Study on the surface density of surface-active substances through total-reflection X-ray absorption fine structure...

Area of Science:

• Surface Science
• Physical Chemistry
• Materials Science

Background:

• The total-reflection X-ray absorption fine structure (XAFS) method has been used to study adsorption at interfaces.
• Understanding surfactant adsorption is crucial for various applications.

Purpose of the Study:

• To validate the total-reflection XAFS method for determining surface concentrations of dodecyltrimethylammonium bromide (DTAB) in the presence of NaBr.
• To extend the application of XAFS to a nonionic amphiphile, 6-bromo-1-hexanol (BrC6OH).

Main Methods:

• Utilizing Br K-edge absorption jump values from total-reflection XAFS spectra to evaluate surface bromide ion concentrations.
• Comparing XAFS-derived surface concentrations with those estimated from surface tension measurements.
• Applying the method to both cationic (DTAB) and nonionic (BrC6OH) amphiphiles.

Main Results:

• XAFS-derived surface concentrations for DTAB closely matched surface tension-derived values up to the critical micelle concentration (cmc), confirming method applicability with added salt.
• The XAFS method also accurately determined surface concentrations for the nonionic amphiphile 6-bromo-1-hexanol.
• Excellent agreement was observed between XAFS and surface tension methods for both surfactant types.

Conclusions:

• The total-reflection XAFS method is a reliable technique for quantifying surface concentrations of cationic surfactants and nonionic amphiphiles, with or without added salts.
• The study confirms the versatility and accuracy of XAFS in surface chemistry research.
• Differences between XAFS and surface tension derived values are discussed for DTAB systems.