
• The range of this short-ranged attraction varied with particle type and electrolyte: 0.3 nm (KCl), 1.0 nm (SL-SL, N6), 0.6 nm (AL-SL, N6), and 0.3 nm (AL-AL, N6).

Conclusions:

• DLVO theory, when accounting for asymmetric electrolytes and charge regulation, provides a good framework for long-range forces.
• A significant short-ranged attraction, not explained by classical DLVO theory, is present between latex particles.
• The range of this non-DLVO attraction is modulated by the presence of multivalent cations, suggesting complex surface interactions.