XAS STUDY OF SOLUBILIZATION LOCI OF BROMINATED MOLECULES IN AQUEOUS MICELLAR SOLUTIONS

X-ray absorption measurements performed on various systems containing brominated hydrocarbons showed remarkable differences in the bromine K-edge spectra recorded in polar or nonpolar media. For this reason, the brominated hydrocarbons can be used to monitor the polarity of the medium in which they are buried. By determining the coordination of the bromine atom, information on the interactions between the probe molecule and several systems such as micelles, macromolecules, membranes, solvent molecules, and host molecules in inclusion compounds can be obtained. Brominated hydrocarbons in aqueous micellar solutions of sodium and rubidium deoxycholate and sodium dodecyl sulfate have been investigated by means of the XAS technique. Experimental and calculated EXAFS spectra and Fourier transform functions are presented. The results are supported by the XANES experimental spectra. As already found for 2-bromopropane, the polarity of the solubilization loci of bromoethane in sodium and rubidium deoxycholate micellar solutions decreases by increasing the solute concentration. On the contrary, in micellar solutions of sodium dodecyl sulfate, these molecules are embedded in a more apolar environment. The polarity of the spectra of 2-bromopropane changes with the probe concentration, while bromoethane presents a marked apolar coordination which seems to be independent from the probehost ratio. The bromine intermolecular coordination in 1-bromobutane has been found to be apolar in both classes of surfactants. Functionalized surfactants with -ene, -yne, or bromo as the terminal group of the alkyl chain were used by different researchers to investigate micellar structures. In fact, for a micellar aggregate with a hydrocarbon core and an outer region containing the polar heads, the chain terminal group may be buried in the micelle core or placed in the head group region of the micelle, in contact with water. In the latter case, the alkyl chain flexibility may give rise to a chain folded conformation which increases the probability of finding the terminal group at the micelle-water interface. EXAFS data analysis of aqueous micellar solutions of sodium and rubidium 12-bromo dodecyl sulfate and sodium 1 I-bromoundecanoate at the Br K-edge has been accomplished. The chemical constitution of the locus of solubilization of the terminal bromine has been determined, and it has been found to be apolar for the dodecyl sulfate salts and polar in the case of sodium 1 1-bromoundecanoate.