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

Nanoconfined water exhibits various interesting properties, which are not only of fundamental importance but also of practical use. Because reverse micelles (RMs) provide versatile ways to prepare nanoconfined water, the understanding of their physicochemical properties is essential for developing efficient applications. Although the water properties in the RMs could be affected by its interaction with the RM interface, the details have not been well understood. This study focuses on the local structures of Br- in hexadecyltrimethylammonium bromide (HTAB) RMs formed in chloroform and 10% hexanol/heptane. The dependence in Br- hydration on the molar ratio of water to HTAB (w) is investigated using X-ray absorption fine structure (XAFS). These systems cover a wide range of w values (0-30) and allow us to study the impact of this parameter on the local structure of Br- at the RM interface, which comprises water, surfactant headgroups, and organic solvent components. The presence of multiple scattering paths complicates the XAFS spectra and makes it difficult to analyze them using standard fitting methods. The linear combination of the spectra corresponding to the individual scattering paths captures the molecular processes that occur at the RM interface upon increasing w. The maximum hydration number of Br- is found to be 4.5 at w > 15, suggesting that although most of the ions remain at the interface as partly hydrated ions, some of them dissociate as completely hydrated ones.

Reverse micelle chromatography for evaluation of partition of organic solutes to micellar pseudophases.

The evaluation of solute partition from an organic solvent to a reverse micelle (RM) is critical for designing effective reaction systems and for synthesizing functional materials using the nano water-phase in the RM. Although spectroscopic methods have been extensively employed for determining the partition constants of solutes in the RM systems, their applications are limited to the case, in which the background absorption is low and the spectroscopic features of the solute are effectively varied by its partitioning to RMs. This paper proposes a novel chromatographic method to overcome this limitation of conventional methods. In the present system, a size-exclusion chromatographic column is used with RM solutions as mobile phases. The RM is excluded from the stationary phase and, therefore, is eluted first. A solute is eluted with the retention volume determined by its affinities to the stationary phase and the RM. The elution volumes of a solute measured by varying the concentrations of RM in the mobile phase allow us to determine its partition constant to the RM. The partition of phenols in hexadecyltrimethylammonium chloride and bromide RMs in chloroform is successfully evaluated without interferences from the UV-absorption of the media. The larger partition constant is confirmed for a smaller water/surfactant molar ratio (w) and for chloride than bromide as the RM counterion. This suggests that the nature of water is more strongly influenced by confinement in the RM cores for smaller w and the chloride counterion because water molecules are strongly imbibed by the interface.