title: Kinetic asymmetry in the gel-liquid crystalline state transitions of DDAB vesicles studied by differential scanning calorimetry.
abstract: Didodecyldimethylammonium bromide (DDAB) (1.0 mM) vesicles in water were investigated by differential scanning calorimetry (DSC) to highlight the existing kinetic asymmetry in the gel-liquid crystalline (LC) state transitions. The experiments were performed in the range of temperature, scan rate and pre-scanning time 5-45 degrees C, 15-90 degrees C/h and 0-16 h, respectively, in the up- and down-scanning modes. Depending on the input parameters and number of heating-cooling cycles, the DSC thermograms exhibit a sharp peak, a broad band or a flat shape. A melting temperature T(m)=15.6-16.0 degrees C, given by the peak position, was obtained independently of the scan rate used in the up-scanning mode. The data reveal that DDAB vesicles exhibit much slower kinetics for the LC to gel state than for the opposite transition. Such an asymmetry is supported by: (a) the absence of peak for shorter pre-scanning times but longer scan rates, (b) the increasing intensity of the DSC peak with increasing pre-scanning time and decreasing scan rate, and (c) the complete absence of peak in the down-scan mode. Longer pre-scanning time, however, yields crystal precipitates due to a Krafft phenomenon, which also reduces the peak intensity. The overall results depend on whether the sample is fresh or not, that is, after some heating-cooling cycles, the melting peak requires a longer pre-scanning time to be detected. The kinetic asymmetry explains, for example, the lack of any DSC melting peak reported for "non-fresh" DDAB vesicles, which was as yet unexplained.
authors.affiliation: Group of Colloid Physics, Physics Department, IBILCE/UNESP, São José do Rio Preto, SP, Brazil. eloi@ibilce.unesp.br
authors.forename: Eloi
authors.initials: E
authors.lastname: Feitosa
mesh_terms: D002152:Calorimetry, Differential Scanning; D002412:Cations / Q000737:chemistry; D044367:Phase Transition; D000644:Quaternary Ammonium Compounds / Q000737:chemistry*; D044366:Transition Temperature
chemical_list: D002412:Cations; D000644:Quaternary Ammonium Compounds; C046112:didodecyldimethylammonium