Adsorption of single and mixed ionic surfactants at fluid interfaces

Area of Science:

• Physical Chemistry
• Surface Science
• Colloid Science

Background:

• Two models exist for surfactant ionization effects on surface pressure (π).
• These include a molecular model with Gouy-Chapman theory and a two-dimensional (2D) solution approach.
• The 2D approach treats the surface as electroneutral, with adsorption as the sum of monolayer and electrical double-layer excesses.

Purpose of the Study:

• To compare and reconcile two distinct theoretical approaches for describing surfactant ionization effects on surface pressure.
• To provide arguments for the negligible contribution of electrical double layers to surface pressure in many surfactant solutions.
• To explain experimental observations in both single and mixed ionic surfactant solutions.

Main Methods:

• Analysis of surface pressure (π) data based on molecular models and 2D solution theory.
• Application of Gouy-Chapman theory and consideration of counterion binding.
• Evaluation of experimental results for single and mixed ionic surfactant solutions with varying electrolyte concentrations.

Main Results:

• Both molecular and 2D solution models can adequately describe experimental surface pressure data.
• Surface pressure (π) and adsorption are often independent of salt concentration (cXY) at a given mean ionic activity.
• Double-layer contributions to surface pressure are frequently negligible, particularly in mixed surfactant systems.

Conclusions:

• The two models provide consistent interpretations of experimental data.
• Negligible double-layer effects are attributed to significant ion binding and the vanishing contribution of the diffuse double layer for 1:1 electrolytes.
• The 2D electroneutral solution approach offers a simplified explanation for features in mixed surfactant solutions.