Counterions under a Surface-Adsorbed Cationic Surfactant Monolayer: Structure and Thermodynamics

Area of Science:

• Physical Chemistry
• Surface Science
• Materials Science

Background:

• Surface adsorption of ionic surfactants is crucial for life sciences and technology.
• Atomic-resolution studies of noncrystalline adsorbed films are limited, leading to uncertainty in charge density and theoretical models.

Purpose of the Study:

• To determine the surface-normal charge distribution of adsorbed cetyltrimethylammonium bromide (CTAB) films.
• To compare experimental findings with existing theoretical models like Gouy-Chapman and Stern layer models.
• To refine understanding of surfactant adsorption thermodynamics.

Main Methods:

• X-ray reflectivity measurements of CTAB films in aqueous solutions at varying temperatures and concentrations.
• Integration of published neutron reflectivity data.
• Analysis using Langmuir and Kralchevsky adsorption models.

Main Results:

• Ångström-scale surface-normal structure of CTAB films was obtained.
• Charge distribution is inconsistent with the Gouy-Chapman model.
• Findings align with a compact Stern layer model of condensed counterions.
• Experimental adsorption thermodynamics match classical adsorption models.

Conclusions:

• The study provides atomic-scale structural insights into ionic surfactant adsorption.
• A compact Stern layer model accurately describes counterion condensation.
• The Kralchevsky model successfully reproduces experimental observations and allows parameter resolution.