Hydration Structures of Bromides on Cationic Micelles

Abstract

The peripheral structures of bromides on dodecyltrimethylammonium bromide (DTAB) and hexadecyltrimethylammonium bromide (HTAB) micelles have been studied by X-ray absorption fine structure (XAFS) at the Br-K edge. The XAFS spectra indicate that water is a dominant scattering group for Br- even in these micellar solutions. However, the oscillation intensity decreases with increasing micellar concentration, suggesting that the bromides are dehydrated to some extent when they are bound to the micelles. A XAFS analysis routine gives unusually short Br-O (water) distances and is inapplicable to the present systems. This comes from the structure of the first coordination shell, in which two or more scattering paths are involved. The second scattering group is obviously the head group of the surfactants forming the micelles. The detailed analysis has allowed us to estimate the hydration number of the bromides bound on the DTAB and HTAB micelles (N = ca. 4.2). The assumption that all of the bromides form direct ion-associates with the head groups causes the contradiction to the results of the XAFS analyses. This strongly implies that some of the bromides partitioned into the micelle are completely hydrated as far as their first coordination shell is concerned. Assuming that the maximum hydration number of the bromides bound to the head groups of the micelle is three, 40% of the bromides partitioned into the micelle are completely hydrated.