Single-particle thermal diffusion of charged colloids: double-layer theory in a temperature gradient

Area of Science:

• Colloid Science
• Physical Chemistry
• Soft Matter Physics

Background:

• Thermophoresis describes particle movement in response to temperature gradients.
• The double-layer structure around charged colloids significantly influences their thermophoretic behavior.
• Existing models often simplify the complex forces within the double layer.

Purpose of the Study:

• To calculate the double-layer contribution to the thermal diffusion coefficient of charged spherical colloids.
• To extend the Debye-Hückel theory to account for arbitrary double-layer thickness under a temperature gradient.
• To compare theoretical predictions with experimental data.

Main Methods:

• Extended Debye-Hückel theory incorporating a temperature gradient.
• Analysis of three distinct forces: electrostatic energy (F W), electric force (Fel), and solvent-friction force (Fsol).
• Derivation of explicit expressions for the thermal diffusion coefficient.

Main Results:

• Identified three key forces contributing to thermophoresis in charged colloids.
• Showed that F W aligns with irreversible-thermodynamics predictions.
• Demonstrated that Fel and Fsol capture double-layer asymmetry effects beyond traditional treatments.
• Derived expressions for the thermal diffusion coefficient applicable to various double-layer thicknesses.

Conclusions:

• The study provides a more comprehensive model for colloid thermophoresis by including electric and solvent-friction forces.
• The derived expressions enhance understanding of thermal diffusion coefficients in charged colloids.
• The findings offer improved theoretical predictions that can be validated against experimental results.