| Summary: | 碩士 === 國立成功大學 === 化學工程學系碩博士班 === 90 === This study investigated the effects of composition on the spontaneous formation of catanionic vesicles in the mixed surfactant systems of an anionic surfactant, sodium dodecylsulfate (SDS), and a cationic surfactant, dodecyltrimethylammonium bromide (DTMAB), tetradecyltrimethyl- ammonium bromide (TTMAB), or hexadecyltrimethylammonium bromide (HTMAB). The formation of microstructures in the mixed surfactant systems was analyzed by the measurements of particle size and zeta potential and by the observation with transmission electron microscopy.
The mixed anionic/cationic surfactant systems with a molar ratio of 1:1 were prepared first, and their equilibrium surface tension behavior was studied by the Wilhelmy plate method. The results demonstrated that the mixed surfactants had a greater ability of lowering surface tension in comparison with that of single surfactants. Moreover, the significant slope changes in the equilibrium surface tension versus concentration curves were apparently due to the solubility limits of the mixed surfactants.
For the mixed SDS/DTMAB systems, crystallites easily formed over a wide range of compositions and the amount of spontaneously-formed vesicles was small. However, when the molar fraction of SDS was in excess of 0.5, only vesicles or micelles were detected. For the mixed systems of SDS with TTMAB or HTMAB, the turbidity of the mixtures strongly depended on the molar ratio of the single surfactants. When the molar fraction for one of the surfactants was higher than 0.5, the turbidity of the mixtures became lower and the transition from crystallites to vesicles or micelles was detected by particle size measurements. In the filtrated solutions of the mixtures, the spontaneously-formed vesicles with a higher absolute value of zeta potential were always found with a larger size. In addition, when the SDS molar fraction became higher, the zeta potential of the vesicles changed from positive values to negative ones. For the mixed SDS/DTMAB systems with symmetry in hydrocarbon chain length, it appears that compact structures, such as crystallites, could easily form. However, the particle size and zeta potential measurements indicated that for the mixed surfactants with asymmetry in hydrocarbon chain length, the dominant formation of vesicles or micelles resulted.
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