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, bromi de, 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 incre ase 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 ene rgy for vesicle formation is, perhaps surprisingly, largely unaffected by a n increase in temperature, as a consequence of a compensating change in bot h standard entropy and standard enthalpy of vesicle formation. Interestingl y, standard isobaric heat capacities of vesicle formation are negative, lar ge 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. (C) 2000 Academic Press.