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The effect of counterion nature and concentration on phase transition, bilayer structure, vesicle size, vesicle internal volume per mole of amphiphile, and surface potential is evaluated for cationic vesicles composed of dioctadecyldimethylammonium (DODA) acetate, chloride, or bromide. Over a range of ionic strengths (0−5 mM monovalent salt), no interdigitation was detected in the bilayer structure for the three DODA counterions. The preferential type of aggregate formed from self-assembly of DODA salts is a large vesicle composed of a single traditional and noninterdigitated bilayer. Vesicle size and zeta-potentials were inversely related, i.e., an increase in zeta-potential was accompanied by a decrease in vesicle size. The largest zeta-potentials and smallest sizes were obtained for bilayer vesicles of DODA acetate which have the largest and more hydrated counterion. The effect of ionic strength (0−5 mM NaAc, NaCl, or NaBr as monovalent salt) was a slight decrease followed by a significant increase in vesicle size as a function of salt concentration. The results for counterion effects on vesicle size agree with predictions from the self-assembly model by Israelachvili and co-workers.

Ribeiro, pela orientação competente durante todo o decorrer do trabalho e por ter me iniciado na pesquisa cientifica. Ao Professor Dr. Miguel Jafelicci Júnior, da Universidade Estadual Paulista (UNESP)-campus de Araraquara-pela determinação da área superficial específica das partículas de sílica. À Sandra Micelli Sicchierolli, por ter me apresentado a linha de pesquisa do laboratório. A todos que passaram pelo grupo no decorrer do trabalho, Lílian Rumi