DSC Investigations of DDAB,DTAB and DHAB Vesicle Aqueous Solutions in Presence of SDS

Interactions of anionic surfactant sodium dodecyl sulphate (SDS) with vesicles formed by synthetic dialkyldimethylammonium bromides i.e. didodecyldimethylammonium bromide (DDAB), ditetradecyldimethylammonium bromide (DTAB), dihexadecyldimethylammonium bromide (DHAB) have been examined by using differential scanning microcalorimetry and electron transmission microscopy. The temperatures and standard enthalpies characterising gel to liquid-crystal transitions increase significantly with increase of SDS concentration for all investigated systems. It means that incorporation of SDS monomers in these vesicular bilayers significantly stabilises their gel states. Moreover, in case of DDAB and DTAB vesicle systems added SDS limits kinetic features of recorded isobaric heat capacity dependencies on temperature observed earlier for the pure vesicular solutions. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effects of Added Urea and Alkylureas on Gel to Liquid-Crystal Transitions in Doab Vesicles

The gel to liquid-crystal transition for vesicles in aqueous solution formed by dimethyldi-n-octadecylammonium bromide (DOAB) occurs at 44.7°C. 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. This revised version was published online in August 2006 with corrections to the Cover Date.     

Determination of compositions of NaI solid solvates obtained from the solutions in mixtures of water with acetonitrile and N,N-dimethylformamide at 25°C

The thermogravimetric method was applied to determine the compositions of solid NaI existing in equilibrium with saturated salt solutions over the whole range of the compositions of the mixtures of water with AN and DMF at 25°C. It was concluded that NaI is preferentially hydrated in water-rich region only for both investigated systems, i.e. forx _AN<0.15 andx _DMF<0.25. Above the mentioned organic solvent content the significant differences in solvation of NaI are observed for both systems. In water-AN system for 0.15<x _AN<0.95 NaI is weakly preferentially hydrated and the composition of the solvate is: NaI·H₂O·CH₃CN. In pure AN, NaI is not solvated. In water-DMF system in the ranges of composition 0.25<x _DMF<0.50 and 0.50<x _DMF<0.80 two types of mixed solvates are observed: NaI·4H₂O·2DMF, and NaI·2H₂O·2DMF, respectively. So, NaI is strongly solvated by both components of mixed solvent. Forx _DMF>0.80 like in pure DMF, NaI is solvated by two molecules of DMF.     

Viscosities of NaI in water-formamide and water-N,N-dimethylformamide mixtures from 5 to 45°C

Water-N,N-dimethylformamide mixtures show viscosity maxima near 30 mole percent DMF at all temperatures studied, whereas the viscosity of water-formamide mixtures is an increasing monotonic function of the mole percent formamide in the mixtures. This fundamental difference in their viscosity behavior must be attributed mainly to differences in the hydrogen bonding characteristics of the two amides. In spite of this difference, the effect on the viscosity of added NaI to these two aqueous mixtures is surprisingly similar over the temperature range studied.     

The effect of alkyltrimethylammonium bromides on the thermal stability of dioctadecyldimethylammonium bromide (DOAB) vesicles in aqueous solutions

The influence of alkyl chain length in alkyltrimethylammonium bromides on the gel to liquid crystal transitions in DOAB versicles is examined using differential scanning microcalorimetric data. The changes in melting temperature, patch number and standard enthalpy of melting for DOAB vesicles depend strongly on added surfactants and their concentration. The data show that vesicles are readily penetrated by surfactant molecules when the vesicles are in the liquid crystal state and the penetration is facile when the length of alkyl chains in both surfactants and vesicles are comparable. Furthermore, the vesicles are made up of domains that differ in composition.