Hydration of bromide at reverse micelle interfaces studied by X-ray absorption fine structure

Area of Science:

• Physical Chemistry
• Materials Science
• Nanotechnology

Background:

• Nanoconfined water exhibits unique properties crucial for fundamental science and practical applications.
• Reverse micelles (RMs) are versatile systems for creating nanoconfined water, but their interfacial properties require deeper understanding.
• The interaction between water and the RM interface influences water properties, with details remaining elusive.

Purpose of the Study:

• To investigate the local structure and hydration of bromide (Br-) ions within hexadecyltrimethylammonium bromide (HTAB) reverse micelles.
• To determine how the molar ratio of water to surfactant (w) affects Br- hydration at the RM interface.
• To elucidate the impact of interfacial components (water, surfactant headgroups, solvent) on ion behavior.

Main Methods:

• Utilized X-ray absorption fine structure (XAFS) spectroscopy to probe the local environment of Br- ions.
• Studied HTAB RMs in chloroform and 10% hexanol/heptane mixtures across a wide range of water-to-surfactant ratios (w = 0-30).
• Employed a linear combination of spectra representing individual scattering paths to analyze complex XAFS data, overcoming limitations of standard fitting methods.

Main Results:

• Observed significant changes in Br- local structure and hydration as a function of the water-to-surfactant ratio (w).
• Identified a maximum hydration number of 4.5 for Br- ions at w > 15.
• Demonstrated that most ions remain at the interface as partly hydrated species, while a fraction dissociates into fully hydrated ions.

Conclusions:

• The hydration state of Br- ions is strongly dependent on the water content within HTAB reverse micelles.
• At higher water ratios, a portion of bromide ions become fully hydrated, indicating dissociation from the interface.
• These findings provide critical insights into the interfacial chemistry and ion behavior in nanoconfined aqueous systems.