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  • Author or Editor: P. Cullis x
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Abstract  

An enormous amount of detailed information can be obtained concerning macromolecules in aqueous solution using data obtained from differential scanning and titration microcalorimetry. This claim is supported by reference to examples taken from recent work concerned with micelles (e.g. CTAB), vesicles (e.g. DOAB and DDP) and enzymes (e.g. CAT and DNA gyrase).

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Abstract  

The gel to liquid-crystal transition for vesicles in aqueous solution formed by dimethyldi-n-octadecylammonium bromide (DOAB) occurs at 44.7C. Moreover, the shapes of the scans recorded by a sensitive DSC microcalorimeter are very similar when the vesicular solutions are prepared starting with solid DOAB and comparable amounts of either solid urea or solid alkylureas. Therefore, the DOAB vesicles in aqueous solution accommodate this class of solutes without marked changes in the melting temperature and the enthalpy of the transition. The contrast with effects of added surfactants and simple organic solutes such as THF and ethanol is particularly significant.

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In a titration calorimetric study an aqueous solution held in a syringe and containing hexadecyltrimethylammonium bromide (CTAB; 15.4×10−3 mol dm−3) is injected in aliquots (5×l0−3 dm3) into a sample cell containing initially water. Analysis of the data shows that thecmc equals 0.97×l0−3 dm−3 and the enthalpy of micelle formation equals −10.3 kJ mol−1. When the solution in the syringe is replaced by a mixed surfactant solution, CTAB+dodecyltrimethylammonium bromide, at the same total concentration of surfactant, thecmc of CTAB decreases gradually with increasing mole fraction of DOTAB but the enthalpy of CTAB micelle formation is hardly affected. We conclude, therefore, that incorporation of DOTAB monomers into the CTAB micelles stabilizes entropically the CTAB micelles.

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