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.     

Frequency dependence of the freezing temperature in spin glasses: A comparative study of (La, Gd)B6, (Y, Gd)Al2 and (La, Gd)Al2

The frequency dependence of the freezing temperatureT _f() is determined for the dilute spin glass systems (La, Gd)B₆ and (Y, Gd)Al₂ in the frequency range 10–1,000 Hz. While for (La, Gd)B₆,T _f() is found to be weak, for (Y, Gd)Al₂ T _f() is even stronger than for the previously studied system (La, Gd)Al₂. Both, measurements of the temperature dependence of the susceptibility nearT _f and calculations of the RKKY pair interaction, suggest that this difference is correlated with a different sign of the nearest-neighbor interaction, which appears to be antiferromagnetic for (La, Gd)B₆ and ferromagnetic for (Y, Gd)Al₂ as well as (La, Gd)Al₂.