Ionic Specificity in Rapid Coagulation of Silica Nanoparticles

Area of Science:

• Colloid and surface science
• Physical chemistry

Background:

• The Smoluchowski theory is the standard for estimating rapid coagulation rates of colloidal particles in electrolytes.
• This theory has limitations as it neglects particle size and ion specificity, particularly the effects of adsorbed molecular layers.
• Accurate modeling requires accounting for structured layers of water, ions, and hydrated ions on colloidal surfaces.

Purpose of the Study:

• To investigate the rapid coagulation rates of silica nanoparticles in concentrated electrolyte solutions.
• To determine the influence of particle diameter and ion hydration energy on coagulation rates.
• To explore deviations from the Smoluchowski theory in nanoparticle systems.

Main Methods:

• Measurement of rapid coagulation rates using a low-angle light-scattering apparatus.
• Systematic variation of silica nanoparticle size (D_p).
• Use of concentrated chloride and potassium salt solutions to study ion effects.

Main Results:

• Coagulation rate (K_E^R) decreased exponentially with decreasing particle size and increasing negative Gibbs free energy of hydration (-ΔG_hyd) for both cations and anions.
• Silica nanoparticles below ~70 nm in 1 M KNO3 and KSCN solutions exhibited no coagulation, contrary to larger particles (>100 nm) and other potassium solutions.
• Observed phenomena were explained through proposed mechanisms addressing nanoparticle-specific interactions.

Conclusions:

• The Smoluchowski theory's limitations are evident for nanoparticles due to neglected size and ion-specific effects.
• Particle size and ion hydration energy significantly impact nanoparticle coagulation rates.
• Specific ion-electrolyte interactions can lead to unexpected coagulation behavior in concentrated solutions.