The kinetics and mechanism of vesicle self-assembly in aqueous SDS/DTAB mixtures

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Vesicles may form spontaneously when aqueous solutions of cationic and anionic n-alkyl surfactants, are mixed together at certain concentration ratios. These vesicles once formed are stable and grow to a finite size. We have examined the kinetic processes involved in such vesicle formation using sodium dodecyl sulfate (SDS) and dodecyl trimethylammonium bromide (DTAB) mixed at a weight ratio of 1.6 to 1.0. A major aspect of our investigation makes use of the acid-base and solvatochromic probe, 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio) phenoxide, ET(30). ET(30) resides in the interfacial region of self-assembled surfactant aggregates and therefore can be used to "report" on the interfacial dielectric constant and electrostatic potential of the vesicle surface as it spontaneously self-assembles. Our results suggest that the interfacial environment of the vesicles that are formed is quickly established, in the order of seconds. Further, it was found that the rate of formation of the vesicle interface was strongly influenced by electrostatic interactions. High electrostatic potentials were measured (> -100 mV) on the membrane surfaces. The morphology of the vesicle intermediates was determined by cryo-TEM and growth was found to occur over a period of days or longer. A micelle- to-vesicle transition mechanism consistent with current understanding of surfactant self-assembly thermodynamics is proposed. © 2013 The authors mentioned in the table of contents. All rights reserved.

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