Adsorption parameters and phase behaviour of non-ionic surfactants at liquid interfaces

Area of Science:

• Physical Chemistry
• Surface Science
• Colloid Science

Background:

• Established adsorption models (van der Waals, Frumkin) show inconsistencies with experimental data for surfactants at liquid interfaces.
• These inconsistencies include inaccurate prediction of adsorption parameters like hard-disc area (α) and attraction parameter (β).
• Existing models fail to accurately represent phase behavior and molecular structure-dependent adsorption.

Purpose of the Study:

• To investigate alternative adsorption models suitable for liquid interfaces.
• To develop a model that accurately predicts surfactant adsorption parameters based on molecular structure and medium conditions.
• To classify surfactant adsorption behavior based on derived parameters.

Main Methods:

• Evaluation of less common adsorption models, focusing on the sticky disc model.
• Development of a new model for lateral attraction to explain the dependence of the attraction parameter (β).
• Application of experimental data and the law of corresponding states to determine critical points.

Main Results:

• The sticky disc model successfully describes adsorption behavior for homologous surfactant series at water|air and water|oil interfaces.
• The hard-disc area (α) derived from the sticky disc model is interface-independent and consistent with crystallographic data.
• A proposed lateral attraction model shows a non-linear relationship between β and factors like chain length, head group area, and temperature.

Conclusions:

• The sticky disc model provides a more accurate framework for understanding surfactant adsorption at liquid interfaces.
• The derived lateral attraction model allows for the classification of surfactants into cohesive/non-cohesive and sub-critical/super-critical categories based on β values.
• This work offers a predictive tool for surfactant behavior, crucial for applications in various chemical processes.