title: Morphology and local organization of water-containing (1R,2S)-dodecyl(2-hydroxy-1-methyl-2-phenylethyl)dimethylammonium bromide reverse micelles dispersed in toluene.
abstract: The structure of (1R,2S)-dodecyl(2-hydroxy-1-methyl-2-phenylethyl)dimethylammonium bromide (DMEB) reverse micelles, at various water-to-surfactant molar ratio (Rw, Rw = [water]/[DMEB]) and DMEB concentrations, has been investigated by small angle X-ray scattering (SAXS) and extended X-ray absorption fine structure spectroscopy (EXAFS). SAXS data of dry reverse micelles are consistent with a model of spherical hydrophilic core surrounded by DMEB alkyl chains whose polydispersity decreases significantly with surfactant concentration. By adding water, a sphere to cylinder transition occurs leading to a one-dimensional growth of reverse micellar cores with Rw and surfactant concentration. The observed behavior was taken as an indication that water molecules are confined in the core of DMEB reverse micelles, quite uniformly distributed among them and mainly located among surfactant head groups. EXAFS data allow to focus within the hydrophilic micellar core to solve the short range local environment around the Br(-) counterion and to follow its changing with surfactant concentration and Rw. Analysis of Fourier transform of the EXAFS spectra indicates the existence of a local order nearby the bromide ions; pointing toward a quite structured hydrophilic core of DMEB reverse micelles. However, as a consequence of the fluid nature of reverse micelles, such local order is lower than that found in the lamellar structure of solid DMEB. Water confinement within the reverse micellar cores induces an increase of the local disorder suggesting an enhancement of the micellar core dynamics.
authors.affiliation: Netherlands Organization for Scientific Research (NWO), ESRF-The European Synchrotron, CS40220 38043 Grenoble, Cedex 9, France.
authors.forename: Alessandro
authors.initials: A
authors.lastname: Longo
authors.affiliation: Dipartimento Fisica e Chimica Universitá di Palermo Viale delle Scienze Ed. 17, 90128 Palermo, Italy.
authors.forename: Francesco
authors.initials: F
authors.lastname: Giannici
authors.affiliation: Netherlands Organization for Scientific Research (NWO), ESRF-The European Synchrotron, CS40220 38043 Grenoble, Cedex 9, France.
authors.forename: Giuseppe
authors.initials: G
authors.lastname: Portale
authors.affiliation: Dutch-Belgian Beamline (DUBBLE), ESRF-The European Synchrotron, CS40220 38043 Grenoble, Cedex 9, France.
authors.forename: Dipanjan
authors.initials: D
authors.lastname: Banerjee
authors.affiliation: CNR-ISMN, Consiglio Nazionale delle Ricerche - Istituto per lo Studio dei Materiali Nanostrutturati, Via Salaria km 29.300, 00015 Monterotondo Stazione (RM), Italy.
authors.forename: Pietro
authors.initials: P
authors.lastname: Calandra
authors.affiliation: Dipartimento STEBICEF, Universit degli Studi di Palermo, Viale delle Scienze Parco dOrleans II, 90128 Palermo, Italy.
authors.forename: Vincenzo
authors.initials: V
authors.lastname: Turco Liveri