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.     

Hydrolyzable poly(ester-urethane) networks from L-lysine diisocyanate and D,L-lactide/ϵ-caprolactone homo- and copolyester triols

Bioabsorbable poly(ester-urethane) networks were synthesized from ethyl 2,6-diisocyanatohexanoate (L -lysine diisocyanate) (LDI) and a series of polyester triols. LDI was synthesized by refluxing L-lysine monohydrochloride with ethanol to form the ester, which was subsequently refluxed with 1,1,1,3,3,3-hexamethyldisilazane to yield a silazane-protected intermediate. This product was then phosgenated using triphosgene. Polyester triols were synthesized from D,L-lactide, ?-caprolactone, or comonomer mixtures thereof, using glycerol as initiator and stannous octoate as catalyst. Polyurethane networks were cured using [NCO]/[OH] = 1.05 and stannous octoate (0.05 wt %) for 24 h at room temperature and pressure and 24 h at 50°C and 0.1 mm Hg. LDI-based polyurethane networks were totally amorphous and possessed very low sol contents. Networks based on poly (D,L-lactide) triols were rigid (T_g ∽ 60°C) with ultimate tensile strengths of ～ 40–70 MPa, tensile moduli of ～ 1.2–2.0 GPa, and ultimate elongations of ～ 4–10%. Networks based on ?-caprolactone triols were low-modulus elastomers with tensile strengths and moduli of ～ 1–4 MPa and ～ 3–6 GPa, respectively, and ultimate elongations of ～ 50–300%. Networks based on copolymers displayed physical properties consistent with monomer composition and were tougher than the networks based on the homopolymers. Tensile strengths for the copolymers were ～ 3–25 MPa with ultimate elongations up to 600%. Hydrolytic degradation under simulated physiological conditions showed that D ,L -lactide homopolymer networks were the most resistant to degradation, undergoing virtually no change in mass or physical properties for 60 days. ?-Caprolactone-based networks were resistant to degradation for 40 days, and high-lactide copolymer-based networks suffered substantial losses in physical properties after only 3 days. © 1994 John Wiley & Sons, Inc.     

7-azasteroid analogues—I

Although 7-methoxy- and 6,7-dimethoxy-2-methyl-4-vinyl-isocarbostyrils could not be isolated and characterised, they have been generated in situ and shown to undergo cyclo-addition reactions with typical dienophiles. The enol acetate of 2-methyl-4-acetylisocarbostyril has also been trapped as its adduct with maleic anhydride and with p-benzoquinone.