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, th e brominated hydrocarbons can be used to monitor the polarity of the m edium in which they are buried. By determining the coordination of the bromine atom, information on the interactions between the probe molec ule and several systems such as micelles, macromolecules, membranes, s olvent molecules, and host molecules in inclusion compounds can be obt ained. Brominated hydrocarbons in aqueous micellar solutions of sodium and rubidium deoxycholate and sodium dodecyl sulfate have been invest igated by means of the XAS technique. Experimental and calculated EXAF S spectra and Fourier transform functions are presented. The results a re 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 in creasing the solute concentration. On the contrary, in micellar soluti ons of sodium dodecyl sulfate, these molecules are embedded in a more a polar environment. The polarity of the spectra of 2-bromopropane cha nges with the probe concentration, while bromoethane presents a marked apolar coordination which seems to be independent from the probe-host ratio. The bromine intermolecular coordination in 1-bromobutane has b een found to be apolar in both classes of surfactants. Functionalized surfactants with -ene, -yne, or bromo as the terminal group of the alk yl chain were used by different researchers to investigate micellar st ructures. In fact, for a micellar aggregate with a hydrocarbon core an d 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 c hain flexibility may give rise to a chain folded conformation which in creases the probability of finding the terminal group at the micelle-w ater interface. EXAFS data analysis of aqueous micellar solutions of s odium and rubidium 12-bromo dodecyl sulfate and sodium 11-bromoundecan oate at the Br K-edge has been accomplished. The chemical constitution of the locus of solubilization of the terminal bromine has been deter mined, and it has been found to be apolar for the dodecyl sulfate salt s and polar in the case of sodium 11-bromoundecanoate.