Surfactant adsorption onto interfaces: measuring the surface excess in time.

Area of Science:

• Physical Chemistry
• Colloid and Surface Science
• Materials Science

Background:

• Characterizing surfactants with very low critical micellar concentrations (CMC) is challenging.
• Traditional methods for determining surface properties often require complex models or indirect measurements.
• Understanding dynamic surface properties is crucial for applications involving interfaces.

Purpose of the Study:

• To introduce a direct and simplified method for measuring equilibrium and dynamic surface properties of surfactant solutions.
• To validate the proposed method using well-characterized nonionic (C12E6) and ionic (CTAB) surfactants.
• To accurately determine surfactant diffusion coefficients from dynamic surface tension measurements.

Main Methods:

• Utilized a pendant drop tensiometer to measure surface tension variations.
• Studied solutions of hexaethylene glycol monododecyl ether (C12E6) and hexadecyl trimethyl ammonium bromide (CTAB) with sodium bromide.
• Analyzed surface tension as a function of surface concentration and time.

Main Results:

• The method directly yielded surface tension versus surface concentration, aligning with Gibbs adsorption equation predictions.
• Time-dependent surface tension measurements revealed a diffusion-controlled adsorption process for both surfactants.
• Extracted diffusion coefficients for C12E6 and CTAB showed excellent agreement with literature values.

Conclusions:

• The pendant drop tensiometry method provides a direct, model-independent approach to determine equilibrium and dynamic surface properties for low CMC surfactants.
• The method accurately quantifies surfactant adsorption kinetics and allows for reliable determination of diffusion coefficients.
• This technique offers a robust validation of surfactant behavior at interfaces.