A Titration Microcalorimetric Study of the Effects of Halide Counterions on Vesicle-Forming Aggregation in Aqueous Solution of Branched-Chain Alkylpyridinium Surfactants

Titration microcalorimetry is used to study the influences of iodide, bromide, and chloride counterions on the aggregation of vesicle-forming 1-methyl-4-(2-pentylheptyl)pyridinium halide surfactants. Formation of vesicles by these surfactants was characterised using transmission electron microscopy. When the counterion is changed at 303 K through the series iodide, bromide, to chloride, the critical vesicular concentration (cvc) increases and the enthalpy of vesicle formation changes from exo- to endothermic. With increase in temperature to 333 K, vesicle formation becomes strongly exothermic. Increasing the temperature leads to a decrease in enthalpy and entropy of vesicle formation for all three surfactants. However the standard Gibbs energy for vesicle formation is, perhaps surprisingly, largely unaffected by an increase in temperature, as a consequence of a compensating change in both standard entropy and standard enthalpy of vesicle formation. Interestingly, standard isobaric heat capacities of vesicle formation are negative, large in magnitude but not strikingly dependent on the counterion. We conclude that the driving force for vesicle formation can be understood in terms of overlap of the thermally labile hydrophobic hydration shells of the alkyl chains. Copyright 2000 Academic Press.     

Temperature dependence of second critical micelle concentration of dodecyldimethylbenzylammonium bromide in aqueous solution

The specific conductivity of dodecyldimethylbenzylammonium bromide (C12BBr) in aqueous solutions, in the temperature range of 15 to 40 °C, has been measured as a function of molality. The two breaks which were found on the conductivity against molality plots were attributed to the critical micelle concentration, cmc, and second critical micelle concentration, 2^nd cmc, respectively. The ratio of the slopes, S, of the three linear fragments on the plots, S2/S1 and S3/S1, was attributed to the degree of ionization of the micelles at cmc and 2^nd cmc respectively. It was shown that the values of the 2^nd cmc estimated above 27 °C are only apparent due to thermal disintegration of the micelles. In the temperature range of 15 to 27 °C, the values of the 2^nd cmc increase gradually and the plot of the 2^nd cmc against temperature is concave. The ratio of 2^nd cmc/cmc for C12BBr at 25 °C amounts to 15 and appears to be high compared to the literature values for other surfactants. For comparative purposes the cmc and 2^nd cmc values were also estimated conductometrically for decyldimethylbenzylammonium bromide (C10BBr) at 25 °C. The 2^nd cmc value for this surfactant is higher compared to the value for the C12 homologue by a factor of 2.6.The standard Gibbs free energies of micellization at cmc and at the 2^nd cmc were estimated from the experimental data for both surfactants at 25 °C.     

Effects of Adsorbed Polyaniline on Redox Process on MoO3 Surface

The effects exhibited by adsorbed conducting polyaniline on the redox process on a molybdenum oxide surface were studied. Thermogravimetric results indicate a 4% polyaniline deposition. Cyclic voltammograms of the adsorbed polymer on MoO3 show that polyaniline exerts remarkable effects on the molybdenum blue oxidation-reduction process, with oxidation and reduction potentials of 0.33 and 0.18 V, respectively. This effect strongly enhances the electrode response, and can be used as an important tool in qualitative and/or quantitative determinations of molybdenum in solution as well as in any substrate. Copyright 1999 Academic Press.     

Supershell structure in alkali metal nanowires

Nanowires are formed by indenting and subsequently retracting two pieces of sodium metal. Their cross section gradually reduces upon retraction and the diameters can be obtained from the conductance. In previous work we have demonstrated that when one constructs a histogram of diameters from large numbers of indentation-retraction cycles such histograms show a periodic pattern of stable nanowire diameters due to shell structure in the conductance modes. Here, we report the observation of a modulation of this periodic pattern, in agreement with predictions of a supershell structure.     

Effect of alkyl chain asymmetry on catanionic mixtures of hydrogenated and fluorinated surfactants

In this work we studied and compared the physicochemical properties of the catanionic mixtures cetyltrimethyl-ammonium bromide–sodium dodecanoate, cetyltrimethyl-ammonium bromide–sodium perfluorodacanoate, octyltrimethylammonium bromide–sodium perfluorodacanoate and cetyltrimethyl-ammonium bromide–sodium octanoate by a combination of rheological, transmission electron microscopy (TEM) and polarized optical microscopy measurements. The binary mixtures of the surfactants have been analyzed at different mixed ratios and total concentration of the mixture. Mixtures containing a perfluorinated surfactant are able to form lamellar liquid crystals and stable spontaneous vesicles. Meanwhile, system containing just hydrogenated surfactants form hexagonal phases or they are arranged in elongated aggregates.     

Pion Electro-Production in the Region of Low-Lying P11 and S11 Resonances

A coupled-channel approach incorporating quasi-bound quark-model states has been used to calculate pion-nucleon scattering amplitudes in the P11 and S11 partial waves and the corresponding pion electro-production amplitudes. The method allows for the extraction of the resonant part of the amplitudes even in the presence of different decay channels and a strong mixing of neighbouring resonances. We demonstrate the important role of the pion cloud at large distances, as opposed to the short-distance behaviour governed by the quark core.     

Thermal stability of lysozyme and myoglobin in the presence of anionic surfactants

The interactions of lysozyme and myoglobin with anionic surfactants (hydrogenated and fluorinated), at surfactant concentrations below the critical micelle concentration, in aqueous solution were studied using spectroscopic techniques. The temperature conformational transition of globular proteins by anionic surfactants was analysed as a function of denaturant concentration through absorbance measurements at 280 nm. Changes in absorbance of protein-surfactant system with temperature were used to determine the unfolding thermodynamics parameters, melting temperature, T _m, enthalpy, ΔH _m, entropy, ΔS _m and the heat capacity change, ΔC _p, between the native and denatured states.